[PATCH] git: fix unused code path for cat-file stderr reset

[Eric Wong]

We haven't used _bidi_pipe idempotently in a while, so
the stderr was never getting reset on reads.
This isn't fully useful when using async eeeae20893a25956
(imap: use git-cat-file asynchronously, 2020-06-10)

So instead of truncating it on reads, we'll truncate
immediately after reading and rely on O_APPEND to keep
new writes at the end.

Fortunately, this stderrr error checking isn't used
outside of solver (which is synchronous).
---
 lib/PublicInbox/Git.pm | 23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index 53887e3d..eb88aa48 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -136,24 +136,20 @@ sub object_format {
 
 sub last_check_err {
        my ($self) = @_;
-       my $fh = $self->{err_c} or return;
-       sysseek($fh, 0, 0) or $self->fail("sysseek failed: $!");
-       defined(sysread($fh, my $buf, -s $fh)) or
-                       $self->fail("sysread failed: $!");
+       my $fh = $self->{err_c} or return '';
+       sysseek($fh, 0, 0) or $self->fail("sysseek: $!");
+       my $size = -s $fh or return '';
+       sysread($fh, my $buf, $size) // $self->fail("sysread: $!");
+       truncate($fh, 0) or $self->fail("truncate: $!");
        $buf;
 }
 
 sub _bidi_pipe {
        my ($self, $batch, $in, $out, $pid, $err) = @_;
-       if ($self->{$pid}) {
-               if (defined $err) { # "err_c"
-                       my $fh = $self->{$err};
-                       sysseek($fh, 0, 0) or $self->fail("sysseek failed: $!");
-                       truncate($fh, 0) or $self->fail("truncate failed: $!");
-               }
+       if (defined $self->{$pid}) {
+               Carp::cluck("BUG: self->{$pid} exists unexpectedly");
                return;
        }
-
        pipe(my ($out_r, $out_w)) or $self->fail("pipe failed: $!");
        my $rdr = { 0 => $out_r, pgid => 0 };
        my $gd = $self->{git_dir};
@@ -169,9 +165,8 @@ sub _bidi_pipe {
                        'cat-file', "--$batch");
        if ($err) {
                my $id = "git.$self->{git_dir}.$batch.err";
-               my $fh = tmpfile($id) or $self->fail("tmpfile($id): $!");
-               $self->{$err} = $fh;
-               $rdr->{2} = $fh;
+               $self->{$err} = $rdr->{2} = tmpfile($id, undef, 1) or
+                                               $self->fail("tmpfile($id): $!");
        }
        # see lib/PublicInbox/ProcessPipe.pm for why we don't use that here
        my ($in_r, $p) = popen_rd(\@cmd, undef, $rdr);