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authorLibravatar Jeff King <peff@peff.net>2016-06-30 05:07:54 -0400
committerLibravatar Junio C Hamano <gitster@pobox.com>2016-07-01 10:17:39 -0700
commit48860819e80260abae480407d0bd75dd58e70bb7 (patch)
treeaa6d9940e317423ce6025f17320f7e091b29ebb4 /gitweb/static/git-logo.png
parentGit 2.9 (diff)
downloadtgif-48860819e80260abae480407d0bd75dd58e70bb7.tar.xz
t9300: factor out portable "head -c" replacement
It is sometimes useful to be able to read exactly N bytes from a pipe. Doing this portably turns out to be surprisingly difficult in shell scripts. We want a solution that: - is portable - never reads more than N bytes due to buffering (which would mean those bytes are not available to the next program to read from the same pipe) - handles partial reads by looping until N bytes are read (or we see EOF) - is resilient to stray signals giving us EINTR while trying to read (even though we don't send them, things like SIGWINCH could cause apparently-random failures) Some possible solutions are: - "head -c" is not portable, and implementations may buffer (though GNU head does not) - "read -N" is a bash-ism, and thus not portable - "dd bs=$n count=1" does not handle partial reads. GNU dd has iflags=fullblock, but that is not portable - "dd bs=1 count=$n" fixes the partial read problem (all reads are 1-byte, so there can be no partial response). It does make a lot of write() calls, but for our tests that's unlikely to matter. It's fairly portable. We already use it in our tests, and it's unlikely that implementations would screw up any of our criteria. The most unknown one would be signal handling. - perl can do a sysread() loop pretty easily. On my Linux system, at least, it seems to restart the read() call automatically. If that turns out not to be portable, though, it would be easy for us to handle it. That makes the perl solution the least bad (because we conveniently omitted "length of code" as a criterion). It's also what t9300 is currently using, so we can just pull the implementation from there. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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