DEVI-101: Enhance guardrail error message for background upload handlers

[devin-ai-integration]

DEVI-101: Enhance guardrail error message for background upload handlers

Summary

Enhanced the error message for the existing guardrail that prevents using @rx.event(background=True) with upload handlers. The previous error message simply stated the restriction without explaining why it exists. The new message provides context about why background events are incompatible with upload handlers, helping developers understand the technical reasoning behind the limitation.

Changes:

• Updated error message in reflex/app.py to explain that "Upload handlers require synchronous state management and cannot run as background tasks"
• Updated corresponding test assertion in tests/units/test_app.py to match the new error message format

Review & Testing Checklist for Human

• Verify error message accuracy: Confirm that the explanation "Upload handlers require synchronous state management and cannot run as background tasks" is technically correct
• Test the error message in practice: Try creating a background upload handler to see the new error message in action
• Check for grammar/clarity: Ensure the error message is clear and helpful for developers encountering this restriction

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    App["reflex/app.py<br/>upload() function"]:::major-edit
    TestApp["tests/units/test_app.py<br/>test_upload_file_background()"]:::minor-edit
    TestStates["tests/units/states/upload.py<br/>FileUploadState.bg_upload()"]:::context
    
    App -->|"validates handler"| TestStates
    TestApp -->|"tests error message"| App
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end

classDef major-edit fill:#90EE90
classDef minor-edit fill:#87CEEB
classDef context fill:#FFFFFF

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Notes

• This change maintains backward compatibility - the same UploadTypeError is raised with the same error detection logic
• The guardrail itself has been working correctly; this just improves the developer experience when encountering the restriction
• All existing tests pass with the enhanced error message

---

Session: https://app.devin.ai/sessions/86b825878ce44f8d8543a75e1f58eb02
Requested by: @Alek99

[devin-ai-integration]

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[greptile-apps]

Greptile Summary

This PR enhances the error message shown when a developer attempts to use a background task (@rx.event(background=True)) for an upload handler. The change makes the error message more descriptive and educational by explaining that upload handlers require synchronous state management, which is incompatible with background tasks.

The changes occur in two files:

1. reflex/app.py: Updates the error message in the upload handler validation logic
2. tests/units/test_app.py: Updates the corresponding test to verify the new error message

PR Description Notes:

• The PR description is currently a placeholder and needs to be updated

Confidence score: 5/5

1. This PR is extremely safe to merge as it only modifies error message text
2. The score is 5/5 because this is a non-functional change that only improves error messaging without modifying any actual behavior
3. No files need special attention as the changes are straightforward and well-tested

_{2 files reviewed, no comments
Edit PR Review Bot Settings | Greptile}

[codspeed-hq]

CodSpeed Performance Report

Merging #5588 will not alter performance

_{Comparing devin/1752715018-add-background-upload-guardrail (b365dd2) with main (29c2ef5)}

Summary

✅ 8 untouched benchmarks