Re: [PATCH] Don't fflush(stdout) when it's not helpful
- From
Theodore Tso <tytso@mit.edu>
- Date
- Jun 29, 2007, 17:40 UTC
- Message-ID
- <20070629174046.GC16268@thunk.org>
- In-Reply-To
- <alpine.LFD.0.98.0706290851480.8675@woody.linux-foundation.org>
On Fri, Jun 29, 2007 at 09:06:22AM -0700, Linus Torvalds wrote:
Show 5 quoted lines
> I think that patch looks fine, but I also think that there is a more > fundamental problem with this approach: > > - all these patches basically break the whole _point_ of Jim's original > reason for wanting this!
Yeah, I pointed that out in my first patch. It had seemed that interactivity over a pipe was considered more important, though when we started talking about things. :-)
It looks like from my reading of the standard that ferror(f) should not change the state of the file handle f. So the following patch I think should work; it checks ferror(f), and if it indicates that there is an error, we try a flush to get the error message. I've tested under Linux and it gives the correct error message in the "git log > /mnt/full-filesystem" case, and I believe it should DTRT on other systems.
Comments?
- Ted
commit 93a96f94028106687412acbb771bb18ee7ec5560 Author: Theodore Ts'o <tytso@mit.edu> Date: Thu Jun 28 14:10:58 2007 -0400
Don't fflush(stdout) when it's not helpful
This patch arose from a discussion started by Jim Meyering's patch
whose intention was to provide better diagnostics for failed writes.
Linus proposed a better way to do things, which also had the added
benefit that adding a fflush() to git-log-* operations and incremental
git-blame operations could improve interactive respose time feel, at
the cost of making things a bit slower when we aren't piping the
output to a downstream program.
This patch skips the fflush() calls when stdout is a regular file, or
if the environment variable GIT_FLUSH is set to "0". This latter can
speed up a command such as:
GIT_FLUSH=0 strace -c -f -e write time git-rev-list HEAD | wc -l
a tiny amount.
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>diff --git a/Documentation/git.txt b/Documentation/git.txt index 20b5b7b..8269148 100644 --- a/Documentation/git.txt +++ b/Documentation/git.txt @@ -396,6 +396,16 @@ other 'GIT_PAGER':: This environment variable overrides `$PAGER`. +'GIT_FLUSH':: + If this environment variable is set to "1", then commands such + as git-blame (in incremental mode), git-rev-list, git-log, + git-whatchanged, etc., will force a flush of the output stream + after each commit-oriented record have been flushed. If this + variable is set to "0", the output of these commands will be done + using completely buffered I/O. If this environment variable is + not set, git will choose buffered or record-oriented flushing + based on whether stdout appears to be redirected to a file or not. + 'GIT_TRACE':: If this variable is set to "1", "2" or "true" (comparison is case insensitive), git will print `trace:` messages on diff --git a/builtin-blame.c b/builtin-blame.c index f7e2c13..da23a6f 100644 --- a/builtin-blame.c +++ b/builtin-blame.c @@ -1459,6 +1459,7 @@ static void found_guilty_entry(struct blame_entry *ent) printf("boundary\n"); } write_filename_info(suspect->path); + maybe_flush_or_die(stdout, "stdout"); } } diff --git a/builtin-rev-list.c b/builtin-rev-list.c index 813aadf..86db8b0 100644 --- a/builtin-rev-list.c +++ b/builtin-rev-list.c @@ -100,7 +100,7 @@ static void show_commit(struct commit *commit) printf("%s%c", buf, hdr_termination); free(buf); } - fflush(stdout); + maybe_flush_or_die(stdout, "stdout"); if (commit->parents) { free_commit_list(commit->parents); commit->parents = NULL; diff --git a/cache.h b/cache.h index ed83d92..0525c4e 100644 --- a/cache.h +++ b/cache.h @@ -532,6 +532,8 @@ extern char git_default_name[MAX_GITNAME]; extern const char *git_commit_encoding; extern const char *git_log_output_encoding; +/* IO helper functions */ +extern void maybe_flush_or_die(FILE *, const char *); extern int copy_fd(int ifd, int ofd); extern int read_in_full(int fd, void *buf, size_t count); extern int write_in_full(int fd, const void *buf, size_t count); diff --git a/log-tree.c b/log-tree.c index 0cf21bc..ced3f33 100644 --- a/log-tree.c +++ b/log-tree.c @@ -408,5 +408,6 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit) shown = 1; } opt->loginfo = NULL; + maybe_flush_or_die(stdout, "stdout"); return shown; } diff --git a/write_or_die.c b/write_or_die.c index 5c4bc85..e125e11 100644 --- a/write_or_die.c +++ b/write_or_die.c @@ -1,5 +1,45 @@ #include "cache.h" +/* + * Some cases use stdio, but want to flush after the write + * to get error handling (and to get better interactive + * behaviour - not buffering excessively). + * + * Of course, if the flush happened within the write itself, + * we've already lost the error code, and cannot report it any + * more. So we just ignore that case instead (and hope we get + * the right error code on the flush). + * + * If the file handle is stdout, and stdout is a file, then skip the + * flush entirely since it's not needed. + */ +void maybe_flush_or_die(FILE *f, const char *desc) +{ + static int skip_stdout_flush = -1; + struct stat st; + char *cp; + + if (f == stdout) { + if (skip_stdout_flush < 0) { + cp = getenv("GIT_FLUSH"); + if (cp) + skip_stdout_flush = (atoi(cp) == 0); + else if ((fstat(fileno(stdout), &st) == 0) && + S_ISREG(st.st_mode)) + skip_stdout_flush = 1; + else + skip_stdout_flush = 0; + } + if (skip_stdout_flush && !ferror(f)) + return; + } + if (fflush(f)) { + if (errno == EPIPE) + exit(0); + die("write failure on %s: %s", desc, strerror(errno)); + } +} + int read_in_full(int fd, void *buf, size_t count) { char *p = buf;