fix(tail): reduce ring buffer and byte-mode buffer caps from 64/32 MiB to 5 MiB

[thieman]

Problem

tail had two memory limits well above 10 MiB per invocation:

M-1: Ring buffer cap was 64 MiB

MaxRingBytes = 64 << 20 — a single tail -n <large> on a file with long lines could hold 64 MiB of live data in the ring buffer simultaneously.

M-2: Byte-mode circular buffer cap was 32 MiB

tail -c <large> allocated a circular buffer capped at 32 MiB.

Both limits are far above the project's 10 MiB per-invocation guideline.

Fix

Both caps reduced to 5 MiB (5 << 20). MaxTotalReadBytes (the read cap for infinite streams like pipes) is unchanged — that controls how much total data is consumed from an infinite source before bailing, which is a different concern from how much live data is held in memory.

Test plan

• tail -n 100 file still works correctly
• tail -c 1000 file still works correctly
• tail -n 1000000 file correctly returns the last lines within the 5 MiB cap
• Existing tail scenario tests pass

🤖 Generated with Claude Code

[thieman]

@codex review this PR

[thieman]

Self-Review Summary — PR #148: Reduce tail memory limits to 5 MiB

Overall assessment: safe to merge.

This PR reduces two memory caps in the tail builtin:

• MaxRingBytes (line-mode ring buffer): 64 MiB → 5 MiB
• MaxBytesBuffer (byte-mode circular buffer): 32 MiB → 5 MiB
• Bonus cleanup: replaces a 3-line if/else with max() builtin (semantically identical)

All constants, package-level doc comments, and inline function comments are updated consistently. No security concerns — this change is strictly a memory limit reduction. Sandbox integrity is unaffected.

Findings summary

#	Priority	File	Finding
1		builtins/tail/builtin_tail_pentest_test.go:447	Stale comment says "32 MiB" after MaxBytesBuffer was reduced to 5 MiB
2		builtins/tail/tail_test.go	No test covers MaxRingBytes (5 MiB ring-byte cap) being exceeded
3		builtins/tail/tail_test.go	No test covers MaxBytesBuffer overflow error path (byte-mode buffer limit)

Coverage summary

Code path	Scenario test	Go test	Status
MaxRingBytes constant reduction	—	—	No direct test
MaxBytesBuffer constant reduction	—	—	No direct test
max() refactor (suppressPos)	tests/scenarios/cmd/tail/headers/	tail_test.go	Covered via header behavior tests
Ring line-count overflow error	—	tail_test.go:TestTailRingOverflowError	Covered
Byte-mode overflow error	—	—	Missing

Positive observations

• Constants, doc comments, and inline comments are all updated consistently — no stale references remain in tail.go itself.
• The max() refactor is a clean simplification with identical semantics.
• Memory reduction is in the right direction for the project's safety goals.

[thieman]

MaxBytesBuffer comment could mention the overflow scenario

The constant comment describes the buffer's role correctly, but it doesn't mention that when count > MaxBytesBuffer and input exceeds MaxBytesBuffer, readLastBytes returns an error. Consider adding a note matching the style of MaxRingBytes above:

Suggested change

	const MaxBytesBuffer = 5 << 20 // 5 MiB
	// MaxBytesBuffer is the maximum size of the circular byte buffer used in
	// last-N-bytes mode. If the input exceeds this size and the requested count
	// also exceeds it, tail returns an error rather than silently truncating.
	const MaxBytesBuffer = 5 << 20 // 5 MiB

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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