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GH-39149: [C++][Compute] Fix replace_substring hang on empty pattern - #55442
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…ttern Return Invalid for an empty pattern in replace_substring, and treat any negative max_replacements as unlimited in replace_substring_regex. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Rationale for this change
replace_substringwith an empty pattern never returns. With an empty pattern, the search matches at the current position, the cursor advances by the pattern length (0), and with the default unlimitedmax_replacementsthe loop appends the replacement forever. Memory grows until the process is killed (about 100 MB/s on a one-element array). This still happens on main.replace_substring_regexhas a related hang: with a pattern that can match empty text and a negativemax_replacementsother than -1 (for example -3), the counter never reaches 0 and the position doesn't advance.pandas worked around the first case on its side (pandas-dev/pandas#64941, pandas-dev/pandas#65147).
What changes are included in this PR?
replace_substring: returnInvalidwhen the pattern is empty. This follows the suggestion in GH-39149: [C++] replace_substring_regex hangs indefinitely for negative max_replacements #39264 to start with an error, since other systems disagree on the expected result: Python, Java and R'sgsubinsert the replacement at every position, Postgres leaves the string unchanged, and stringr raises.replace_substring_regex: treat any negativemax_replacementsas unlimited, using the existing global-replace path that -1 already takes.Are these changes tested?
Yes. New cases in
scalar_string_test.cc. On a Linux Release build of main (7299fdc):arrow-compute-scalar-type-test: 361/361 pass before and after the change*ReplaceSubstring*: 12/12 pass with the changeAre there any user-facing changes?
Yes.
replace_substringwith an empty pattern now raisesArrowInvalid. Before, it hung with unlimited replacements, and with a positivemax_replacementsit returned the replacement repeated at the start (for example"-ab"). Negativemax_replacementsother than -1 inreplace_substring_regexnow behaves like -1 instead of hanging.This PR contains a "Critical Fix". The empty-pattern case loops forever and allocates memory without bound, which can bring down the process.
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