fix

example.env

@@ -80,6 +80,13 @@ OXICLOUD_SERVER_HOST=127.0.0.1
 # higher = less background DB work. Minimum enforced: 30s.
 #OXICLOUD_STORAGE_USAGE_RECONCILE_SECS=600
 
+# How often (milliseconds) the background job drains storage.tree_etag_dirty
+# and bumps folder tree ETags (default: 500). Write paths only enqueue bump
+# requests — this is the upper bound on how stale an ancestor folder's ETag
+# (WebDAV/NextCloud collection sync signal) can be after a change. Lower =
+# fresher sync detection, higher = fewer background UPDATEs. Minimum: 100.
+#OXICLOUD_TREE_ETAG_FLUSH_MS=500
+
 # Allow multiple processes to bind to the same port (SO_REUSEPORT).
 # DISABLED by default — leaving this off means a second accidental instance
 # will fail immediately with "address already in use", which is the safe behaviour.

migrations/20260627000000_async_tree_etag_queue.sql

@@ -0,0 +1,196 @@
+-- Asynchronous tree-ETag bumps: enqueue-only triggers + background flusher.
+--
+-- The statement-level triggers from `20260626000000_tree_etag_statement_triggers`
+-- still updated ancestor folder rows inside the writer's own transaction,
+-- locking them with `SELECT … ORDER BY id FOR UPDATE`. That lock conflicts
+-- with the FOR KEY SHARE lock the FK RI check on files.folder_id takes on
+-- the parent folder row at INSERT time — a shared-to-exclusive upgrade on
+-- the same hot row. Two concurrent uploads into one folder form a mutual
+-- cycle; PostgreSQL's deadlock detector (deadlock_timeout = 1 s) aborts one
+-- victim per cycle. Measured on a real bulk upload: 363 deadlocks, 31 % of
+-- uploads failed with HTTP 500, throughput collapsed from a demonstrated
+-- ~185 files/s burst to ~1 resolved transaction per second.
+--
+-- The fix removes ALL folder-row locking from user-facing write paths:
+--
+--   * The four bump functions now only INSERT the affected lpath targets
+--     into `storage.tree_etag_dirty` — a plain heap append with no unique
+--     constraints, taking zero shared row locks. Parallel uploads now only
+--     share FOR KEY SHARE locks on the parent folder (mutually compatible),
+--     so N-way concurrent writes cannot conflict, by construction.
+--   * A single background task in the app (`TreeEtagFlushService`, on the
+--     maintenance pool) drains the queue every ~500 ms and applies ONE
+--     batched ancestor UPDATE with deterministic id-order locking.
+--
+-- Semantics preserved from 20260626000000 (see that migration's header):
+--   * depth guard: bumps fired from inside another trigger's DML
+--     (FK cascades, the lpath cascade rewrite) are skipped;
+--   * UPDATE value filters: only DAV-observable column changes count
+--     (the EXIF media_sort_date sync never bumps);
+--   * file moves cover the OLD parent chain as well as the NEW one;
+--   * folder events bump STRICT ancestors only (self-exclusion).
+--
+-- Semantic delta, decided deliberately: ancestor ETags become eventually
+-- consistent (≤ ~1 flush interval after commit) instead of same-transaction.
+-- No reader requires same-transaction freshness: collection ETags are only
+-- compared across successive PROPFIND polls (seconds apart), and no mutation
+-- response embeds a freshly bumped ANCESTOR etag (folder create/rename/move
+-- responses return the row's OWN tree_modified_at, which bumps never touch).
+-- The flusher orders its bump strictly after the writer's commit, so an ETag
+-- can never change before the content that caused it is visible.
+
+-- ── Dirty queue ──────────────────────────────────────────────────────
+-- Each row is one "bump request", dual-keyed:
+--   * `lpath`     — the target chain CAPTURED at mutation time. Covers
+--                   chains whose folders are deleted or moved away by
+--                   flush time (the old location's surviving ancestors
+--                   still get their bump — unrecoverable from an id).
+--   * `folder_id` — the same target folder's id, re-resolved to its
+--                   CURRENT lpath at flush time. Covers the converse
+--                   race: a folder MOVED inside the flush window has its
+--                   whole subtree's lpaths rewritten, so the captured
+--                   lpath no longer prefix-matches it and the pending
+--                   bump would otherwise be silently lost (a sync client
+--                   would never discover the change).
+-- The flusher bumps the inclusive ancestor closure of the UNION of both.
+-- File events target their parent folder; folder events target their
+-- PARENT (subpath drops the last label), preserving self-exclusion —
+-- one uniform inclusive queue semantic.
+--
+-- Append-only between flushes; the flusher deletes what it processes.
+-- Logged (not UNLOGGED): a crash must not lose bumps. Duplicates are
+-- expected and welcome; the flusher dedups. No FK on folder_id and no
+-- indexes beyond the PK: enqueue must stay a pure append,
+-- contention-free under any write concurrency.
+CREATE TABLE IF NOT EXISTS storage.tree_etag_dirty (
+    id        BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
+    lpath     ltree NOT NULL,
+    folder_id UUID
+);
+
+-- Self-heal for databases that applied an earlier draft of THIS
+-- migration (folder_id added after initial rollout of the queue).
+ALTER TABLE storage.tree_etag_dirty ADD COLUMN IF NOT EXISTS folder_id UUID;
+
+-- ── File side: INSERT / DELETE ───────────────────────────────────────
+-- Both triggers alias their transition table to `changed_rows`; one body
+-- serves both events. Reading fo.lpath is a plain MVCC read — no locks.
+-- Root-level files (folder_id IS NULL) have no ancestors; a parent row
+-- deleted in the same statement drops out of the JOIN (the folder-side
+-- trigger of the outer statement covers the surviving ancestors).
+CREATE OR REPLACE FUNCTION storage.bump_tree_from_files_stmt()
+RETURNS TRIGGER LANGUAGE plpgsql AS $$
+BEGIN
+    IF pg_trigger_depth() > 1 THEN
+        RETURN NULL;
+    END IF;
+
+    INSERT INTO storage.tree_etag_dirty (lpath, folder_id)
+    SELECT DISTINCT fo.lpath, fo.id
+      FROM (SELECT DISTINCT folder_id
+              FROM changed_rows
+             WHERE folder_id IS NOT NULL) c
+      JOIN storage.folders fo ON fo.id = c.folder_id;
+
+    RETURN NULL;
+END;
+$$;
+
+-- ── File side: UPDATE ────────────────────────────────────────────────
+-- Union of OLD and NEW parent chains so a move invalidates both the
+-- source and the destination collection ETags. The value filter keeps
+-- the 20260626000000 semantics: only DAV-observable changes enqueue
+-- (PostgreSQL forbids `AFTER UPDATE OF <cols>` with transition tables,
+-- so the filter lives here).
+CREATE OR REPLACE FUNCTION storage.bump_tree_from_files_stmt_upd()
+RETURNS TRIGGER LANGUAGE plpgsql AS $$
+BEGIN
+    IF pg_trigger_depth() > 1 THEN
+        RETURN NULL;
+    END IF;
+
+    WITH changed AS (
+        SELECT o.folder_id AS old_folder_id, n.folder_id AS new_folder_id
+          FROM old_rows o
+          JOIN new_rows n USING (id)
+         WHERE (o.name, o.folder_id, o.blob_hash, o.size,
+                o.mime_type, o.is_trashed, o.updated_at)
+               IS DISTINCT FROM
+               (n.name, n.folder_id, n.blob_hash, n.size,
+                n.mime_type, n.is_trashed, n.updated_at)
+    )
+    INSERT INTO storage.tree_etag_dirty (lpath, folder_id)
+    SELECT DISTINCT fo.lpath, fo.id
+      FROM (SELECT old_folder_id AS folder_id
+              FROM changed WHERE old_folder_id IS NOT NULL
+            UNION
+            SELECT new_folder_id
+              FROM changed WHERE new_folder_id IS NOT NULL) c
+      JOIN storage.folders fo ON fo.id = c.folder_id;
+
+    RETURN NULL;
+END;
+$$;
+
+-- ── Folder side: INSERT / DELETE ─────────────────────────────────────
+-- Strict ancestors only: enqueue the PARENT's lpath (subpath drops the
+-- last label), so the inclusive queue semantic covers parent + ancestors
+-- but never the folder itself. Root folders (nlevel = 1) enqueue nothing.
+-- The lpath value is captured HERE, at mutation time — by flush time the
+-- row may be gone (purge) or rewritten (move), and the OLD chain would
+-- be unrecoverable from a folder id.
+CREATE OR REPLACE FUNCTION storage.bump_tree_from_folders_stmt()
+RETURNS TRIGGER LANGUAGE plpgsql AS $$
+BEGIN
+    IF pg_trigger_depth() > 1 THEN
+        RETURN NULL;
+    END IF;
+
+    INSERT INTO storage.tree_etag_dirty (lpath, folder_id)
+    SELECT DISTINCT subpath(lpath, 0, nlevel(lpath) - 1), parent_id
+      FROM changed_rows
+     WHERE lpath IS NOT NULL
+       AND nlevel(lpath) > 1;
+
+    RETURN NULL;
+END;
+$$;
+
+-- ── Folder side: UPDATE ──────────────────────────────────────────────
+-- Union of OLD and NEW parent chains (a move bumps the chain it left and
+-- the chain it joined), same value filter as 20260626000000: descendant
+-- path/lpath rewrites by `trg_folders_cascade_path` change none of the
+-- compared columns and additionally run at depth 2.
+CREATE OR REPLACE FUNCTION storage.bump_tree_from_folders_stmt_upd()
+RETURNS TRIGGER LANGUAGE plpgsql AS $$
+BEGIN
+    IF pg_trigger_depth() > 1 THEN
+        RETURN NULL;
+    END IF;
+
+    WITH changed AS (
+        SELECT o.lpath AS old_lpath, o.parent_id AS old_parent_id,
+               n.lpath AS new_lpath, n.parent_id AS new_parent_id
+          FROM old_rows o
+          JOIN new_rows n USING (id)
+         WHERE (o.name, o.parent_id, o.is_trashed, o.updated_at)
+               IS DISTINCT FROM
+               (n.name, n.parent_id, n.is_trashed, n.updated_at)
+    )
+    INSERT INTO storage.tree_etag_dirty (lpath, folder_id)
+    SELECT DISTINCT subpath(c.lpath, 0, nlevel(c.lpath) - 1), c.parent_id
+      FROM (SELECT old_lpath AS lpath, old_parent_id AS parent_id
+              FROM changed WHERE old_lpath IS NOT NULL
+            UNION
+            SELECT new_lpath, new_parent_id
+              FROM changed WHERE new_lpath IS NOT NULL) c
+     WHERE nlevel(c.lpath) > 1;
+
+    RETURN NULL;
+END;
+$$;
+
+-- The six statement-level triggers from 20260626000000 keep their names
+-- and wiring — they reference these functions by name, so replacing the
+-- bodies above is the whole swap. `trg_folders_cascade_path` and the
+-- BEFORE path triggers are untouched.

src/common/config.rs

@@ -248,6 +248,12 @@ pub struct StorageConfig {
     /// quota fresh for all mutations without recomputing on the request path.
     /// Default: 600 (10 min). Env: `OXICLOUD_STORAGE_USAGE_RECONCILE_SECS`.
     pub usage_reconcile_secs: u64,
+    /// Interval (milliseconds) of the background job that drains
+    /// `storage.tree_etag_dirty` and bumps folder `tree_modified_at`
+    /// (collection ETags). Write paths only enqueue — this is the upper
+    /// bound on how stale an ancestor folder's ETag can be after a change.
+    /// Default: 500. Env: `OXICLOUD_TREE_ETAG_FLUSH_MS`.
+    pub tree_etag_flush_ms: u64,
     /// Which blob storage backend to use (`local`, `s3`, or `azure`).
     pub backend: StorageBackendType,
     /// S3-compatible backend configuration (used when `backend == S3`).
@@ -390,6 +396,7 @@ impl Default for StorageConfig {
             upload_temp_dir: None,
             chunk_dir: None,
             usage_reconcile_secs: 600, // 10 minutes
+            tree_etag_flush_ms: 500,
             backend: StorageBackendType::Local,
             s3: None,
             azure: None,
@@ -1291,6 +1298,13 @@ impl AppConfig {
             config.storage.usage_reconcile_secs = val;
         }
 
+        // Tree-ETag dirty-queue flush cadence
+        if let Ok(ms) = env::var("OXICLOUD_TREE_ETAG_FLUSH_MS").map(|v| v.parse::<u64>())
+            && let Ok(val) = ms
+        {
+            config.storage.tree_etag_flush_ms = val;
+        }
+
         // Storage backend selection
         if let Ok(backend) = env::var("OXICLOUD_STORAGE_BACKEND") {
             match backend.to_lowercase().as_str() {

src/common/di.rs

@@ -657,6 +657,25 @@ impl AppServiceFactory {
         service
     }
 
+    /// Starts the tree-ETag flush job (requires database).
+    ///
+    /// The statement triggers on `storage.files`/`storage.folders` only
+    /// enqueue bump requests into `storage.tree_etag_dirty` (so user-facing
+    /// writes take no folder-row locks); this job is the single drainer that
+    /// turns them into `tree_modified_at` updates. It must run whenever the
+    /// database is up — the triggers are always installed, and an undrained
+    /// queue grows unboundedly while folder ETags freeze. Fire-and-forget on
+    /// the maintenance pool, like the trash cleanup job.
+    fn start_tree_etag_flush_job(&self, maintenance_pool: &Arc<PgPool>) {
+        let service =
+            crate::infrastructure::services::tree_etag_flush_service::TreeEtagFlushService::new(
+                maintenance_pool.clone(),
+                self.config.storage.tree_etag_flush_ms,
+            );
+        service.start_flush_job();
+        tracing::info!("Tree-ETag flush service initialized");
+    }
+
     /// Builds the complete AppState using all factory services.
     ///
     /// This is the main entry point that replaces all manual logic in `main.rs`.
@@ -745,6 +764,8 @@ impl AppServiceFactory {
             storage_usage_service =
                 Some(self.create_storage_usage_service(&repos, &pool, &maintenance_pool));
 
+            self.start_tree_etag_flush_job(&maintenance_pool);
+
             // User-lifecycle dispatcher. Hook order is registration order;
             // document dependencies inline if/when any arise. Today:
             //   1. AuditLifecycleHook             — fires first so the

src/domain/entities/folder.rs

@@ -227,11 +227,15 @@ impl Folder {
         self.owner_id
     }
 
-    /// Latest descendant-write timestamp, maintained by a Postgres
-    /// trigger that walks the ltree ancestor chain on every file or
-    /// folder write inside this folder's subtree. See migration
-    /// `20260625000000_folder_tree_modified_at.sql` for the trigger
-    /// definition.
+    /// Latest descendant-write timestamp. Statement-level Postgres
+    /// triggers enqueue every file/folder write into
+    /// `storage.tree_etag_dirty`; the background `TreeEtagFlushService`
+    /// drains the queue (default every 500 ms) and bumps the ltree
+    /// ancestor chain in one batched UPDATE. Eventually consistent:
+    /// the bump lands within ~one flush interval AFTER the change is
+    /// committed, never before. See migration
+    /// `20260627000000_async_tree_etag_queue.sql` for the rationale
+    /// (user-facing writes must take zero folder-row locks).
     pub fn tree_modified_at(&self) -> u64 {
         self.tree_modified_at
     }
@@ -257,19 +261,26 @@ impl Folder {
     /// - The 16-char UUID prefix gives the folder its identity
     ///   component — keeps two empty same-mtime folders distinct.
     /// - `tree_modified_at` (Unix seconds) is the actual signal:
-    ///   bumped by trigger whenever ANY descendant (file or
-    ///   sub-folder, at any depth) is created, modified, deleted,
-    ///   or moved. This is the contract NextCloud's sync engine
-    ///   relies on — "did anything change inside this collection
-    ///   since I last looked?". Until this column existed, the
-    ///   answer was always "no" because the folder UUID never
-    ///   changed; clients had to do periodic deep PROPFIND walks
-    ///   to discover web-uploaded files.
+    ///   bumped whenever ANY descendant (file or sub-folder, at any
+    ///   depth) is created, modified, deleted, or moved. This is the
+    ///   contract NextCloud's sync engine relies on — "did anything
+    ///   change inside this collection since I last looked?". Until
+    ///   this column existed, the answer was always "no" because the
+    ///   folder UUID never changed; clients had to do periodic deep
+    ///   PROPFIND walks to discover web-uploaded files.
+    /// - The bump is asynchronous: triggers enqueue, the
+    ///   `TreeEtagFlushService` applies (≤ ~one flush interval after
+    ///   commit, monotonic — two flushes in the same wall-clock
+    ///   second still yield distinct values). Clients only compare
+    ///   etags across successive polls, so the short lag is
+    ///   unobservable; what matters is the bump never precedes the
+    ///   change becoming visible.
     /// - Renaming the folder itself does NOT change the etag's
-    ///   identity portion (UUID is stable across renames). The
-    ///   trigger does bump `tree_modified_at` on rename via the
-    ///   folder-side trigger, so the etag still changes — which is
-    ///   correct, the parent collection's listing changed.
+    ///   identity portion (UUID is stable across renames). A rename
+    ///   does enqueue the ancestor chain, so the PARENT's etag still
+    ///   changes — which is correct, the parent collection's listing
+    ///   changed; the folder's own value stays untouched
+    ///   (self-exclusion).
     pub fn compute_etag(id: &str, tree_modified_at: u64) -> String {
         let prefix: String = id.chars().take(16).collect();
         format!("{}-{}", prefix, tree_modified_at)

src/infrastructure/repositories/pg/file_blob_write_repository.rs

@@ -20,6 +20,7 @@ use crate::domain::entities::file::File;
 use crate::domain::services::path_service::StoragePath;
 
 use super::folder_db_repository::FolderDbRepository;
+use super::transaction_utils::retry_on_deadlock;
 use crate::infrastructure::services::dedup_service::DedupService;
 
 /// File write repository backed by PostgreSQL metadata + blob storage.
@@ -157,24 +158,28 @@ impl FileBlobWriteRepository {
         modified_at: Option<i64>,
     ) -> Result<(String, i64), DomainError> {
         // Atomic CTE: capture old hash then update in one round-trip, no TOCTOU.
-        let (old_hash, updated_at) = match sqlx::query_as::<_, (String, i64)>(
-            r#"
-            WITH old AS (
-                SELECT id, blob_hash FROM storage.files WHERE id = $3::uuid FOR UPDATE
+        // Deadlock victims (40P01) retry before the compensation below runs —
+        // a successful retry must keep the new blob reference alive.
+        let (old_hash, updated_at) = match retry_on_deadlock("files.swap_blob_hash", || {
+            sqlx::query_as::<_, (String, i64)>(
+                r#"
+                WITH old AS (
+                    SELECT id, blob_hash FROM storage.files WHERE id = $3::uuid FOR UPDATE
+                )
+                UPDATE storage.files f
+                   SET blob_hash = $1, size = $2,
+                       updated_at = COALESCE(to_timestamp($4), NOW())
+                  FROM old
+                 WHERE f.id = old.id
+                RETURNING old.blob_hash, EXTRACT(EPOCH FROM f.updated_at)::bigint
+                "#,
             )
-            UPDATE storage.files f
-               SET blob_hash = $1, size = $2,
-                   updated_at = COALESCE(to_timestamp($4), NOW())
-              FROM old
-             WHERE f.id = old.id
-            RETURNING old.blob_hash, EXTRACT(EPOCH FROM f.updated_at)::bigint
-            "#,
-        )
-        .bind(new_hash)
-        .bind(new_size)
-        .bind(file_id)
-        .bind(modified_at.map(|t| t as f64))
-        .fetch_optional(self.pool.as_ref())
+            .bind(new_hash)
+            .bind(new_size)
+            .bind(file_id)
+            .bind(modified_at.map(|t| t as f64))
+            .fetch_optional(self.pool.as_ref())
+        })
         .await
         {
             Ok(Some(row)) => row,
@@ -236,23 +241,30 @@ impl FileBlobWriteRepository {
         let is_new_blob = !dedup_result.was_deduplicated();
         let blob_hash = dedup_result.hash().to_string();
 
-        let row = match sqlx::query_as::<_, (String, i64, i64)>(
-            r#"
-            INSERT INTO storage.files (name, folder_id, user_id, blob_hash, size, mime_type, category_order)
-            VALUES ($1, $2::uuid, $3, $4, $5, $6, $7)
-            RETURNING id::text,
-                      EXTRACT(EPOCH FROM created_at)::bigint,
-                      EXTRACT(EPOCH FROM updated_at)::bigint
-            "#,
-        )
-        .bind(&name)
-        .bind(&folder_id)
-        .bind(user_id)
-        .bind(&blob_hash)
-        .bind(size as i64)
-        .bind(&content_type)
-        .bind(category_order_for(&name, &content_type))
-        .fetch_one(self.pool.as_ref())
+        // Deadlock victims (40P01) retry before the compensation below runs —
+        // a successful retry must keep the blob reference alive. The final
+        // attempt's error falls through untouched so the 23505 mapping holds
+        // (a retried INSERT can legitimately lose to a concurrent identical
+        // upload).
+        let row = match retry_on_deadlock("files.insert", || {
+            sqlx::query_as::<_, (String, i64, i64)>(
+                r#"
+                INSERT INTO storage.files (name, folder_id, user_id, blob_hash, size, mime_type, category_order)
+                VALUES ($1, $2::uuid, $3, $4, $5, $6, $7)
+                RETURNING id::text,
+                          EXTRACT(EPOCH FROM created_at)::bigint,
+                          EXTRACT(EPOCH FROM updated_at)::bigint
+                "#,
+            )
+            .bind(&name)
+            .bind(&folder_id)
+            .bind(user_id)
+            .bind(&blob_hash)
+            .bind(size as i64)
+            .bind(&content_type)
+            .bind(category_order_for(&name, &content_type))
+            .fetch_one(self.pool.as_ref())
+        })
         .await
         {
             Ok(row) => row,
@@ -372,46 +384,48 @@ impl FileWritePort for FileBlobWriteRepository {
         // Single round-trip; blob content is NOT copied (dedup makes this zero-copy).
         let target_fid = target_folder_id.clone();
 
-        let row = sqlx::query_as::<
-            _,
-            (
-                String,
-                String,
-                Option<String>,
-                i64,
-                String,
-                i64,
-                i64,
-                String,
-            ),
-        >(
-            r#"
-            WITH src AS (
-                SELECT name, folder_id, user_id, blob_hash, size, mime_type, category_order
-                  FROM storage.files
-                 WHERE id = $1::uuid AND NOT is_trashed
-            ),
-            new_file AS (
-                INSERT INTO storage.files (name, folder_id, user_id, blob_hash, size, mime_type, category_order)
-                SELECT name,
-                       COALESCE($2::uuid, folder_id),
-                       user_id,
-                       blob_hash,
-                       size,
-                       mime_type,
-                       category_order
-                  FROM src
-                RETURNING id::text, name, folder_id::text, size, mime_type,
-                          EXTRACT(EPOCH FROM created_at)::bigint,
-                          EXTRACT(EPOCH FROM updated_at)::bigint,
-                          blob_hash
+        let row = retry_on_deadlock("files.copy", || {
+            sqlx::query_as::<
+                _,
+                (
+                    String,
+                    String,
+                    Option<String>,
+                    i64,
+                    String,
+                    i64,
+                    i64,
+                    String,
+                ),
+            >(
+                r#"
+                WITH src AS (
+                    SELECT name, folder_id, user_id, blob_hash, size, mime_type, category_order
+                      FROM storage.files
+                     WHERE id = $1::uuid AND NOT is_trashed
+                ),
+                new_file AS (
+                    INSERT INTO storage.files (name, folder_id, user_id, blob_hash, size, mime_type, category_order)
+                    SELECT name,
+                           COALESCE($2::uuid, folder_id),
+                           user_id,
+                           blob_hash,
+                           size,
+                           mime_type,
+                           category_order
+                      FROM src
+                    RETURNING id::text, name, folder_id::text, size, mime_type,
+                              EXTRACT(EPOCH FROM created_at)::bigint,
+                              EXTRACT(EPOCH FROM updated_at)::bigint,
+                              blob_hash
+                )
+                SELECT * FROM new_file
+                "#,
             )
-            SELECT * FROM new_file
-            "#,
-        )
-        .bind(file_id)
-        .bind(&target_fid)
-        .fetch_optional(self.pool.as_ref())
+            .bind(file_id)
+            .bind(&target_fid)
+            .fetch_optional(self.pool.as_ref())
+        })
         .await
         .map_err(|e| {
             if let sqlx::Error::Database(ref db_err) = e
@@ -557,23 +571,25 @@ impl FileWritePort for FileBlobWriteRepository {
         // The write-behind cache will call update_file_content later.
         let placeholder_hash = "0000000000000000000000000000000000000000000000000000000000000000";
 
-        let row = sqlx::query_as::<_, (String, i64, i64)>(
-            r#"
-            INSERT INTO storage.files (name, folder_id, user_id, blob_hash, size, mime_type, category_order)
-            VALUES ($1, $2::uuid, $3, $4, $5, $6, $7)
-            RETURNING id::text,
-                      EXTRACT(EPOCH FROM created_at)::bigint,
-                      EXTRACT(EPOCH FROM updated_at)::bigint
-            "#,
-        )
-        .bind(&name)
-        .bind(&folder_id)
-        .bind(user_id)
-        .bind(placeholder_hash)
-        .bind(size as i64)
-        .bind(&content_type)
-        .bind(category_order_for(&name, &content_type))
-        .fetch_one(self.pool.as_ref())
+        let row = retry_on_deadlock("files.insert_deferred", || {
+            sqlx::query_as::<_, (String, i64, i64)>(
+                r#"
+                INSERT INTO storage.files (name, folder_id, user_id, blob_hash, size, mime_type, category_order)
+                VALUES ($1, $2::uuid, $3, $4, $5, $6, $7)
+                RETURNING id::text,
+                          EXTRACT(EPOCH FROM created_at)::bigint,
+                          EXTRACT(EPOCH FROM updated_at)::bigint
+                "#,
+            )
+            .bind(&name)
+            .bind(&folder_id)
+            .bind(user_id)
+            .bind(placeholder_hash)
+            .bind(size as i64)
+            .bind(&content_type)
+            .bind(category_order_for(&name, &content_type))
+            .fetch_one(self.pool.as_ref())
+        })
         .await
         .map_err(|e| DomainError::internal_error("FileBlobWrite", format!("deferred: {e}")))?;
 

src/infrastructure/repositories/pg/folder_db_repository.rs

@@ -12,6 +12,7 @@ use sqlx::PgPool;
 use std::sync::Arc;
 use uuid::Uuid;
 
+use super::transaction_utils::retry_on_deadlock;
 use crate::application::dtos::folder_dto::{FolderResourceCursor, FolderResourceRow};
 use crate::common::errors::DomainError;
 use crate::domain::entities::folder::Folder;
@@ -450,20 +451,25 @@ impl FolderRepository for FolderDbRepository {
         // The BEFORE UPDATE trigger recomputes path/lpath for this row;
         // the AFTER UPDATE cascade trigger then batch-updates all
         // descendants in a single UPDATE using the GiST lpath index.
-        let row = sqlx::query_as::<_, FolderRow>(
-            r#"
-            UPDATE storage.folders
-               SET name = $1, updated_at = NOW()
-             WHERE id = $2::uuid AND NOT is_trashed
-            RETURNING id::text, name, path, parent_id::text, user_id,
-                      EXTRACT(EPOCH FROM created_at)::bigint,
-                      EXTRACT(EPOCH FROM updated_at)::bigint,
-                       EXTRACT(EPOCH FROM tree_modified_at)::bigint
-            "#,
-        )
-        .bind(&new_name)
-        .bind(id)
-        .fetch_optional(self.pool())
+        // That multi-row rewrite can deadlock against the tree-ETag
+        // flusher's id-ordered ancestor bump — retry instead of failing
+        // the user's operation (40P01 only; 23505 still maps below).
+        let row = retry_on_deadlock("folders.rename", || {
+            sqlx::query_as::<_, FolderRow>(
+                r#"
+                UPDATE storage.folders
+                   SET name = $1, updated_at = NOW()
+                 WHERE id = $2::uuid AND NOT is_trashed
+                RETURNING id::text, name, path, parent_id::text, user_id,
+                          EXTRACT(EPOCH FROM created_at)::bigint,
+                          EXTRACT(EPOCH FROM updated_at)::bigint,
+                           EXTRACT(EPOCH FROM tree_modified_at)::bigint
+                "#,
+            )
+            .bind(&new_name)
+            .bind(id)
+            .fetch_optional(self.pool())
+        })
         .await
         .map_err(|e| {
             if let sqlx::Error::Database(ref db_err) = e
@@ -486,20 +492,23 @@ impl FolderRepository for FolderDbRepository {
         // The BEFORE UPDATE trigger recomputes path/lpath for this row;
         // the AFTER UPDATE cascade trigger then batch-updates all
         // descendants in a single UPDATE using the GiST lpath index.
-        let row = sqlx::query_as::<_, FolderRow>(
-            r#"
-            UPDATE storage.folders
-               SET parent_id = $1::uuid, updated_at = NOW()
-             WHERE id = $2::uuid AND NOT is_trashed
-            RETURNING id::text, name, path, parent_id::text, user_id,
-                      EXTRACT(EPOCH FROM created_at)::bigint,
-                      EXTRACT(EPOCH FROM updated_at)::bigint,
-                       EXTRACT(EPOCH FROM tree_modified_at)::bigint
-            "#,
-        )
-        .bind(new_parent_id)
-        .bind(id)
-        .fetch_optional(self.pool())
+        // Retried on deadlock vs the tree-ETag flusher (see rename_folder).
+        let row = retry_on_deadlock("folders.move", || {
+            sqlx::query_as::<_, FolderRow>(
+                r#"
+                UPDATE storage.folders
+                   SET parent_id = $1::uuid, updated_at = NOW()
+                 WHERE id = $2::uuid AND NOT is_trashed
+                RETURNING id::text, name, path, parent_id::text, user_id,
+                          EXTRACT(EPOCH FROM created_at)::bigint,
+                          EXTRACT(EPOCH FROM updated_at)::bigint,
+                           EXTRACT(EPOCH FROM tree_modified_at)::bigint
+                "#,
+            )
+            .bind(new_parent_id)
+            .bind(id)
+            .fetch_optional(self.pool())
+        })
         .await
         .map_err(|e| DomainError::internal_error("FolderDb", format!("move: {e}")))?
         .ok_or_else(|| DomainError::not_found("Folder", id))?;
@@ -511,24 +520,30 @@ impl FolderRepository for FolderDbRepository {
         // Delete all files whose folder is anywhere in the subtree.
         // Uses the GiST-indexed ltree `<@` operator — O(log N) vs the
         // O(depth × N) recursive CTE it replaces.
-        sqlx::query(
-            "DELETE FROM storage.files \
-              WHERE folder_id IN ( \
-                  SELECT id FROM storage.folders \
-                   WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid) \
-              )",
-        )
-        .bind(id)
-        .execute(self.pool())
+        // Both statements retried on deadlock vs the tree-ETag flusher's
+        // id-ordered ancestor bump (multi-row exclusive locks).
+        retry_on_deadlock("folders.delete_files", || {
+            sqlx::query(
+                "DELETE FROM storage.files \
+                  WHERE folder_id IN ( \
+                      SELECT id FROM storage.folders \
+                       WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid) \
+                  )",
+            )
+            .bind(id)
+            .execute(self.pool())
+        })
         .await
         .map_err(|e| DomainError::internal_error("FolderDb", format!("delete files: {e}")))?;
 
         // Then delete the folder (CASCADE will remove descendant folders)
-        let result = sqlx::query("DELETE FROM storage.folders WHERE id = $1::uuid")
-            .bind(id)
-            .execute(self.pool())
-            .await
-            .map_err(|e| DomainError::internal_error("FolderDb", format!("delete: {e}")))?;
+        let result = retry_on_deadlock("folders.delete", || {
+            sqlx::query("DELETE FROM storage.folders WHERE id = $1::uuid")
+                .bind(id)
+                .execute(self.pool())
+        })
+        .await
+        .map_err(|e| DomainError::internal_error("FolderDb", format!("delete: {e}")))?;
 
         if result.rows_affected() == 0 {
             return Err(DomainError::not_found("Folder", id));
@@ -570,22 +585,24 @@ impl FolderRepository for FolderDbRepository {
         // Child files and sub-folders are implicitly hidden because their
         // ancestor is trashed — list queries already filter NOT is_trashed,
         // and folder navigation won't reach a trashed folder's children.
-        let result = sqlx::query_scalar::<_, i64>(
-            r#"
-            WITH trash_folder AS (
-                UPDATE storage.folders
-                   SET is_trashed = TRUE,
-                       trashed_at = NOW(),
-                       original_parent_id = parent_id,
-                       updated_at = NOW()
-                 WHERE id = $1::uuid AND NOT is_trashed
-                RETURNING 1
+        let result = retry_on_deadlock("folders.trash", || {
+            sqlx::query_scalar::<_, i64>(
+                r#"
+                WITH trash_folder AS (
+                    UPDATE storage.folders
+                       SET is_trashed = TRUE,
+                           trashed_at = NOW(),
+                           original_parent_id = parent_id,
+                           updated_at = NOW()
+                     WHERE id = $1::uuid AND NOT is_trashed
+                    RETURNING 1
+                )
+                SELECT COUNT(*) FROM trash_folder
+                "#,
             )
-            SELECT COUNT(*) FROM trash_folder
-            "#,
-        )
-        .bind(folder_id)
-        .fetch_one(self.pool())
+            .bind(folder_id)
+            .fetch_one(self.pool())
+        })
         .await
         .map_err(|e| DomainError::internal_error("FolderDb", format!("trash: {e}")))?;
 
@@ -607,23 +624,25 @@ impl FolderRepository for FolderDbRepository {
         // The BEFORE UPDATE trigger recomputes path/lpath when
         // original_parent_id is restored; the cascade trigger
         // batch-updates all descendants via the GiST lpath index.
-        let result = sqlx::query_scalar::<_, i64>(
-            r#"
-            WITH restore_folder AS (
-                UPDATE storage.folders
-                   SET is_trashed = FALSE,
-                       trashed_at = NULL,
-                       parent_id = COALESCE(original_parent_id, parent_id),
-                       original_parent_id = NULL,
-                       updated_at = NOW()
-                 WHERE id = $1::uuid AND is_trashed
-                RETURNING 1
+        let result = retry_on_deadlock("folders.restore", || {
+            sqlx::query_scalar::<_, i64>(
+                r#"
+                WITH restore_folder AS (
+                    UPDATE storage.folders
+                       SET is_trashed = FALSE,
+                           trashed_at = NULL,
+                           parent_id = COALESCE(original_parent_id, parent_id),
+                           original_parent_id = NULL,
+                           updated_at = NOW()
+                     WHERE id = $1::uuid AND is_trashed
+                    RETURNING 1
+                )
+                SELECT COUNT(*) FROM restore_folder
+                "#,
             )
-            SELECT COUNT(*) FROM restore_folder
-            "#,
-        )
-        .bind(folder_id)
-        .fetch_one(self.pool())
+            .bind(folder_id)
+            .fetch_one(self.pool())
+        })
         .await
         .map_err(|e| DomainError::internal_error("FolderDb", format!("restore: {e}")))?;
 
@@ -636,25 +655,30 @@ impl FolderRepository for FolderDbRepository {
 
     async fn delete_folder_permanently(&self, folder_id: &str) -> Result<(), DomainError> {
         // Delete all files whose folder is anywhere in the subtree
-        // (GiST ltree index, same pattern as delete_folder).
-        sqlx::query(
-            "DELETE FROM storage.files \
-              WHERE folder_id IN ( \
-                  SELECT id FROM storage.folders \
-                   WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid) \
-              )",
-        )
-        .bind(folder_id)
-        .execute(self.pool())
+        // (GiST ltree index, same pattern as delete_folder — both
+        // statements retried on deadlock vs the tree-ETag flusher).
+        retry_on_deadlock("folders.perm_delete_files", || {
+            sqlx::query(
+                "DELETE FROM storage.files \
+                  WHERE folder_id IN ( \
+                      SELECT id FROM storage.folders \
+                       WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid) \
+                  )",
+            )
+            .bind(folder_id)
+            .execute(self.pool())
+        })
         .await
         .map_err(|e| DomainError::internal_error("FolderDb", format!("perm delete files: {e}")))?;
 
         // Then permanently delete folder — CASCADE handles descendant folders
-        let result = sqlx::query("DELETE FROM storage.folders WHERE id = $1::uuid")
-            .bind(folder_id)
-            .execute(self.pool())
-            .await
-            .map_err(|e| DomainError::internal_error("FolderDb", format!("perm delete: {e}")))?;
+        let result = retry_on_deadlock("folders.perm_delete", || {
+            sqlx::query("DELETE FROM storage.folders WHERE id = $1::uuid")
+                .bind(folder_id)
+                .execute(self.pool())
+        })
+        .await
+        .map_err(|e| DomainError::internal_error("FolderDb", format!("perm delete: {e}")))?;
 
         if result.rows_affected() == 0 {
             return Err(DomainError::not_found("Folder", folder_id));

src/infrastructure/repositories/pg/mod.rs

@@ -16,7 +16,7 @@ mod session_pg_repository;
 mod settings_pg_repository;
 mod share_pg_repository;
 mod subject_group_pg_repository;
-mod transaction_utils;
+pub(crate) mod transaction_utils;
 mod user_pg_repository;
 
 // ── Blob-storage repositories ──

src/infrastructure/repositories/pg/transaction_utils.rs

@@ -1,6 +1,7 @@
 use sqlx::{Error as SqlxError, PgPool, Postgres, Transaction};
+use std::future::Future;
 use std::sync::Arc;
-use tracing::{debug, error, info};
+use tracing::{debug, error, info, warn};
 
 /// Helper function to execute database operations in a transaction
 /// Takes a database pool and a closure that will be executed within a transaction
@@ -56,3 +57,128 @@ where
         }
     }
 }
+
+/// True when the error is a PostgreSQL deadlock abort (SQLSTATE `40P01`).
+///
+/// Deadlock victims are safe to re-run when the statement is a single
+/// autocommit round-trip: the aborted implicit transaction left nothing
+/// behind, and PostgreSQL chose this session as the victim precisely so
+/// the competing transaction could finish — a retry usually succeeds
+/// immediately.
+pub fn is_deadlock(err: &SqlxError) -> bool {
+    matches!(err, SqlxError::Database(db) if db.code().as_deref() == Some("40P01"))
+}
+
+/// Re-run `op` while it fails with an error matching `should_retry`, up to
+/// 3 retries with a short growing backoff. Errors that don't match the
+/// predicate — and the final attempt's error — are returned untouched, so
+/// callers' existing error mapping (e.g. `23505` → already-exists) still
+/// sees exactly what it expects.
+pub async fn retry_when<T, E, F, Fut, P>(
+    operation_name: &str,
+    should_retry: P,
+    op: F,
+) -> Result<T, E>
+where
+    F: Fn() -> Fut,
+    Fut: Future<Output = Result<T, E>>,
+    P: Fn(&E) -> bool,
+{
+    const BACKOFF_MS: [u64; 3] = [10, 50, 150];
+
+    let mut attempt = 0;
+    loop {
+        match op().await {
+            Ok(value) => return Ok(value),
+            Err(e) if attempt < BACKOFF_MS.len() && should_retry(&e) => {
+                warn!(
+                    "Retryable failure on {} (attempt {}/{}), backing off {}ms",
+                    operation_name,
+                    attempt + 1,
+                    BACKOFF_MS.len() + 1,
+                    BACKOFF_MS[attempt]
+                );
+                tokio::time::sleep(std::time::Duration::from_millis(BACKOFF_MS[attempt])).await;
+                attempt += 1;
+            }
+            Err(e) => return Err(e),
+        }
+    }
+}
+
+/// [`retry_when`] specialised to PostgreSQL deadlocks (`40P01`) — the only
+/// transient SQLSTATE our single-statement write paths can hit.
+pub async fn retry_on_deadlock<T, F, Fut>(operation_name: &str, op: F) -> Result<T, SqlxError>
+where
+    F: Fn() -> Fut,
+    Fut: Future<Output = Result<T, SqlxError>>,
+{
+    retry_when(operation_name, is_deadlock, op).await
+}
+
+#[cfg(test)]
+mod tests {
+    use super::retry_when;
+    use std::sync::atomic::{AtomicU32, Ordering};
+
+    #[derive(Debug, PartialEq)]
+    enum FakeError {
+        Transient,
+        Fatal,
+    }
+
+    #[tokio::test]
+    async fn retries_transient_errors_until_success() {
+        let calls = AtomicU32::new(0);
+        let result = retry_when(
+            "test",
+            |e| *e == FakeError::Transient,
+            || {
+                let n = calls.fetch_add(1, Ordering::SeqCst);
+                async move {
+                    if n < 2 {
+                        Err(FakeError::Transient)
+                    } else {
+                        Ok(n)
+                    }
+                }
+            },
+        )
+        .await;
+        assert_eq!(result, Ok(2));
+        assert_eq!(calls.load(Ordering::SeqCst), 3);
+    }
+
+    #[tokio::test]
+    async fn gives_up_after_max_attempts_returning_last_error() {
+        let calls = AtomicU32::new(0);
+        let result: Result<(), _> = retry_when(
+            "test",
+            |e| *e == FakeError::Transient,
+            || {
+                calls.fetch_add(1, Ordering::SeqCst);
+                async { Err(FakeError::Transient) }
+            },
+        )
+        .await;
+        assert_eq!(result, Err(FakeError::Transient));
+        // 1 initial attempt + 3 retries
+        assert_eq!(calls.load(Ordering::SeqCst), 4);
+    }
+
+    #[tokio::test]
+    async fn non_matching_errors_are_not_retried() {
+        let calls = AtomicU32::new(0);
+        let result: Result<(), _> = retry_when(
+            "test",
+            |e| *e == FakeError::Transient,
+            || {
+                calls.fetch_add(1, Ordering::SeqCst);
+                async { Err(FakeError::Fatal) }
+            },
+        )
+        .await;
+        assert_eq!(result, Err(FakeError::Fatal));
+        assert_eq!(calls.load(Ordering::SeqCst), 1);
+    }
+}

src/infrastructure/services/mod.rs

@@ -29,6 +29,7 @@ pub mod thumbnail_service;
 #[cfg(test)]
 mod thumbnail_service_test;
 pub mod trash_cleanup_service;
+pub mod tree_etag_flush_service;
 pub mod webdav_lock_service;
 pub mod wopi_discovery_service;
 pub mod zip_service;

src/infrastructure/services/tree_etag_flush_service.rs

@@ -0,0 +1,173 @@
+use sqlx::PgPool;
+use std::sync::Arc;
+use tracing::{debug, error, info, instrument};
+
+use crate::infrastructure::repositories::pg::transaction_utils::retry_on_deadlock;
+
+/// Background drainer for `storage.tree_etag_dirty` — the asynchronous half
+/// of the tree-ETag bump (see migration `20260627000000_async_tree_etag_queue`).
+///
+/// The statement triggers on `storage.files` / `storage.folders` only append
+/// "bump request" lpaths to the queue, taking zero folder-row locks, so any
+/// number of concurrent uploads never conflict with each other. This service
+/// turns those requests into actual `tree_modified_at` updates: every
+/// `interval_ms` it drains the queue and bumps the deduplicated ancestor
+/// closure in ONE batched UPDATE with deterministic id-order locking.
+///
+/// Correctness invariants this relies on:
+///   * The drain (DELETE) and the bump (UPDATE) are one statement — one
+///     transaction. A crash mid-flush rolls both back and the queue rows
+///     survive; the first flush after startup drains any leftovers, so no
+///     bump is ever lost (a lost bump means sync clients never discover
+///     the change).
+///   * Queue rows are dual-keyed (`lpath` captured at mutation time,
+///     `folder_id` re-resolved at flush time) and the target set is the
+///     UNION of both — so neither a folder deleted/moved away (captured
+///     lpath wins) nor a folder re-rooted by a move inside the flush
+///     window (resolved lpath wins) can lose its pending bump.
+///   * The bump is monotonic per folder:
+///     `GREATEST(NOW(), tree_modified_at + interval '1 second')`. Folder
+///     ETags have whole-second granularity (`{id}-{epoch_seconds}`, see
+///     `Folder::compute_etag`); two flushes inside the same wall-clock
+///     second must still produce two distinct ETags or a client polling
+///     between them would permanently miss the second change.
+///   * The flusher is the ONLY writer of `tree_modified_at`, runs as a
+///     single instance, and locks victims in id order — so it cannot
+///     deadlock against itself. It can still lose a race against a folder
+///     move's descendant lpath cascade, which is why the statement runs
+///     under [`retry_on_deadlock`]; a retried flush re-drains the intact
+///     queue, invisible to users.
+pub struct TreeEtagFlushService {
+    pool: Arc<PgPool>,
+    interval_ms: u64,
+}
+
+/// Rows drained from the queue per statement. Bounds the lock footprint of
+/// one flush under burst load (bulk trash purge, recursive copy); leftovers
+/// are picked up by the in-tick drain loop or the next tick.
+const DRAIN_BATCH: i64 = 5_000;
+
+/// Max drain statements per tick, so a pathological backlog cannot
+/// monopolise the maintenance connection forever within one tick.
+const MAX_BATCHES_PER_TICK: u32 = 8;
+
+impl TreeEtagFlushService {
+    pub fn new(pool: Arc<PgPool>, interval_ms: u64) -> Self {
+        Self {
+            pool,
+            // Floor the cadence so a misconfiguration can't busy-loop the
+            // maintenance pool.
+            interval_ms: interval_ms.max(100),
+        }
+    }
+
+    /// Spawn the flush loop. Fire-and-forget: the loop logs and survives
+    /// every error (an exited loop would silently freeze all folder ETags),
+    /// and the first flush runs immediately to drain rows left over from a
+    /// previous run.
+    #[instrument(skip(self))]
+    pub fn start_flush_job(&self) {
+        let pool = self.pool.clone();
+        let interval_ms = self.interval_ms;
+        info!(
+            "Starting tree-ETag flush job (every {}ms, batch {})",
+            interval_ms, DRAIN_BATCH
+        );
+        tokio::spawn(async move {
+            let mut ticker = tokio::time::interval(std::time::Duration::from_millis(interval_ms));
+            // If a flush overruns the interval, fire the next one a full
+            // interval later instead of bursting to catch up.
+            ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+            loop {
+                ticker.tick().await;
+                match Self::flush(&pool).await {
+                    Ok((0, _)) => {}
+                    Ok((drained, bumped)) => {
+                        debug!(
+                            "Tree-ETag flush: drained {} queue row(s), bumped {} folder(s)",
+                            drained, bumped
+                        );
+                    }
+                    Err(e) => {
+                        error!("Tree-ETag flush failed (queue preserved, will retry): {e}");
+                    }
+                }
+            }
+        });
+    }
+
+    /// Drain the dirty queue and bump the ancestor closure. Returns
+    /// `(queue rows drained, folder rows bumped)` summed over the batches
+    /// processed this tick.
+    async fn flush(pool: &PgPool) -> Result<(i64, i64), sqlx::Error> {
+        let mut total_drained = 0i64;
+        let mut total_bumped = 0i64;
+
+        for _ in 0..MAX_BATCHES_PER_TICK {
+            let (drained, bumped) = retry_on_deadlock("tree_etag_flush", || {
+                sqlx::query_as::<_, (i64, i64)>(
+                    r#"
+                    WITH drained AS (
+                        DELETE FROM storage.tree_etag_dirty
+                         WHERE id IN (SELECT id
+                                        FROM storage.tree_etag_dirty
+                                       ORDER BY id
+                                       LIMIT $1)
+                        RETURNING lpath, folder_id
+                    ),
+                    targets AS (
+                        -- Captured chain: covers target folders deleted or
+                        -- moved away since enqueue (the old location's
+                        -- surviving ancestors still get their bump).
+                        SELECT lpath FROM drained
+                        UNION
+                        -- Flush-time resolution: a folder MOVED since
+                        -- enqueue had its subtree's lpaths rewritten, so
+                        -- the captured lpath no longer matches it — its id
+                        -- resolves to the CURRENT chain instead. Without
+                        -- this, a bump queued just before a move would be
+                        -- silently lost and sync clients would never
+                        -- discover the change.
+                        SELECT fo.lpath
+                          FROM storage.folders fo
+                          JOIN drained d ON fo.id = d.folder_id
+                    ),
+                    victims AS (
+                        -- `lpath @> target` = the target folder itself plus
+                        -- every ancestor (GiST-indexed). Folder rows deleted
+                        -- since enqueue simply don't match. Lock in id order
+                        -- so overlapping closures cannot deadlock.
+                        SELECT f.id
+                          FROM storage.folders f
+                         WHERE EXISTS (SELECT 1 FROM targets t
+                                        WHERE f.lpath @> t.lpath)
+                         ORDER BY f.id
+                           FOR NO KEY UPDATE
+                    ),
+                    bumped AS (
+                        UPDATE storage.folders f
+                           SET tree_modified_at =
+                                   GREATEST(NOW(), f.tree_modified_at + interval '1 second')
+                          FROM victims v
+                         WHERE f.id = v.id
+                        RETURNING f.id
+                    )
+                    SELECT (SELECT COUNT(*) FROM drained)::bigint,
+                           (SELECT COUNT(*) FROM bumped)::bigint
+                    "#,
+                )
+                .bind(DRAIN_BATCH)
+                .fetch_one(pool)
+            })
+            .await?;
+
+            total_drained += drained;
+            total_bumped += bumped;
+            if drained < DRAIN_BATCH {
+                break;
+            }
+        }
+
+        Ok((total_drained, total_bumped))
+    }
+}

src/interfaces/nextcloud/webdav_handler.rs

@@ -1211,9 +1211,9 @@ pub fn write_folder_response<W: std::io::Write>(
             .unwrap_or_else(Utc::now);
 
     write_text_element(xml, "d:getlastmodified", &modified_at.to_rfc2822())?;
-    // Route through `FolderDto::etag` (= `Folder::etag()`, currently
-    // the folder UUID — see the entity for the documented v1 formula
-    // and the follow-up plan to make it descendant-aware).
+    // Route through `FolderDto::etag` (= `Folder::etag()`: the
+    // descendant-aware `{id[..16]}-{tree_modified_at}` — see the
+    // entity for the formula and the async-bump freshness contract).
     write_text_element(xml, "d:getetag", &format!("\"{}\"", folder.etag))?;
     write_text_element(xml, "d:getcontenttype", "httpd/unix-directory")?;
     write_text_element(xml, "d:getcontentlength", "0")?;