[PATCH v2 1/5] merge-tree --stdin: flush stdout to avoid deadlock

From: Phillip Wood via GitGitGadget <gitgitgadget@gmail.com>
Date: Feb 18, 2025, 16:24 UTC
Message-ID: <3b3179785098580f3336bb24bdbaf0aa1366bfcd.1739895879.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.1862.v2.git.1739895879.gitgitgadget@gmail.com>

From: Phillip Wood <phillip.wood@dunelm.org.uk>

If a process tries to read the output from "git merge-tree --stdin" before it closes merge-tree's stdin then it deadlocks. This happens because merge-tree does not flush its output before trying to read another line of input and means that it is not possible to cherry-pick a sequence of commits using "git merge-tree --stdin". Fix this by calling maybe_flush_or_die() before trying to read the next line of input. Flushing the output after each merge does not seem to affect the performance, any difference is lost in the noise even after increasing the number of runs.

$ git rev-list --merges --parents -n100 origin/master |
	sed 's/^[^ ]* //' >/tmp/merges
$ hyperfine -L flush 0,1 --warmup 1 --runs 30 \
	'GIT_FLUSH={flush} ./git merge-tree --stdin </tmp/merges'
Benchmark 1: GIT_FLUSH=0 ./git merge-tree --stdin </tmp/merges
  Time (mean ± σ):     546.6 ms ±  11.7 ms    [User: 503.2 ms, System: 40.9 ms]
  Range (min … max):   535.9 ms … 567.7 ms    30 runs

Benchmark 2: GIT_FLUSH=1 ./git merge-tree --stdin </tmp/merges
  Time (mean ± σ):     546.9 ms ±  12.0 ms    [User: 505.9 ms, System: 38.9 ms]
  Range (min … max):   529.8 ms … 570.0 ms    30 runs

Summary
  'GIT_FLUSH=0 ./git merge-tree --stdin </tmp/merges' ran
    1.00 ± 0.03 times faster than 'GIT_FLUSH=1 ./git merge-tree --stdin </tmp/merges'

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 builtin/merge-tree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index 9a6c8b4e4cf..57f4340faba 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -18,6 +18,7 @@
 #include "tree.h"
 #include "config.h"
 #include "strvec.h"
+#include "write-or-die.h"
 
 static int line_termination = '\n';
 
@@ -623,6 +624,7 @@ int cmd_merge_tree(int argc,
 			} else {
 				die(_("malformed input line: '%s'."), buf.buf);
 			}
+			maybe_flush_or_die(stdout, "stdout");
 
 			if (result < 0)
 				die(_("merging cannot continue; got unclean result of %d"), result);

-- 
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