cmd/goyacc: don't panic on grammar files ending without a newline by shaggyinsomniac · Pull Request #671 · golang/tools

shaggyinsomniac · 2026-08-28T02:53:25Z

getrune keeps EOF sticky in the single-slot peekrune buffer (it returns EOF without clearing the slot). When gettok's IDENTIFIER/IDENTCOLON look-ahead reaches end-of-file and ungets the EOF rune, ungetrune hits its single-slot "2nd unget" panic — a 10-byte grammar with no trailing newline crashes:

$ printf '%%\nS: A; B' > v2.y && goyacc -o /dev/null v2.y
panic: ungetc - 2nd unget

goroutine 1 [running]:
main.ungetrune(...)
	cmd/goyacc/yacc.go:3160
main.gettok()
	cmd/goyacc/yacc.go:990

Make ungetting EOF a no-op: an exhausted reader returns EOF again on the next read regardless, so behavior for well-formed files is unchanged. With the fix the example reports the proper illegal rule: missing semicolon or | error and exits 1, and a valid grammar compiles to identical output.

getrune keeps EOF sticky in peekrune (it returns EOF without clearing the slot), so when gettok's IDENTIFIER/IDENTCOLON look-ahead reaches end-of-file and ungets the EOF rune, ungetrune hit its single-slot "2nd unget" panic. A 10-byte grammar such as %% S: A; B (with no trailing newline) crashed with panic: ungetc - 2nd unget Make ungetting EOF a no-op: an exhausted reader returns EOF again on the next read regardless, so behavior for well-formed files is unchanged. The example above now reports the proper syntax error ("illegal rule: missing semicolon or |"), and a valid grammar compiles to identical output.

gopherbot · 2026-08-28T03:23:21Z

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gopherbot · 2026-08-28T04:00:25Z

Message from Gopher Robot:

Patch Set 1:

(1 comment)