fix: release the stream when a bitmap preview cannot be decoded

[oc-tmueller]

Summary

Bitmap::getThumbnail() opens the file and closes it only on the success path — the catch around getResizedPreview() returns before the fclose(). Every failed decode therefore leaks one file descriptor for the lifetime of the process.

Flagged as out of scope in #41827 ("that's a resource leak, not a security defect, and deserves its own focused PR"), so this is that PR. It targets master and is independent of the #41827/#41834 chain.

Why it matters more than it looks

A failed decode is not an edge case. Any content ImageMagick has no coder for lands in that catch, and #41834 makes throwing a designed outcome — if a build does not register a provider's coder, the pin throws rather than falling back to content sniffing. So occ preview pre-generation, or a cron preview job, over a directory of .heic/.psd files leaks a handle per file until EMFILE.

Second defect, same three lines

$file->fopen('r') was unchecked. A storage that cannot open the file returns false, and stream_get_contents(false) raises a TypeError — an \Error, so it escapes the catch (\Exception) directly underneath and surfaces as a 500 instead of the missing preview every other failure here degrades to. Confirmed, not theorised:

TypeError: stream_get_contents(): Argument #1 ($stream) must be of type resource, false given
  lib/private/Preview/Bitmap.php:89
  lib/private/Preview/Bitmap.php:51

What changed

• fclose() moves into a finally, so it runs on both paths.
• A false return from fopen() is handled explicitly and logged.

Tests

New tests/lib/Preview/BitmapStreamTest.php, 3 cases. It asserts the contract directly via is_resource($stream) on the caller's own handle rather than counting descriptors, so it is portable rather than Linux-only.

Confirmed RED before the fix on owncloudci/php:8.3 — 1 failure (the stream must be closed on the failure path) and 1 error (the TypeError above); the success-path case passed unfixed, as the control. GREEN after: 3 tests, 5 assertions.

Verification

• tests/lib/Preview/ on owncloudci/php:8.3: 55 tests, 152 assertions, 0 failures.
• make test-php-style: 0 of 2435 files need fixing.
• php -l clean under PHP 7.4, so this backports to the 10.x line without syntax changes.

Note on sequencing

#41827 modifies these same lines. Whichever lands second needs a trivial rebase — the conflict is confined to the try/catch/fclose block.

🤖 Generated with Claude Code