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.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/git.txt b/Documentation/git.txt
index 20b5b7bb48..826914837b 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