fix: avoid quadratic re-scan of comments after a value#1689
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OurReader::readComment() decides whether a comment should be attached to the previous value (commentAfterOnSameLine) by scanning the input from the end of that value up to the comment with containsNewLine(). lastValueEnd_ only advances when a new value is read, so a long run of comments after a value (e.g. during error recovery, or a value followed by many comments) made every comment re-scan the same growing prefix, giving O(n^2) parse time. A jsoncpp_fuzzer testcase took ~18s for a 400KB input. A comment can only ever be on the same line as the last value if no newline separates them, and the gap to inspect only grows as further comments are consumed, so once the gap has been examined for the first comment it never needs to be examined again. Mark lastValueHasAComment_ after the first comment following a value so subsequent comments skip the scan. Parsing the testcase drops from ~18s to ~56ms with identical output. Add a regression test that parses a value followed by a large number of trailing comments and requires it to complete well under a generous time bound.
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OurReader::readComment()decides whether a comment attaches to the previous value (commentAfterOnSameLine) by scanning from the end of that value to the comment viacontainsNewLine(lastValueEnd_, commentBegin).lastValueEnd_only advances when a new value is read, so a long run of comments after a value (error recovery, or a value followed by many comments) re-scans the same growing prefix for every comment -- O(n^2).A comment can only be on the same line as the last value when no newline separates them, and that gap only grows as more comments are consumed, so it never needs re-examining after the first comment. Setting
lastValueHasAComment_after the first comment makes the rest skip the scan. Output is unchanged.Found via a
jsoncpp_fuzzertimeout (Chromium issue 521541633): a 400KB input scanned 2.24 GB across 8384containsNewLinecalls and took ~18s. After the fix: 122 KB / 1 call, ~56ms.Adds
CharReaderTest/parseCommentsAfterValueNotQuadratic(a value followed by many trailing comments, bounded parse time): fails multi-second without the fix, passes in ms with it. Existing tests unaffected.