The %YAML directive takes exactly one parameter, the version, and at most
one may appear per document. The scanner checked only that a valid version
was present: it ignored any further tokens, so '%YAML 1.2 foo' was
accepted, and it did not track that a %YAML directive had already been
seen, so a repeated directive was accepted too. It now rejects an extra
parameter (allowing only a trailing comment) and a second %YAML directive,
resetting the per-document flag at each document boundary. A well-formed
directive, with or without a trailing comment, is unaffected.
Measured against the official YAML test-suite, parse-level conformance
rises from 251/333 (75.4%) to 253/333 (76.0%) with no regression.
Mihaela M
4c77b17f9f