Merge pull request #32530 from overleaf/mg-stuck-sync-repro

Detect and auto-clear stuck sync states

GitOrigin-RevId: 680861a33e42432dab7d40ad421980b707eb6089

services/project-history/app/js/Errors.js

@@ -1,5 +1,7 @@
 import OError from '@overleaf/o-error'
 
+export const SYNC_ONGOING_ERROR_MESSAGE = 'sync ongoing'
+
 export class NotFoundError extends OError {}
 export class BadRequestError extends OError {}
 export class SyncError extends OError {}

services/project-history/app/js/HttpController.js

@@ -374,6 +374,7 @@ export function getResyncPending(req, res, next) {
     if (err) return next(err)
     res.json({
       resyncPending: state.isSyncOngoing(),
+      syncStuck: state.isSyncStuck(),
     })
   })
 }

services/project-history/app/js/RetryManager.js

@@ -4,6 +4,7 @@ import logger from '@overleaf/logger'
 import OError from '@overleaf/o-error'
 import * as UpdatesProcessor from './UpdatesProcessor.js'
 import * as SyncManager from './SyncManager.js'
+import { SYNC_ONGOING_ERROR_MESSAGE } from './Errors.js'
 import * as WebApiManager from './WebApiManager.js'
 import * as RedisManager from './RedisManager.js'
 import * as ErrorRecorder from './ErrorRecorder.js'
@@ -44,9 +45,11 @@ promises.retryFailures = async (options = {}) => {
   } else if (failureType === 'hard') {
     const batch = await getFailureBatch(hardErrorSelector, limit)
     const result = await retryFailureBatch(batch, timeout, async failure => {
-      await resyncProject(failure.project_id, {
-        hard: failureRequiresHardResync(failure),
-      })
+      // Ongoing-sync failures always use soft resync to preserve sync state.
+      // SyncManager needs existing state to detect and clear stuck syncs.
+      const hard =
+        failureRequiresHardResync(failure) && !isOngoingSyncFailure(failure)
+      await resyncProject(failure.project_id, { hard })
     })
     return result
   }
@@ -62,6 +65,11 @@ function softErrorSelector(failure) {
 }
 
 function hardErrorSelector(failure) {
+  // Ongoing-sync failures are always retried via soft resync.
+  // SyncManager's stuck detection handles the retry limit (stuckClearCount).
+  if (isOngoingSyncFailure(failure)) return true
+
+  // Other failures: retry hard/repeated ones, but stop after MAX_RESYNC_ATTEMPTS
   return (
     (isHardFailure(failure) || isRepeatedFailure(failure)) &&
     !isStuckFailure(failure)
@@ -84,6 +92,10 @@ function isRepeatedFailure(failure) {
   return failure.attempts > 3
 }
 
+export function isOngoingSyncFailure(failure) {
+  return failure.error?.includes(SYNC_ONGOING_ERROR_MESSAGE) ?? false
+}
+
 function isStuckFailure(failure) {
   return (
     failure.resyncAttempts != null &&

services/project-history/app/js/SyncManager.js

@@ -8,7 +8,11 @@ import logger from '@overleaf/logger'
 import Metrics from '@overleaf/metrics'
 import OError from '@overleaf/o-error'
 import { File, Range } from 'overleaf-editor-core'
-import { NeedFullProjectStructureResyncError, SyncError } from './Errors.js'
+import {
+  NeedFullProjectStructureResyncError,
+  SYNC_ONGOING_ERROR_MESSAGE,
+  SyncError,
+} from './Errors.js'
 import { db, ObjectId } from './mongodb.js'
 import * as SnapshotManager from './SnapshotManager.js'
 import * as LockManager from './LockManager.js'
@@ -30,6 +34,8 @@ import { isInsert, isDelete } from './Utils.js'
  */
 const MAX_RESYNC_HISTORY_RECORDS = 100 // keep this many records of previous resyncs
 const EXPIRE_RESYNC_HISTORY_INTERVAL_MS = 90 * 24 * 3600 * 1000 // 90 days
+const SYNC_STUCK_TIMEOUT_MS = 4 * 60 * 60 * 1000 // 4 hours
+const MAX_STUCK_CLEAR_ATTEMPTS = 5
 
 const keys = Settings.redis.lock.key_schema
 
@@ -93,7 +99,42 @@ async function startResyncWithoutLock(projectId, options) {
 
   const syncState = await getResyncState(projectId)
   if (syncState.isSyncOngoing()) {
-    throw new OError('sync ongoing')
+    if (syncState.isSyncStuck()) {
+      const stuckDocPaths = Array.from(syncState.resyncDocContents)
+      const stuckClearCount = syncState.stuckClearCount
+
+      if (stuckClearCount >= MAX_STUCK_CLEAR_ATTEMPTS) {
+        await _recordStuckClearInSyncState(projectId, stuckDocPaths)
+        Metrics.inc('project_history_sync_stuck_permanent')
+        // Log error only on first permanent-stuck detection to avoid spam
+        if (stuckClearCount === MAX_STUCK_CLEAR_ATTEMPTS) {
+          logger.error(
+            {
+              projectId,
+              stuckClearCount: stuckClearCount + 1,
+              stuckDocPaths,
+              resyncPendingSince: syncState.resyncPendingSince,
+            },
+            'sync permanently stuck — exceeded auto-clear limit'
+          )
+        }
+        throw new OError('sync permanently stuck')
+      }
+
+      logger.warn(
+        {
+          projectId,
+          stuckClearCount: stuckClearCount + 1,
+          stuckDocPaths,
+          resyncPendingSince: syncState.resyncPendingSince,
+        },
+        'sync stuck, clearing state and restarting'
+      )
+      Metrics.inc('project_history_sync_stuck_cleared')
+      await _recordStuckClearInSyncState(projectId, stuckDocPaths)
+    } else {
+      throw new OError(SYNC_ONGOING_ERROR_MESSAGE)
+    }
   }
   syncState.setOrigin(options.origin || { kind: 'history-resync' })
   syncState.startProjectStructureSync()
@@ -167,7 +208,12 @@ async function setResyncState(projectId, syncState) {
     update.$set.expiresAt = new Date(
       Date.now() + EXPIRE_RESYNC_HISTORY_INTERVAL_MS
     )
-    update.$unset = { resyncPendingSince: 1 }
+    update.$unset = {
+      resyncPendingSince: 1,
+      stuckClearCount: 1,
+      lastStuckClearAt: 1,
+      lastStuckDocPaths: 1,
+    }
   }
 
   // apply the update
@@ -184,6 +230,17 @@ async function clearResyncState(projectId) {
   })
 }
 
+async function _recordStuckClearInSyncState(projectId, stuckDocPaths) {
+  await db.projectHistorySyncState.updateOne(
+    { project_id: new ObjectId(projectId) },
+    {
+      $inc: { stuckClearCount: 1 },
+      $set: { lastStuckClearAt: new Date(), lastStuckDocPaths: stuckDocPaths },
+      $unset: { resyncPendingSince: 1 },
+    }
+  )
+}
+
 /**
  * @param {string} projectId
  * @param {Date} date
@@ -223,6 +280,7 @@ async function skipUpdatesDuringSync(projectId, updates) {
     if (!shouldSkipUpdate) {
       filteredUpdates.push(update)
     } else {
+      Metrics.inc('project_history_sync_update_skipped')
       logger.debug({ projectId, update }, 'skipping update due to resync')
     }
   }
@@ -297,7 +355,10 @@ class SyncState {
     resyncCount,
     resyncPendingSince,
     lastUpdated,
-    history
+    history,
+    stuckClearCount,
+    lastStuckClearAt,
+    lastStuckDocPaths
   ) {
     this.projectId = projectId
     this.resyncProjectStructure = resyncProjectStructure
@@ -307,6 +368,9 @@ class SyncState {
     this.resyncPendingSince = resyncPendingSince
     this.lastUpdated = lastUpdated
     this.history = history
+    this.stuckClearCount = stuckClearCount
+    this.lastStuckClearAt = lastStuckClearAt
+    this.lastStuckDocPaths = lastStuckDocPaths
   }
 
   static fromRaw(projectId, rawSyncState) {
@@ -336,6 +400,9 @@ class SyncState {
       }
     }
     const lastUpdated = rawSyncState.lastUpdated
+    const stuckClearCount = rawSyncState.stuckClearCount ?? 0
+    const lastStuckClearAt = rawSyncState.lastStuckClearAt
+    const lastStuckDocPaths = rawSyncState.lastStuckDocPaths
     return new SyncState(
       projectId,
       resyncProjectStructure,
@@ -344,7 +411,10 @@ class SyncState {
       resyncCount,
       resyncPendingSince,
       lastUpdated,
-      history
+      history,
+      stuckClearCount,
+      lastStuckClearAt,
+      lastStuckDocPaths
     )
   }
 
@@ -459,6 +529,20 @@ class SyncState {
   isSyncOngoing() {
     return this.isProjectStructureSyncing() || this.isAnyDocContentSyncing()
   }
+
+  isSyncStuck() {
+    if (!this.isSyncOngoing()) {
+      return false
+    }
+    if (!this.resyncPendingSince) {
+      // No timestamp recorded — treat long-running syncs without a timestamp
+      // as potentially stuck (legacy state from before this field was added)
+      return true
+    }
+    return (
+      Date.now() - this.resyncPendingSince.getTime() > SYNC_STUCK_TIMEOUT_MS
+    )
+  }
 }
 
 class SyncUpdateExpander {

services/project-history/test/acceptance/js/SyncTests.js

@@ -2385,6 +2385,204 @@ describe('Syncing with web and doc-updater', function () {
           )
         })
       })
+
+      describe('stuck sync state (missing resyncDocContent updates)', function () {
+        beforeEach(function () {
+          MockHistoryStore()
+            .get(`/api/projects/${historyId}/latest/history`)
+            .reply(200, {
+              chunk: {
+                history: {
+                  snapshot: {
+                    files: {
+                      'main.tex': {
+                        hash: '0a207c060e61f3b88eaee0a8cd0696f46fb155eb',
+                        stringLength: 3,
+                      },
+                    },
+                  },
+                  changes: [],
+                },
+                startVersion: 0,
+              },
+            })
+
+          MockHistoryStore()
+            .get(
+              `/api/projects/${historyId}/blobs/0a207c060e61f3b88eaee0a8cd0696f46fb155eb`
+            )
+            .reply(200, 'a\nb')
+        })
+
+        it('should get stuck when resyncDocContent update is missing', async function () {
+          // Step 1: Start a resync
+          await ProjectHistoryClient.resyncHistory(this.project_id)
+
+          // Step 2: Push resyncProjectStructure update (lists docs to sync)
+          // but do NOT push the corresponding resyncDocContent update
+          const structureUpdate = {
+            projectHistoryId: historyId,
+            resyncProjectStructure: {
+              docs: [{ path: '/main.tex' }],
+              files: [],
+            },
+            meta: {
+              ts: this.timestamp,
+            },
+          }
+          await ProjectHistoryClient.pushRawUpdate(
+            this.project_id,
+            structureUpdate
+          )
+
+          // Step 3: Flush — processes structure update, adds /main.tex to resyncDocContents
+          // This succeeds (204) but leaves sync in an incomplete state
+          await ProjectHistoryClient.flushProject(this.project_id)
+
+          // Step 4: Verify the sync state is stuck
+          const syncState = await ProjectHistoryClient.getSyncState(
+            this.project_id
+          )
+          expect(syncState).to.not.be.null
+          expect(syncState.resyncProjectStructure).to.equal(false)
+          expect(syncState.resyncDocContents).to.deep.equal(['/main.tex'])
+          expect(syncState.resyncPendingSince).to.be.a('date')
+          // Sync thinks it's ongoing because resyncDocContents is not empty
+
+          // Step 5: Push a normal text update (simulating a user edit)
+          const textUpdate = {
+            doc: this.doc_id,
+            op: [{ p: 3, i: '\nc' }],
+            v: 1,
+            meta: {
+              ts: this.timestamp,
+              pathname: '/main.tex',
+              doc_length: 3,
+            },
+          }
+          await ProjectHistoryClient.pushRawUpdate(this.project_id, textUpdate)
+
+          // Step 6: Flush again — the text update should be silently skipped
+          // because the doc is in resyncDocContents (sync "ongoing")
+          await ProjectHistoryClient.flushProject(this.project_id)
+
+          // Step 7: Verify the sync state is STILL stuck (text update was skipped)
+          const syncStateAfter = await ProjectHistoryClient.getSyncState(
+            this.project_id
+          )
+          expect(syncStateAfter.resyncProjectStructure).to.equal(false)
+          expect(syncStateAfter.resyncDocContents).to.deep.equal(['/main.tex'])
+
+          // Step 7b: Verify resync-pending reports ongoing but not yet stuck
+          const pending = await ProjectHistoryClient.getResyncPending(
+            this.project_id
+          )
+          expect(pending.resyncPending).to.equal(true)
+          expect(pending.syncStuck).to.equal(false)
+
+          // Step 8: Verify a normal resync FAILS because sync is "ongoing"
+          try {
+            await fetchNothing(
+              `http://127.0.0.1:3054/project/${this.project_id}/resync`,
+              {
+                method: 'POST',
+                json: { origin: { kind: 'test-origin' } },
+              }
+            )
+            assert.fail('resync should have failed')
+          } catch (error) {
+            if (error instanceof RequestFailedError) {
+              expect(error.response.status).to.equal(500)
+            } else {
+              throw error
+            }
+          }
+        })
+
+        it('should recover with a hard resync (force=true)', async function () {
+          // Set up the stuck state (same as above)
+          await ProjectHistoryClient.resyncHistory(this.project_id)
+
+          const structureUpdate = {
+            projectHistoryId: historyId,
+            resyncProjectStructure: {
+              docs: [{ path: '/main.tex' }],
+              files: [],
+            },
+            meta: {
+              ts: this.timestamp,
+            },
+          }
+          await ProjectHistoryClient.pushRawUpdate(
+            this.project_id,
+            structureUpdate
+          )
+          await ProjectHistoryClient.flushProject(this.project_id)
+
+          // Verify stuck
+          const syncState = await ProjectHistoryClient.getSyncState(
+            this.project_id
+          )
+          expect(syncState.resyncDocContents).to.deep.equal(['/main.tex'])
+
+          // Mock the web resync endpoint for the hard resync
+          MockWeb()
+            .post(`/project/${this.project_id}/history/resync`)
+            .reply(204)
+
+          // Hard resync should clear the stuck state and succeed
+          const response = await fetchNothing(
+            `http://127.0.0.1:3054/project/${this.project_id}/resync?force=true`,
+            {
+              method: 'POST',
+              json: { origin: { kind: 'test-origin' } },
+            }
+          )
+          expect(response.status).to.equal(204)
+
+          // After hard resync, sync state should be cleared or reset
+          const syncStateAfter = await ProjectHistoryClient.getSyncState(
+            this.project_id
+          )
+          // Hard resync clears and restarts — at this point the new resync
+          // should be in progress (resyncProjectStructure = true) or cleared
+          // depending on whether web sent updates
+          if (syncStateAfter) {
+            expect(syncStateAfter.resyncProjectStructure).to.equal(true)
+          }
+        })
+
+        it('should auto-recover when stuck (null resyncPendingSince)', async function () {
+          // Inject a stuck sync state: ongoing but no resyncPendingSince
+          // (legacy state from before the timestamp field was added)
+          await ProjectHistoryClient.injectStuckSyncState(this.project_id, [
+            '/main.tex',
+          ])
+
+          // Verify the state is recognised as stuck
+          const pending = await ProjectHistoryClient.getResyncPending(
+            this.project_id
+          )
+          expect(pending.resyncPending).to.equal(true)
+          expect(pending.syncStuck).to.equal(true)
+
+          // Mock the web callback that requestResync triggers
+          MockWeb()
+            .post(`/project/${this.project_id}/history/resync`)
+            .reply(204)
+
+          // A plain resync (no force) should detect stuck, clear, and restart
+          await ProjectHistoryClient.resyncHistory(this.project_id)
+
+          // stuckClearCount incremented and a new structure sync started
+          const syncState = await ProjectHistoryClient.getSyncState(
+            this.project_id
+          )
+          expect(syncState.stuckClearCount).to.equal(1)
+          expect(syncState.resyncProjectStructure).to.equal(true)
+          expect(syncState.lastStuckDocPaths).to.deep.equal(['/main.tex'])
+        })
+      })
     })
   })
 

services/project-history/test/acceptance/js/helpers/ProjectHistoryClient.js

@@ -1,7 +1,7 @@
 import { expect } from 'chai'
 import Settings from '@overleaf/settings'
 import RedisWrapper from '@overleaf/redis-wrapper'
-import { db } from '../../../../app/js/mongodb.js'
+import { db, ObjectId } from '../../../../app/js/mongodb.js'
 import {
   fetchJson,
   fetchJsonWithResponse,
@@ -223,6 +223,32 @@ export async function getFailures() {
   return failures
 }
 
+export async function getSyncState(projectId) {
+  return await db.projectHistorySyncState.findOne({
+    project_id: new ObjectId(projectId),
+  })
+}
+
+export async function getResyncPending(projectId) {
+  return await fetchJson(
+    `http://127.0.0.1:3054/project/${projectId}/resync-pending`
+  )
+}
+
+export async function injectStuckSyncState(projectId, docPaths) {
+  await db.projectHistorySyncState.replaceOne(
+    { project_id: new ObjectId(projectId) },
+    {
+      project_id: new ObjectId(projectId),
+      resyncProjectStructure: false,
+      resyncDocContents: docPaths,
+      stuckClearCount: 0,
+      history: [],
+    },
+    { upsert: true }
+  )
+}
+
 export async function deleteProject(projectId) {
   const response = await fetchNothing(
     `http://127.0.0.1:3054/project/${projectId}`,

services/project-history/test/unit/js/RetryManager/RetryManagerTests.js

@@ -12,6 +12,8 @@ describe('RetryManager', function () {
     this.projectId2 = new ObjectId().toString()
     this.projectId3 = new ObjectId().toString()
     this.projectId4 = new ObjectId().toString()
+    this.projectId5 = new ObjectId().toString()
+    this.projectId6 = new ObjectId().toString()
     this.historyId = 12345
 
     this.WebApiManager = {
@@ -39,13 +41,25 @@ describe('RetryManager', function () {
           },
           {
             project_id: this.projectId3,
-            error: 'sync ongoing',
-            attempts: 10,
+            error: 'OpsOutOfOrderError: doc version out of order',
+            attempts: 5,
             resyncAttempts: 1,
           },
           {
             project_id: this.projectId4,
-            error: 'sync ongoing',
+            error: 'OpsOutOfOrderError: doc version out of order',
+            attempts: 5,
+            resyncAttempts: 2,
+          },
+          {
+            project_id: this.projectId5,
+            error: 'OError: sync ongoing',
+            attempts: 10,
+            resyncAttempts: 1,
+          },
+          {
+            project_id: this.projectId6,
+            error: 'OError: sync ongoing',
             attempts: 10,
             resyncAttempts: 2,
           },
@@ -129,6 +143,24 @@ describe('RetryManager', function () {
         ).not.to.have.been.calledWith(this.projectId4)
       })
 
+      it('should use soft resync for sync ongoing failures regardless of resyncAttempts', function () {
+        expect(this.SyncManager.promises.startResync).to.have.been.calledWith(
+          this.projectId5
+        )
+        expect(
+          this.SyncManager.promises.startHardResync
+        ).not.to.have.been.calledWith(this.projectId5)
+      })
+
+      it('should retry stuck sync ongoing failures via soft resync', function () {
+        expect(this.SyncManager.promises.startResync).to.have.been.calledWith(
+          this.projectId6
+        )
+        expect(
+          this.SyncManager.promises.startHardResync
+        ).not.to.have.been.calledWith(this.projectId6)
+      })
+
       it('should count the unprocessed updates', function () {
         expect(
           this.RedisManager.promises.countUnprocessedUpdates

services/project-history/test/unit/js/SyncManager/SyncManagerTests.js

@@ -194,11 +194,11 @@ describe('SyncManager', function () {
             project_id: new ObjectId(this.projectId),
           },
           sinon.match({
-            $set: {
+            $set: sinon.match({
               resyncProjectStructure: true,
               resyncDocContents: [],
               origin: { kind: 'history-resync' },
-            },
+            }),
             $currentDate: { lastUpdated: true },
             $inc: { resyncCount: 1 },
             $unset: { expiresAt: true },
@@ -212,7 +212,10 @@ describe('SyncManager', function () {
 
     describe('if project structure sync is in progress', function () {
       beforeEach(function () {
-        const syncState = { resyncProjectStructure: true }
+        const syncState = {
+          resyncProjectStructure: true,
+          resyncPendingSince: new Date(),
+        }
         this.db.projectHistorySyncState.findOne.resolves(syncState)
       })
 
@@ -225,7 +228,10 @@ describe('SyncManager', function () {
 
     describe('if doc content sync in is progress', function () {
       beforeEach(async function () {
-        const syncState = { resyncDocContents: ['/foo.tex'] }
+        const syncState = {
+          resyncDocContents: ['/foo.tex'],
+          resyncPendingSince: new Date(),
+        }
         this.db.projectHistorySyncState.findOne.resolves(syncState)
       })
 
@@ -235,6 +241,112 @@ describe('SyncManager', function () {
         ).to.be.rejectedWith('sync ongoing')
       })
     })
+
+    describe('if sync is stuck (no resyncPendingSince — legacy state)', function () {
+      beforeEach(function () {
+        const syncState = { resyncDocContents: ['/foo.tex'] }
+        this.db.projectHistorySyncState.findOne.resolves(syncState)
+      })
+
+      it('records the stuck clear, does not delete state, and starts a new resync', async function () {
+        await this.SyncManager.promises.startResync(this.projectId)
+        expect(
+          this.db.projectHistorySyncState.updateOne
+        ).to.have.been.calledWith(
+          { project_id: new ObjectId(this.projectId) },
+          sinon.match({
+            $inc: { stuckClearCount: 1 },
+            $unset: { resyncPendingSince: 1 },
+          })
+        )
+        expect(this.WebApiManager.promises.requestResync.calledOnce).to.be.true
+        expect(
+          this.Metrics.inc.calledWith('project_history_sync_stuck_cleared')
+        ).to.be.true
+      })
+    })
+
+    describe('if sync is stuck (resyncPendingSince older than timeout)', function () {
+      beforeEach(function () {
+        const fiveHoursAgo = new Date(Date.now() - 5 * 60 * 60 * 1000)
+        const syncState = {
+          resyncDocContents: ['/foo.tex'],
+          resyncPendingSince: fiveHoursAgo,
+        }
+        this.db.projectHistorySyncState.findOne.resolves(syncState)
+      })
+
+      it('records the stuck clear and starts a new resync', async function () {
+        await this.SyncManager.promises.startResync(this.projectId)
+        expect(
+          this.db.projectHistorySyncState.updateOne
+        ).to.have.been.calledWith(
+          { project_id: new ObjectId(this.projectId) },
+          sinon.match({
+            $inc: { stuckClearCount: 1 },
+            $unset: { resyncPendingSince: 1 },
+          })
+        )
+        expect(this.WebApiManager.promises.requestResync.calledOnce).to.be.true
+      })
+    })
+
+    describe('if sync is not stuck (resyncPendingSince within timeout)', function () {
+      beforeEach(function () {
+        const threeHoursAgo = new Date(Date.now() - 3 * 60 * 60 * 1000)
+        const syncState = {
+          resyncDocContents: ['/foo.tex'],
+          resyncPendingSince: threeHoursAgo,
+        }
+        this.db.projectHistorySyncState.findOne.resolves(syncState)
+      })
+
+      it('throws sync ongoing and does not clear', async function () {
+        await expect(
+          this.SyncManager.promises.startResync(this.projectId)
+        ).to.be.rejectedWith('sync ongoing')
+        expect(
+          this.Metrics.inc.calledWith('project_history_sync_stuck_cleared')
+        ).to.be.false
+        expect(
+          this.Metrics.inc.calledWith('project_history_sync_stuck_permanent')
+        ).to.be.false
+      })
+    })
+
+    describe('if sync is permanently stuck (exceeded retry limit)', function () {
+      beforeEach(function () {
+        const syncState = {
+          resyncDocContents: ['/foo.tex'],
+          stuckClearCount: 5,
+        }
+        this.db.projectHistorySyncState.findOne.resolves(syncState)
+      })
+
+      it('throws permanently stuck, emits metric, and records the attempt', async function () {
+        await expect(
+          this.SyncManager.promises.startResync(this.projectId)
+        ).to.be.rejectedWith('sync permanently stuck')
+        expect(
+          this.Metrics.inc.calledWith('project_history_sync_stuck_permanent')
+        ).to.be.true
+        expect(
+          this.db.projectHistorySyncState.updateOne
+        ).to.have.been.calledWith(
+          { project_id: new ObjectId(this.projectId) },
+          sinon.match({ $inc: { stuckClearCount: 1 } })
+        )
+      })
+
+      it('does not attempt a resync', async function () {
+        try {
+          await this.SyncManager.promises.startResync(this.projectId)
+        } catch (e) {
+          // expected
+        }
+        expect(this.WebApiManager.promises.requestResync.called).to.be.false
+      })
+    })
   })
 
   describe('setResyncState', function () {
@@ -265,7 +377,11 @@ describe('SyncManager', function () {
         ).to.have.been.calledWith(
           { project_id: new ObjectId(this.projectId) },
           sinon.match({
-            $set: this.syncState.toRaw(),
+            $set: sinon.match({
+              resyncProjectStructure: true,
+              resyncDocContents: [],
+              origin: { kind: 'history-resync' },
+            }),
             $currentDate: { lastUpdated: true },
             $inc: { resyncCount: 1 },
             $unset: { expiresAt: true },
@@ -289,7 +405,7 @@ describe('SyncManager', function () {
         }
       })
 
-      it('sets the sync state entry in mongo to expire', async function () {
+      it('sets the sync state entry in mongo to expire and clears stuck fields', async function () {
         await this.SyncManager.promises.setResyncState(
           this.projectId,
           this.syncState
@@ -307,6 +423,12 @@ describe('SyncManager', function () {
               expiresAt: new Date(this.now.getTime() + 90 * 24 * 3600 * 1000),
             },
             $currentDate: { lastUpdated: true },
+            $unset: {
+              resyncPendingSince: 1,
+              stuckClearCount: 1,
+              lastStuckClearAt: 1,
+              lastStuckDocPaths: 1,
+            },
           }),
           { upsert: true }
         )
@@ -2005,4 +2127,47 @@ describe('SyncManager', function () {
       })
     })
   })
+
+  describe('stuck sync state reproduction', function () {
+    describe('when resyncDocContents has entries but no resyncDocContent update arrives', function () {
+      beforeEach(function () {
+        // Simulate the stuck state: project structure sync is done,
+        // but doc content sync never completed for /main.tex
+        const stuckSyncState = {
+          resyncProjectStructure: false,
+          resyncDocContents: ['/main.tex'],
+          origin: { kind: 'history-resync' },
+        }
+        this.db.projectHistorySyncState.findOne.resolves(stuckSyncState)
+      })
+
+      it('should skip text updates for a doc stuck in resyncDocContents', async function () {
+        const textUpdate = {
+          doc: new ObjectId(),
+          op: [{ i: 'hello', p: 0 }],
+          meta: {
+            pathname: '/main.tex',
+            doc_length: 0,
+          },
+        }
+        this.UpdateTranslator.isTextUpdate.returns(false)
+        this.UpdateTranslator.isTextUpdate.withArgs(textUpdate).returns(true)
+
+        const { updates: filteredUpdates, syncState } =
+          await this.SyncManager.promises.skipUpdatesDuringSync(
+            this.projectId,
+            [textUpdate]
+          )
+
+        // The text update is silently dropped — this is the stuck state bug
+        expect(filteredUpdates).to.deep.equal([])
+        // Sync state still shows /main.tex as syncing
+        expect(syncState.toRaw().resyncDocContents).to.deep.equal(['/main.tex'])
+        // Metric is emitted for the skipped update
+        expect(
+          this.Metrics.inc.calledWith('project_history_sync_update_skipped')
+        ).to.be.true
+      })
+    })
+  })
 })