Add warning when writing response body multiple times (fixes #4477) by raju-mechatronics · Pull Request #4483 · gin-gonic/gin

raju-mechatronics · 2025-12-28T12:17:20Z

Add warning for multiple response writes

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.

warning works only in debug mode

codecov · 2025-12-28T12:19:10Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.80%. Comparing base (3dc1cd6) to head (7b5727c).
⚠️ Report is 265 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4483      +/-   ##
==========================================
- Coverage   99.21%   98.80%   -0.42%     
==========================================
  Files          42       47       +5     
  Lines        3182     3092      -90     
==========================================
- Hits         3157     3055     -102     
- Misses         17       27      +10     
- Partials        8       10       +2

Flag	Coverage Δ
	`?`
--ldflags="-checklinkname=0" -tags sonic	`98.79% <100.00%> (?)`
-tags go_json	`98.66% <100.00%> (?)`
-tags nomsgpack	`98.72% <100.00%> (?)`
go-1.18	`?`
go-1.19	`?`
go-1.20	`?`
go-1.21	`?`
go-1.25	`98.80% <100.00%> (?)`
go-1.26	`98.73% <100.00%> (?)`
macos-latest	`98.80% <100.00%> (-0.42%)`	⬇️
ubuntu-latest	`98.73% <100.00%> (-0.48%)`	⬇️

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raju-mechatronics · 2025-12-28T12:26:15Z

@appleboy can you please take a look at this pr

Nurysso · 2026-01-01T13:35:52Z

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 · 2026-01-01T13:51:47Z

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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Copilot · 2026-02-28T02:13:24Z

context.go

+	if c.Writer.Written() && IsDebugging() {
+		// Skip warning for SSE and streaming responses (status code -1)
+		if code != -1 {
+			if _, ok := r.(sse.Event); !ok {
+				debugPrint("[WARNING] Response body already written. Attempting to write again with status code %d", code)
+			}


c.Writer.Written() becomes true as soon as headers are written (Size()==0), not only after body bytes are written. This can emit a false warning on the first body write if a handler/middleware flushes headers early (e.g., calls c.Writer.WriteHeaderNow()/Flush()) and then renders JSON. To specifically detect multiple body writes (as the message says), consider checking c.Writer.Size() > 0 (or equivalent) instead of Written().

Copilot · 2026-02-28T02:13:25Z

context.go

 	}

+	if c.Writer.Written() && IsDebugging() {
+		// Skip warning for SSE and streaming responses (status code -1)


The inline comment says “Skip warning for SSE and streaming responses (status code -1)”, but code == -1 is also used for non-streaming renders like render.Redirect, and the logic also has a separate sse.Event type check. Please clarify the comment (and/or consolidate the conditions) so future readers understand why -1 and sse.Event are treated specially.

Suggested change

// Skip warning for SSE and streaming responses (status code -1)

// Skip this warning when:

// - status code is -1, which is used for special/streaming-style renders (including some

// non-streaming helpers like redirects) where multiple writes are expected, and

// - the renderer is an SSE event, since Server-Sent Events are sent as a stream.

Copilot · 2026-02-28T02:13:25Z

context_test.go

+	oldMode := os.Getenv("GIN_MODE")
+	defer os.Setenv("GIN_MODE", oldMode)
+	SetMode(DebugMode)
+


These tests call SetMode(DebugMode) but never restore the previous gin mode, which can leak global state into later tests (the suite defaults to TestMode). Capture the prior Mode() (or assume TestMode) and defer SetMode(...) to restore it after the test.

Copilot · 2026-02-28T02:13:25Z

context_test.go

+	oldMode := os.Getenv("GIN_MODE")
+	defer os.Setenv("GIN_MODE", oldMode)
+	SetMode(DebugMode)


oldMode := os.Getenv("GIN_MODE") / defer os.Setenv(...) is redundant here because the test never modifies the environment variable, and SetMode(...) does not read GIN_MODE after init. Consider removing the env save/restore and instead restoring the previous gin mode via Mode() + SetMode.

Copilot · 2026-02-28T02:13:25Z

context_test.go

+	oldMode := os.Getenv("GIN_MODE")
+	defer os.Setenv("GIN_MODE", oldMode)
+	SetMode(DebugMode)
+


This test calls SetMode(DebugMode) but never restores the previous gin mode, which can leak global state into later tests. Please defer SetMode(...) to restore the mode (usually TestMode) after the assertions complete.

Copilot · 2026-02-28T02:13:25Z

context_test.go

+	oldMode := os.Getenv("GIN_MODE")
+	defer os.Setenv("GIN_MODE", oldMode)
+	SetMode(DebugMode)


As in the previous test, the GIN_MODE env save/restore is unused because the test never changes GIN_MODE, and SetMode does not consult the env var after startup. Prefer removing this and restoring the prior gin mode via Mode()/SetMode instead.

appleboy · 2026-02-28T03:19:07Z

move to the next milestone

raju-mechatronics and others added 3 commits December 28, 2025 18:02

fix(context): added warning for multiple writes to response body

8eb015d

warning works only in debug mode

test(context): add tests for multiple JSON and SSE writes in debug mode

dc128b7

Merge branch 'master' into master

4d32cf9

appleboy added the enhancement label Jan 2, 2026

appleboy added this to the v1.12 milestone Jan 2, 2026

Merge branch 'master' into master

efdb77e

raju-mechatronics mentioned this pull request Jan 17, 2026

feat: add warning for multiple response body writes #4500

Closed

Merge branch 'master' into master

bc46a09

appleboy requested a review from Copilot February 28, 2026 02:09

Copilot started reviewing on behalf of appleboy February 28, 2026 02:10 View session

Copilot AI reviewed Feb 28, 2026

View reviewed changes

appleboy modified the milestones: v1.12, v1.x, v1.13 Feb 28, 2026

Merge branch 'master' into master

7b5727c

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Add warning when writing response body multiple times (fixes #4477)#4483

Add warning when writing response body multiple times (fixes #4477)#4483
raju-mechatronics wants to merge 6 commits intogin-gonic:masterfrom
raju-mechatronics:master

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-		// Skip warning for SSE and streaming responses (status code -1)
+		// Skip this warning when:
+		//   - status code is -1, which is used for special/streaming-style renders (including some
+		//     non-streaming helpers like redirects) where multiple writes are expected, and
+		//   - the renderer is an SSE event, since Server-Sent Events are sent as a stream.

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raju-mechatronics commented Dec 28, 2025

Add warning for multiple response writes

Description

Changes

Example

Checklist

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