Photo shared to Google Messages via share sheet fails to attach — Messages rejects our DiskLruImageCacheFileProvider URI as "private"

[dabudtruck]

⚠️ Before posting ⚠️

• This is a bug, not a question or an enhancement.
• I've searched for similar issues and didn't find a duplicate.
• I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
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Steps to reproduce

Step 1: Open the Nextcloud app, go to Photos, tap a photo, tap the share icon.
Step 2: Choose "Reduced size" or "Original file" (not "Public link").
Step 3: In Android's native share sheet, choose Messages and select a contact/thread.
Step 4: Observe -- Messages opens, but no image attachment appears, no error shown to the user.

Expected behaviour

The image should attach, the same way it does when sharing the identical photo via Signal or Nextcloud Talk.

Actual behaviour

Messages opens, but no image attachment appears. No error is shown to the user.

Confirmed via adb logcat while reproducing -- Messages' own process logs:

E MessagingApp: Assert.fail() called: Cannot send private file content://org.nextcloud.imageCache.provider/Household/Photos/.../IMG_20260106_103619_161.jpg
    at com.android.messaging.ui.conversationlist.ShareIntentActivity.addSharedPartToDraft(SourceFile:244)

This assertion fires before Messages attempts to actually read the file (no preceding SecurityException from opening it) -- it's Messages' own defensive check on the incoming shared Intent, deciding the URI/grant isn't trustworthy and refusing outright, not an OS-level permission wall being tripped mid-read. That points at something in how the share Intent is constructed for content://org.nextcloud.imageCache.provider/... (our DiskLruImageCacheFileProvider) not satisfying whatever Messages checks for (possibly related to ClipData/grant-flag handling, not confirmed at the code level on our side).

Isolation testing done:

Source app	Target app	Result
Nextcloud	Messages	Fails silently -- no attachment
Nextcloud	Nextcloud Talk	Works
Nextcloud	Signal	Works
Default Gallery app	Messages	Works

Reproduced identically on two separate GrapheneOS phones (Pixel 10 / Pixel 10 XL), ruling out a device-specific issue.

Related report: we hit the same Messages-side symptom (identical Assert.fail(): Cannot send private file) from the separate Memories Android app (pulsejet/memories) too, but via a different URI -- Memories routes its share through Android's DownloadManager (content://downloads/all_downloads/) rather than a FileProvider at all. Filed as a separate issue there: pulsejet/memories#1706 -- linking in case the two clients share relevant file-sharing code.

Suspected cause: whatever combination of Intent flags / ClipData construction Messages requires to accept a shared content:// URI as legitimate isn't being satisfied when serving files through DiskLruImageCacheFileProvider. Happy to help narrow this down further or test a patched build if useful.

Android version

17 (GrapheneOS)

Device brand and model

Google Pixel 10 Pro XL / Pixel 10

Stock or custom OS?

Custom (explain in "additional information")

Nextcloud android app version

34.1.0

Nextcloud server version

33.0.6

Using a reverse proxy?

Yes

Android logs

Relevant excerpt from adb logcat -v threadtime captured while reproducing (share icon -> Original file -> Messages -> select contact -> send):

ActivityTaskManager: START u0 {act=android.intent.action.SEND typ=image/jpeg ... cmp=com.android.messaging/.ui.conversationlist.ShareIntentActivity ...} from uid 10195 (com.nextcloud.client) (realCallingUid=10080)
ImageLoader: java.io.IOException: java.lang.SecurityException: Permission Denial: opening provider com.owncloud.android.providers.DiskLruImageCacheFileProvider from ProcessRecord{... com.android.intentresolver/u0a80} (pid=31835, uid=10080) requires that you obtain access using ACTION_OPEN_DOCUMENT or related APIs
MessagingApp: Assert.fail() called: Cannot send private file content://org.nextcloud.imageCache.provider/Household/Photos/.../IMG_20260106_103619_161.jpg
    at com.android.messaging.ui.conversationlist.ShareIntentActivity.addSharedPartToDraft(SourceFile:244)

Note the first SecurityException is from the share sheet's own preview-thumbnail loader (com.android.intentresolver, uid 10080) trying to render a preview -- that happens for any target app, before one is chosen, so it's a separate cosmetic issue. The Assert.fail() from MessagingApp (Messages' own process) is the actual cause of the attach failure.

Server error logs

Additional information

Custom OS: GrapheneOS (a hardened, de-Googled Android fork). Messages app referenced above is com.android.messaging, the built-in AOSP Messaging app that GrapheneOS ships by default (not a Google Play app).

[dabudtruck]

Workaround update: installing Google's actual Messages app (com.google.android.apps.messaging, via GrapheneOS's Sandboxed Google Play) instead of the AOSP com.android.messaging that GrapheneOS ships by default resolves the attach failure -- sharing the same photo from the Nextcloud app to Google's Messages app attaches correctly. Google's Messages app is a separate, independently-developed codebase and doesn't appear to have the same "Cannot send private file" defensive check as the AOSP Messaging app, which is consistent with the theory that this is specific to how the AOSP app validates the shared URI rather than an Android/GrapheneOS permission issue.

No Google account is required either -- Messages has a "Use app without account" option on its sign-in screen that still works fine for this.

Not a fix for the underlying bug (still worth fixing so it works with the AOSP Messaging app and any other strict receiver), but a clean workaround for anyone else hitting this in the meantime. Also filed the same symptom against the Memories app, via a different URI: pulsejet/memories#1706