Thumbnails duplications, missing thumbnails and files failing to download #17918

Investigated all three against the live device, server, app database, and source.
Root causes
1. Duplicate files in /xD/reaction/
The file database itself contained duplicate rows, not the server. For example:
_id 1800  /xD/reaction/001.jpg  remote_id 00427388...
_id 2086  /xD/reaction/001.jpg  remote_id 00427388...
There were 354 duplicate-path groups (580 rows for 292 files in /xD/reaction/).
I reproduced the creation mechanism: forcing two concurrent queryChildDocuments() calls on the same stale folder started two RefreshFolderOperations at once. Both took a database snapshot before either saved, both saw the file as “new”, and both inserted it. FileDataStorageManager.saveFolder() does a check-then-insert without serialization, so it is a race.
2. Broken/missing thumbnails
Two bugs:
- DocumentsStorageProvider.openDocumentThumbnail() returned a generated PNG thumbnail but told the caller its length was file.fileLength — the original file size. Small images often have thumbnails larger than the original, so the thumbnail was truncated and failed to decode. Example: /xD/reaction/001.jpg is 13,645 bytes, but its generated thumbnail file was 60,533 bytes; only 13,645 bytes were exposed.
- DiskLruImageCacheFileProvider.getParcelFileDescriptorForOCFile() called Files.createFile() and only wrote the bitmap inside the same try. If the cache file already existed, createFile() threw and the write was skipped, leaving stale/old thumbnail data forever. Cache files were also keyed only by base filename, so equal filenames in different folders collided.
- Additionally ThumbnailsCacheManager.containsBitmap() dereferenced the disk cache before its async initialization finished, and addBitmapToCache() silently dropped images during startup, so first thumbnails were not cached.
3. /xD/reaction/111665930_..._n.png cannot be opened/shared
This was a device MediaProvider ownership problem, not a Nextcloud logic bug.
The file was registered in MediaProvider’s external.db as:
owner_package_name = com.android.shell
while the app is com.nextcloud.client. The app therefore got EACCES when opening/replacing it. 352 of 593 files under the Nextcloud media directory had the same wrong owner. Because the DB row had an empty etag, DocumentOpener also tried to re-download it; the HTTP download succeeded, but Files.move(..., REPLACE_EXISTING) then failed with EACCES, and opening the existing local file failed too.
Changes made
Source changes in the working tree:
- DocumentLister.kt
- Deduplicates listed/search results by remoteId / decrypted path.
- Coalesces concurrent folder reload jobs per document so repeated queryChildDocuments() calls do not start parallel refreshes.
- FileDataStorageManager.java
- Serializes saveFolder() through a process-wide lock, eliminating the check-then-insert race that created duplicate rows.
- DocumentsStorageProvider.kt
- Returns the actual thumbnail file size via descriptor.statSize instead of the original document size.
- DiskLruImageCacheFileProvider.java
- Always overwrites the generated thumbnail, uses a unique thumbnail filename (remoteId), and reports the real file.
- ThumbnailsCacheManager.java
- containsBitmap() now waits for disk-cache initialization and is null-safe.
- addBitmapToCache() waits for initialization instead of silently dropping thumbnails.
Device fixes applied
- Removed 354 duplicate filelist rows and reparented any affected children. Verified PRAGMA integrity_check = ok; /xD/reaction/ now returns 292 rows, no duplicates.
- Backup left at /data/local/tmp/filelist-before-dedupe.db.
- Repaired MediaProvider ownership for all 593 Nextcloud media files (com.android.shell / old ownership → com.nextcloud.client), preserving the files_update trigger and restarting MediaProvider.
- Deleted and re-downloaded the originally failing 111665930_..._n.png.
Verified content read now succeeds for:
- /xD/reaction/001.jpg — 13,645 bytes
- /xD/reaction/111665930_316871886117476_1713782649053830776_n.png — 92,032 bytes
- /xD/reaction/1708011767812109.jpg — 39,491 bytes
Status / caveat
The device-side duplicate cleanup and MediaProvider repair take effect immediately. The source fixes for duplicate prevention and thumbnails require rebuilding/reinstalling the APK; this environment has no Android SDK/AGP cache, so I could not compile or install the patched build here. The changes are uncommitted for human review as required by the project’s AI contribution policy.
Build · DeepSeek V4.1 Flash · 37m 33s · 47.8 tok/s