pcre2grep: workaround a common optimization in BSD fseek(stdin) #36
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To allow pcre2grep to do an early exit in a resumable way, -m uses fseek on stdin, which is sadly not supported in several platforms. Most of the conflicting issues come from the fact that managing the position while buffering is not trivial, and is therefore an optional feature[1] of POSIX.1-2017 Workaround this by removing the buffer to stdin, if the -m option is being used. There is likely not a significant performance benefit even for the platforms that support it, but it could be conditionally added in that case, later. Fixes: PCRE2Project#10 [1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html
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When BSD's fseek moves the position of a read stream within the
current buffer space, will avoid calling lseek for performance reasons, and therefore will
not update that position in the corresponding file handle, breaking the current implementation for -m (that supports stdin as a shared file)
There is ongoing discussion with NetBSD to change that, but the code has been around this way for decades and is likely to affect other UNIX (ex: AIX has been confirmed affected), so it is better to avoid that.
Fixes: #10