proposal: testing: allow settings flags for the duration of a test

[nobletooth]

Proposal Details

When tests require a certain flag value to work, flag.Set(...) is often chosen as the go-to way.
An example:

import "testing"

func TestSomeFeature(t *testing.T) {
    err := flag.Set("enable_some_feature", "true")
    if err != nil {
        t.Fatal(err)
    }
    // Trigger the feature and make assertions.
}

There are some downsides to this approach, most importantly these:

1. Parallel tests may encounter race conditions and panic when calling the flag.Set function.
2. The flag value is set throughout the test process, i.e. affects other tests are well, while it shouldn't.

I propose having a func (t* testing.T) SetFlag(name, value string) function that makes the flag set goroutine-safe and only affect a single test scope, i.e. the flag is reset to default value when t is done.

[seankhliao]

This is not feasible.
The setters/helpers we've added to testing.T are still global and primarily interact with os features.

flags are a library concern, and modeling that should be done with instanced FlagSets rather than pushing patching global state into testing.