I know this is a pattern in dd-trace-py, and maybe dd-trace-js, but there is no precedence for it in dd-trace-go.

Why do you need to overwrite the env var directly? Couldn't anywhere that wants to query "_DD_ROOT_GO_SESSION_ID" just query globalconfig.RootSessionId()?

The os.Setenv is needed for child process propagation. globalconfig.RootSessionID() only works within the current process — child processes spawned via os/exec are separate binaries that can't call Go functions from the parent.

By setting _DD_ROOT_GO_SESSION_ID in the process environment at init time, any child spawned with default env inheritance (cmd.Env == nil) automatically receives it. When the child starts dd-trace-go, newConfig() reads it from the environment and knows it's part of the same session tree.

This is the core propagation mechanism described in the RFC — it's how Node.js, Python, and Ruby SDKs handle it too (each via their language's equivalent of env var injection into child processes). Without this, every user spawning child processes would need to manually set the env var, which defeats the purpose.

Happy to discuss if there's a Go-idiomatic alternative that achieves the same automatic propagation.

Okay. Let's just put it behind a constant (rootSessionIdEnvVar or similar)

Done — extracted it into a rootSessionIDEnvVar constant at the top of the file. Both the production code and tests now reference the constant instead of the raw string.

init() is very unpopular for go. Can you move this over to some helper function, and call it to set the rootSessionID field on the cfg variable?

e.g, line 21+ becomes:

var cfg = &config{
	analyticsRate: math.NaN(),
	runtimeID:     uuid.New().String(),
	headersAsTags: internal.NewLockMap(map[string]string{}),
	rootSessionID: getRootSessionID()
}

Done — replaced init() with a newConfig() helper that's called inline in the var cfg declaration.

Not exactly 😆

newConfig() is fine but was not required by my previous comment. I meant to abstract away the rootSessionID logic into its own helper, like getRootSessionID.

Good call — extracted the session ID logic into a getRootSessionID(runtimeID) helper. It takes runtimeID as a param since inlining it directly into the struct literal would cause an init cycle (cfg → getRootSessionID → cfg). So newConfig() stays as the thin wrapper that generates the runtimeID and passes it through.

I don't see a rule forbidding os.Setenv, only os.Getenv. Is this comment necessary?

You're right — only os.Getenv and os.LookupEnv are in the forbidigo rules, not os.Setenv. Removed the nolint directive.