testutil

func CaptureStdout ¶

func CaptureStdout(t *testing.T, fn func()) string

CaptureStdout runs fn with os.Stdout redirected to an in-memory pipe and returns whatever fn wrote to stdout.

A goroutine drains the pipe's read end concurrently with fn's writes so callers can emit arbitrarily large output without deadlocking the writer when the OS pipe buffer (~64 KiB on Linux) fills. Without the concurrent reader, an output larger than the pipe buffer blocks the writer inside fn and the test hangs until its timeout.

os.Stdout is restored to its original value before this function returns. t.Cleanup is registered as a safety net so the restoration also happens if fn panics (in which case the explicit restore never runs but the cleanup does, once the testing framework recovers).

Only os.Stdout is redirected; os.Stderr is left untouched. Use a similar helper for stderr if you need it.

The supplied *testing.T is used for t.Helper (so failures are reported at the caller's line) and t.Cleanup. Pass the test's t.

func CaptureStdoutPanicking ¶

func CaptureStdoutPanicking(fn func()) string

CaptureStdoutPanicking is like CaptureStdout but for callers that cannot supply a *testing.T (legacy helpers, helpers used from non-test contexts). On os.Pipe failure it panics with a descriptive message; on a panicking fn, os.Stdout is restored via defer.

Prefer CaptureStdout when *testing.T is available.