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

>
> 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);
> --
> gitgitgadget