05 — Context

Nothing outside a goroutine can stop it. A go statement hands back no handle (A goroutine has no handle), so a caller that wants work to stop has to ask, and the goroutine has to listen. context.Context is Go's standard way of asking. It carries a Done channel that is closed when the work should stop, an Err that says why, and sometimes a deadline, and it is passed down as the first argument of every function on a request's path. The package has been in the standard library since Go 1.7 ↗.

The four lessons follow one cancellation from start to finish. A single cancel reaches a whole tree of contexts. A deadline is the same cancel, fired by a clock. A cancel stops only a goroutine that is listening, and a goroutine whose caller has gone can wait forever. And ctx.Err() loses the reason for the cancel unless the reason is recorded as its cause.

Lesson	The one thing
One cancel reaches every goroutine	a cancel closes Done for the context and everything derived from it, never for its parent; call cancel on every path, and go vet checks that you do
A deadline is a cancel with a clock	the time runs out, Done closes, and Err says DeadlineExceeded; a child can shorten its parent's deadline but never extend it
A leaked goroutine never ends	a goroutine blocked on a send its caller stopped waiting for stays blocked; a buffer of one or a select on ctx.Done() lets it finish
Cancel with a cause	ctx.Err() says only "canceled"; WithCancelCause records why, and context.Cause reads the reason back from any context in the tree

Planned

• context.WithValue is for request-scoped data only — the package's rule that values carry data belonging to a request across API and process boundaries, not optional parameters, and why each key gets an unexported type of its own. The Rust library's stubs Carrying the trace across a boundary ↗ and Instrumenting async code ↗ are the same problem for a trace id, between processes and inside one.
• context.AfterFunc — Go 1.21's way to run a function once a context is canceled, for waking something that cannot select on Done, such as a sync.Cond wait.
• context.WithoutCancel — Go 1.21's context that points to its parent but is never canceled and has no deadline, for work that must finish after its request has gone.
• A context is an argument, not a field — the package's rule against storing a context in a struct, and the Go blog's Contexts and structs ↗.
• signal.NotifyContext — Ctrl-C as a cancel.