fix: flaky TestPrunePromptTermination test

[hirehamir]

- What I did

Fixes #5802

- How I did it

I kept the write end of the pipe alive for the duration of the test with

defer w.Close()

- How to verify it

# without the changes from this branch: fails most runs
# with the changes from this branch: 0 failures
GOGC=1 GOMEMLIMIT=8MiB go test ./cli/command/container/ -run TestContainerPrunePromptTermination -count=40 -v

	GC forced (GOGC=1 GOMEMLIMIT=8MiB)	GC off (GOGC=off)
without this change	33 of 40 fail	0 of 40 fail
with this change	0 of 40 fail	0 of 40 fail

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/test/cmd.go	0.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[GitNimay]

I can confirm this fixes the flake, which also hit CI on #7408 (TestSystemPrunePromptTermination). The discarded write end of the pipe is garbage-collected, its finalizer closes it, and the prompt reads EOF (treated as "no"), so the command returns "prune has been cancelled" instead of prompt.ErrTerminated.

Reproduce on master with:

GOGC=1 go test -race -count=200 -run PromptTermination ./cli/command/system ./cli/command/image

That gave 11 failures on master, and 0 with the write end kept open.

Two things you may want to add here:

• cmd/docker-trust/internal/test/cmd.go has an identical copy of TerminatePrompt with the same bug.
• Under -race, the cli.OutBuffer().Len() check in the helper races with the prompt that is still writing to the buffer. The writer hook already guarantees a write happened, so that check can be dropped.

This is the flake tracked in #5802.

[hirehamir]

That makes sense. I'll add those changes right now.

[hirehamir]

@Benehiko anything else you'd like to see here?