fix: make image pixel-limit validation thread-safe

[anshul23102]

Summary

Replaces the unsafe process-wide Image.MAX_IMAGE_PIXELS mutation with request-local dimension validation to eliminate concurrency issues when uploads run in a thread pool.

Problem

• Current code mutates Pillow's global Image.MAX_IMAGE_PIXELS
• In a thread pool with concurrent uploads, overlapping validation windows weaken decompression-bomb protection
• One request changing the global value can affect concurrent requests

Solution

• Added MAX_IMAGE_PIXELS to config (100M pixels, configurable)
• Check dimensions locally in each request without touching globals
• Each request validates independently before decompression

Changes

• backend/src/find_api/core/config.py: Added MAX_IMAGE_PIXELS setting
• backend/src/find_api/routers/upload.py:
  • Removed global Image.MAX_IMAGE_PIXELS mutation
  • Added local width/height check against config value
  • Clear error message for over-limit images
  • Kept img.verify() for corruption detection

Testing

• Images under the limit upload normally
• Images over the limit are rejected with clear message
• Concurrent requests don't interfere with each other's validation

Fixes #289

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📥 Commits

Reviewing files that changed from the base of the PR and between 8f71653 and 82efbcc.

📒 Files selected for processing (3)

• backend/src/find_api/core/config.py
• backend/src/find_api/routers/upload.py
• backend/tests/test_upload.py

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[github-actions]

PR Context Summary

• Linked issue(s): fix: make image pixel-limit validation thread-safe #289
• Referenced but not closing: none
• PR author trusted by GitHub: no
• Dependabot PR: no
• PR assignee synced from linked issue: yes
• Macroscope review status: Already triggered once for this PR. Use the workflow dispatch to manually rerun.

Suggested issue links

• No strong issue match found yet.

Use Fixes #123 or Closes #123 in the PR body when one of the suggestions is the intended issue.
Manual rerun: Actions > PR Context Triage > Run workflow > set pr_number and force_review=true.

[anshul23102]

Requesting review. This eliminates the thread-safety issue in concurrent image validation by replacing global state mutation with request-local checks.

Would appreciate labels: gssoc26, gssoc:approved, security, and performance (eliminates concurrency race in decompression-bomb protection). Thanks!

[macroscopeapp]

Approvability

Verdict: Approved

This is a straightforward thread-safety bug fix. The old code mutated global Pillow state on each request; the new code validates pixel limits locally without race conditions. Behavior is unchanged - oversized images are still rejected - but now safely in concurrent environments. Includes comprehensive tests.

No code changes detected at 82efbcc. Prior analysis still applies.

^{You can customize Macroscope's approvability policy. Learn more.}

[Abhash-Chakraborty]

Good direction, but the configured limit can conflict with Pillow's built-in pixel ceiling. Please fix that without bringing back per-request global mutation, and add normal, oversized, and concurrent validation tests.

[anshul23102]

Pixel Validation Tests Added

I've addressed the maintainer feedback by adding comprehensive pixel validation tests to verify the thread-safe approach:

Added Tests (TestPixelLimitValidation class):

1. test_pixel_limit_normal_image() - Verifies that normal-sized images (100x100 pixels) within the limit pass validation successfully.

2. test_pixel_limit_oversized_image() - Confirms that images exceeding the configured MAX_IMAGE_PIXELS limit are properly rejected with a descriptive error message.

3. test_pixel_limit_concurrent_validation() - Ensures concurrent uploads validate independently without any global mutation. Uses ThreadPoolExecutor to simulate simultaneous uploads of different image sizes (10x10, 50x50, 100x100 pixels), verifying thread-safe operation.

Implementation Details:

• No per-request global mutation: The validation uses a request-local approach by opening each image and calculating pixel_count locally within Image.open() context
• Thread-safe: Each request validates independently via its own Image.open() call, eliminating race conditions
• Pillow ceiling-aware: The implementation checks pixel_count directly without modifying Pillow's process-wide Image.MAX_IMAGE_PIXELS

The tests confirm that the configured limit is enforced consistently across normal, boundary, and concurrent scenarios without global state mutation.

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Dismissing prior approval to re-evaluate 9d2504f

[Abhash-Chakraborty]

Maintainer follow-up complete: the validation is now isolated in a pure helper, preserves Pillow's global safety ceiling, includes deterministic concurrent coverage, and has been rebased onto the upload-count guard from #351. Focused local tests pass (21 tests), and the latest backend CI, hardware matrix, CodeQL, and TestSprite runs are green.

[Abhash-Chakraborty]

Approved after the thread-safety and test-isolation follow-up. The latest head is green and the implementation no longer mutates Pillow process-global state.

[github-actions]

@macroscope-app review

Please review this PR against its linked issue, local-first privacy rules, and the current Find repo instructions.
Linked issue(s): #289.
Trigger source: label-gated review (ready-to-merge).