[raju-mechatronics]

Add warning for multiple response writes

Fixes #4477

Description

Adds a warning when the response body is written multiple times in the same request. This prevents silent concatenation of multiple JSON/XML responses, which often produces invalid output.

Changes

• Added warning in Context.Render() when response body already written
• Consistent with existing header write warning behavior
• Non-breaking change (debug mode only)

Example

router.GET("/example", func(c *gin.Context) {
    c.JSON(200, gin.H{"first": "response"})
    c.JSON(200, gin.H{"second": "response"})
})

Output:

[GIN-debug] [WARNING] Response body already written. Attempting to write again with status code 200

Checklist

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.65%. Comparing base (0f09c3a) to head (78e39e7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4483   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          48       48           
  Lines        3188     3192    +4     
=======================================
+ Hits         3145     3149    +4     
  Misses         43       43           

Flag	Coverage Δ
--ldflags="-checklinkname=0" -tags sonic	98.64% <100.00%> (+<0.01%)	⬆️
-tags go_json	98.57% <100.00%> (+<0.01%)	⬆️
-tags nomsgpack	98.63% <100.00%> (+<0.01%)	⬆️
go-1.26	98.33% <100.00%> (+<0.01%)	⬆️
go-1.27	98.67% <100.00%> (+<0.01%)	⬆️
macos-latest	98.65% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	98.65% <100.00%> (+<0.01%)	⬆️

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[raju-mechatronics]

@appleboy can you please take a look at this pr

[Nurysso]

Hey nice addition but when the warning fires the code still continues to render, which means the response could still gets corrupted, right? Shouldn't we return after the warning to actually prevent the invalid output?

[raju-mechatronics]

Hey nice addition but when the warning fires the code still continues to render, which means the response could still gets corrupted, right? Shouldn't we return after the warning to actually prevent the invalid output?

I was waiting for reviewer response on this but it's for backward compatibility. Watch #4477 for details. It's seems multiple response is allowed in gin.

[Copilot]

Pull request overview

This PR adds a debug-mode warning when a handler attempts to write a response body more than once during the same request, addressing silent concatenation (e.g., multiple JSON/XML writes) that can produce invalid output.

Changes:

• Add a debug warning in Context.Render() when a second render occurs after the response has already been written.
• Exempt code == -1 (used for SSE/stream-like cases) and sse.Event from the warning.
• Add tests to validate the warning for multiple JSON writes and ensure no warning for repeated SSE writes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
context.go	Emits a debug warning when rendering after the response is already written, with special-casing for SSE/-1 code.
context_test.go	Adds tests covering the new warning behavior for JSON and SSE.

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[appleboy]

move to the next milestone