add a thread safe context that can be used downstream and mutated

[rubensayshi]

It's currently unsafe to use c.Request.Context from the *gin.Context to pass to downstream functions AND do any mutations to the c.Request.Context.
Even when you do c.Request = c.Request.WithContext(...) you'll end up with the go race detector flagging the unsafe reading of the c.Request and writing to c.Request.

And even when you're not really doing anything .WithContext there's a chance that the context pool causes the race detector to get angry

However it would be really nice to be able to put values into the *gin.Context with context.WithValue() in a thread-safe way that allows downstream usage of *gin.Context as context.Context to access those values.
For example, an auth middleware adding some debugging /loggin related values with context.WithValue() will not be lost along the way, or adding a otel.Tracer to the context with additional options etc.

Making all the places where gin touches c.Request thread-safe with a lock would be a significant undertaking and would have an impact on performance (probably talking about nano seconds, but still non zero impact).

With the MR we're adding a thread-safe way to carry around and mutate a context.Context inside *gin.Context without having to protect c.Request.

In practice the c.Request.Context isn't going to be very interesting to wrap anyway, so loosing that as the "root" context probably isn't a issue for anyone.
It's trivial to make the root InternalContext the c.Request.Context when we do .reset() (if ContextWithFallback is enabled), but I wanted to keep this MR as small as possible.
And it would have one quirky downside that if someone uses UseInternalContext and does something like c.Request = c.Request.WithContext(...) then it wouldn't have any effect.

There's a couple of mentions of issues around this before:

• gin.Context data race in most trivial situation #4117
• gin detects DATA RACE with Go1.21, Go1.22 #3931
• Data Race into context when you don't call func (c *Context) Copy() *Context #1118

[appleboy]

@rubensayshi Please update the PR to the latest source code for the CI/CD flow.

[codecov]

Codecov Report

Attention: Patch coverage is 83.92857% with 9 lines in your changes missing coverage. Please review.

Project coverage is 98.75%. Comparing base (3dc1cd6) to head (cd8ad99).
Report is 143 commits behind head on master.

Files with missing lines	Patch %	Lines
context.go	82.69%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4172      +/-   ##
==========================================
- Coverage   99.21%   98.75%   -0.47%     
==========================================
  Files          42       44       +2     
  Lines        3182     3445     +263     
==========================================
+ Hits         3157     3402     +245     
- Misses         17       32      +15     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags "sonic avx"	98.74% <83.92%> (?)
-tags go_json	98.74% <83.92%> (?)
-tags nomsgpack	98.73% <83.92%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.23	98.75% <83.92%> (?)
go-1.24	98.75% <83.92%> (?)
macos-latest	98.75% <83.92%> (-0.47%)	⬇️
ubuntu-latest	98.75% <83.92%> (-0.47%)	⬇️

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[rubensayshi]

happy to increase the coverage if there's an actual chance this would get merged! :D

[RemiBou]

can we get someone to look into it ? it's failing many of our concurrency tests because of the race condition

[appleboy]

I will take a look.

[Copilot]

Pull Request Overview

This PR introduces thread-safe context management capabilities to Gin by adding an internal context mechanism that allows safe mutations without race conditions on c.Request.Context(). The implementation addresses known race detection issues when using context values in concurrent scenarios.

• Adds UseInternalContext feature flag to enable thread-safe internal context management
• Implements WithInternalContext() and InternalContext() methods with proper mutex protection
• Modifies existing context methods to support internal context fallback when enabled

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
context.go	Core implementation of thread-safe internal context with mutex protection and context interface methods
gin.go	Adds UseInternalContext configuration flag and updates comment formatting
context_test.go	Comprehensive test coverage for all internal context methods and edge cases
test_helpers.go	Enhances CreateTestContext with optional configuration functions for testing

Comments suppressed due to low confidence (1)

context.go:1357

• [nitpick] The function name useInternalContext is ambiguous - it reads like an action rather than a predicate. Consider renaming to hasInternalContext or isInternalContextEnabled to better indicate it's a boolean check.

func (c *Context) useInternalContext() bool {

[Copilot]

The comment is incorrect for the useInternalContext function. It should describe that this function checks if the UseInternalContext feature flag is enabled, not about request context fallback.

Suggested change

	// hasRequestContext returns whether c.Request has Context and fallback.
	// useInternalContext checks if the UseInternalContext feature flag is enabled.