cli-plugins/hooks: fix max-message off-by-one

[fallintoplace]

Fixes #7005

What changed

This updates cli-plugins/hooks.ParseTemplate() to enforce the 10-message limit on the actual number of rendered messages instead of on the number of newline separators alone.

It also adds a regression test for the 11-line case.

Why it changed

The current implementation rejects only when strings.Count(out, "\n") > maxMessages and then returns strings.SplitN(out, "\n", maxMessages).

That makes the guard off by one: output with 10 newline characters contains 11 messages, but is still accepted. The extra rendered line is then folded into the last returned element.

Impact

Templates that render to more than 10 messages are now rejected consistently, including the 11-line / 10-newline case.

Valid templates keep the existing split behavior.

Root cause

The limit check counted separators instead of messages.

Validation

• GOPATH=$(mktemp -d) GO111MODULE=off go test github.com/docker/cli/cli-plugins/hooks via a temporary GOPATH layout rooted at this checkout

[thaJeztah]

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Let me know if you need help or more detailed instructions!

[fallintoplace]

@thaJeztah Thank you. I will check later today.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[docker-agent]

Assessment: 🟢 APPROVE

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

[docker-agent]

[medium] Trailing-newline output incorrectly rejected: +1 overcounts messages when output ends with \n

The fix changes the guard from strings.Count(out, "\n") > maxMessages to strings.Count(out, "\n") + 1 > maxMessages. This is correct for output that does not end with a trailing newline, but it overcounts when output does end with \n.

Trigger path: A template that renders to exactly 10 messages each followed by a newline — e.g., "msg1\nmsg2\n...\nmsg10\n" — has 10 newline characters. The new guard computes n = 10 + 1 = 11 > 10 = maxMessages and rejects the output with the error "hook template contains too many messages (11): maximum is 10". Yet the template only produced 10 real messages; the 11th "segment" from strings.Split is just an empty string after the final \n.

Impact: Valid 10-message templates whose rendered output ends with a trailing newline (common when Go text/template templates place a literal newline after the last field) are incorrectly refused. The regression test added in this PR uses strings.Repeat("line\n", 10)+"line" (no trailing newline), so it does not catch this edge case.

Suggested fix: Trim a trailing newline before counting, so an empty final segment is not treated as an extra message:

Suggested change

	if n := strings.Count(out, "\n") + 1; n > maxMessages {
	if n := strings.Count(strings.TrimRight(out, "\n"), "\n") + 1; n > maxMessages {

Confidence	Score
🟢 strong	100/100

[Copilot]

🟢 Approval recommended

The boundary fix is correct, focused, and covered by a regression test.

Pull request overview

Fixes the hook message-limit off-by-one error by counting rendered messages rather than newline separators.

Changes:

• Rejects rendered output containing more than 10 messages.
• Adds regression coverage for the 11-message boundary.

File summaries

File	Description
cli-plugins/hooks/template.go	Corrects message counting.
cli-plugins/hooks/template_test.go	Tests rejection of 11 messages.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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