patchupdate-ref: fix streaming of status updates

> When executing git-update-ref(1) with the `--stdin` flag, then the user
> can queue updates and, since e48cf33b61 (update-ref: implement
> interactive transaction handling, 2020-04-02), interactively drive the
> transaction's state via a set of transactional verbs. This interactivity
> is somewhat broken though: while the caller can use these verbs to drive
> the transaction's state, the status messages which confirm that a verb
> has been processed is not flushed. The caller may thus be left hanging
> waiting for the acknowledgement.
>
> Fix the bug by flushing stdout after writing the status update. Add a
> test which catches this bug.

>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  builtin/update-ref.c  | 14 ++++++++++----
>  t/t1400-update-ref.sh | 32 ++++++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+), 4 deletions(-)
>
> diff --git a/builtin/update-ref.c b/builtin/update-ref.c
> index 6029a80544..a84e7b47a2 100644
> --- a/builtin/update-ref.c
> +++ b/builtin/update-ref.c
> @@ -302,6 +302,12 @@ static void parse_cmd_verify(struct ref_transaction *transaction,
>  	strbuf_release(&err);
>  }
>  
> +static void report_ok(const char *command)
> +{
> +	fprintf(stdout, "%s: ok\n", command);
> +	fflush(stdout);
> +}
> +
>  static void parse_cmd_option(struct ref_transaction *transaction,
>  			     const char *next, const char *end)
>  {
> @@ -317,7 +323,7 @@ static void parse_cmd_start(struct ref_transaction *transaction,
>  {
>  	if (*next != line_termination)
>  		die("start: extra input: %s", next);
> -	puts("start: ok");
> +	report_ok("start");
>  }
>  
>  static void parse_cmd_prepare(struct ref_transaction *transaction,
> @@ -328,7 +334,7 @@ static void parse_cmd_prepare(struct ref_transaction *transaction,
>  		die("prepare: extra input: %s", next);
>  	if (ref_transaction_prepare(transaction, &error))
>  		die("prepare: %s", error.buf);
> -	puts("prepare: ok");
> +	report_ok("prepare");
>  }
>  
>  static void parse_cmd_abort(struct ref_transaction *transaction,
> @@ -339,7 +345,7 @@ static void parse_cmd_abort(struct ref_transaction *transaction,
>  		die("abort: extra input: %s", next);
>  	if (ref_transaction_abort(transaction, &error))
>  		die("abort: %s", error.buf);
> -	puts("abort: ok");
> +	report_ok("abort");
>  }
>  
>  static void parse_cmd_commit(struct ref_transaction *transaction,
> @@ -350,7 +356,7 @@ static void parse_cmd_commit(struct ref_transaction *transaction,
>  		die("commit: extra input: %s", next);
>  	if (ref_transaction_commit(transaction, &error))
>  		die("commit: %s", error.buf);
> -	puts("commit: ok");
> +	report_ok("commit");
>  	ref_transaction_free(transaction);
>  }
>  
> diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
> index 4506cd435b..1e754e258f 100755
> --- a/t/t1400-update-ref.sh
> +++ b/t/t1400-update-ref.sh
> @@ -1598,6 +1598,38 @@ test_expect_success 'transaction cannot restart ongoing transaction' '
>  	test_must_fail git show-ref --verify refs/heads/restart
>  '
>  
> +test_expect_success PIPE 'transaction flushes status updates' '
> +	mkfifo in out &&
> +	(git update-ref --stdin <in >out &) &&
> +
> +	exec 9>in &&
> +	test_when_finished "exec 9>&-" &&
> +
> +	echo "start" >&9 &&
> +	echo "start: ok" >expected &&
> +	read line <out &&
> +	echo "$line" >actual &&
> +	test_cmp expected actual &&
> +
> +	echo "create refs/heads/flush $A" >&9 &&
> +
> +	echo prepare >&9 &&
> +	echo "prepare: ok" >expected &&
> +	read line <out &&
> +	echo "$line" >actual &&
> +	test_cmp expected actual &&
> +
> +	# This must now fail given that we have locked the ref.
> +	test_must_fail git update-ref refs/heads/flush $B 2>stderr &&
> +	grep "fatal: update_ref failed for ref ${SQ}refs/heads/flush${SQ}: cannot lock ref" stderr &&
> +
> +	echo commit >&9 &&
> +	echo "commit: ok" >expected &&
> +	read line <out &&
> +	echo "$line" >actual &&
> +	test_cmp expected actual
> +'
> +
>  test_expect_success 'directory not created deleting packed ref' '
>  	git branch d1/d2/r1 HEAD &&
>  	git pack-refs --all &&

> diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
> index 4506cd435b..1e754e258f 100755
> --- a/t/t1400-update-ref.sh
> +++ b/t/t1400-update-ref.sh
> @@ -1598,6 +1598,38 @@ test_expect_success 'transaction cannot restart ongoing transaction' '
>  	test_must_fail git show-ref --verify refs/heads/restart
>  '
>  
> +test_expect_success PIPE 'transaction flushes status updates' '
> +	mkfifo in out &&
> +	(git update-ref --stdin <in >out &) &&
> +
> +	exec 9>in &&
> +	test_when_finished "exec 9>&-" &&
> +
> +	echo "start" >&9 &&
> +	echo "start: ok" >expected &&
> +	read line <out &&
> +	echo "$line" >actual &&
> +	test_cmp expected actual &&
> +
> +	echo "create refs/heads/flush $A" >&9 &&
> +
> +	echo prepare >&9 &&
> +	echo "prepare: ok" >expected &&
> +	read line <out &&
> +	echo "$line" >actual &&
> +	test_cmp expected actual &&

> I think this test may be racy. I saw a strange failure from it in CI:
> 
>   https://github.com/peff/git/runs/3605506649?check_suite_focus=true#step:5:6734
> 
> I can't reproduce the problem locally with "--stress", but the failure
> there is on macOS (and likewise, a nearby run failed with a timeout just
> for macOS, which could be caused by a racy deadlock).

> On Wed, Sep 15, 2021 at 01:13:25PM -0400, Jeff King wrote:
>
>> I think this test may be racy. I saw a strange failure from it in CI:
>> 
>>   https://github.com/peff/git/runs/3605506649?check_suite_focus=true#step:5:6734
>> 
>> I can't reproduce the problem locally with "--stress", but the failure
>> there is on macOS (and likewise, a nearby run failed with a timeout just
>> for macOS, which could be caused by a racy deadlock).
>
> Ah, I just wasn't trying hard enough. Using --run=1,190 lets it run a
> lot more tightly, and I got a failure pretty quickly. Here's the fix (on
> top of ps/update-ref-batch-flush).
>
> -- >8 --
> Subject: [PATCH] t1400: avoid SIGPIPE race condition on fifo
>
> t1400.190 sometimes fails or even hangs because of the way it uses
> fifos. Our goal is to interactively read and write lines from
> update-ref, so we have two fifos, in and out. We open a descriptor
> connected to "in" and redirect output to that, so that update-ref does
> not see EOF as it would if we opened and closed it for each "echo" call.

> On Wed, Sep 15, 2021 at 01:13:25PM -0400, Jeff King wrote:
> 
> > I think this test may be racy. I saw a strange failure from it in CI:
> > 
> >   https://github.com/peff/git/runs/3605506649?check_suite_focus=true#step:5:6734
> > 
> > I can't reproduce the problem locally with "--stress", but the failure
> > there is on macOS (and likewise, a nearby run failed with a timeout just
> > for macOS, which could be caused by a racy deadlock).
> 
> Ah, I just wasn't trying hard enough. Using --run=1,190 lets it run a
> lot more tightly, and I got a failure pretty quickly. Here's the fix (on
> top of ps/update-ref-batch-flush).
> 
> -- >8 --
> Subject: [PATCH] t1400: avoid SIGPIPE race condition on fifo
> 
> t1400.190 sometimes fails or even hangs because of the way it uses
> fifos. Our goal is to interactively read and write lines from
> update-ref, so we have two fifos, in and out. We open a descriptor
> connected to "in" and redirect output to that, so that update-ref does
> not see EOF as it would if we opened and closed it for each "echo" call.
> 
> But we don't do the same for the output. This leads to a race where our
> "read response <out" has not yet opened the fifo, but update-ref tries
> to write to it and gets SIGPIPE. This can result in the test failing, or
> worse, hanging as we wait forever for somebody to write to the pipe.
> 
> This is the same proble we fixed in 4783e7ea83 (t0008: avoid SIGPIPE

> race condition on fifo, 2013-07-12), and we can fix it the same way, by
> opening a second long-running descriptor.
> 
> Before this patch, running:
> 
>   ./t1400-update-ref.sh --run=1,190 --stress
> 
> failed or hung within a few dozen iterations. After it, I ran it for
> several hundred without problems.
> 
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  t/t1400-update-ref.sh | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
> index 1e754e258f..0d4f73acaa 100755
> --- a/t/t1400-update-ref.sh
> +++ b/t/t1400-update-ref.sh
> @@ -1603,19 +1603,21 @@ test_expect_success PIPE 'transaction flushes status updates' '
>  	(git update-ref --stdin <in >out &) &&
>  
>  	exec 9>in &&
> +	exec 8<out &&
>  	test_when_finished "exec 9>&-" &&
> +	test_when_finished "exec 8<&-" &&
>  
>  	echo "start" >&9 &&
>  	echo "start: ok" >expected &&
> -	read line <out &&
> +	read line <&8 &&
>  	echo "$line" >actual &&
>  	test_cmp expected actual &&
>  
>  	echo "create refs/heads/flush $A" >&9 &&
>  
>  	echo prepare >&9 &&
>  	echo "prepare: ok" >expected &&
> -	read line <out &&
> +	read line <&8 &&
>  	echo "$line" >actual &&
>  	test_cmp expected actual &&
>  
> @@ -1625,7 +1627,7 @@ test_expect_success PIPE 'transaction flushes status updates' '
>  
>  	echo commit >&9 &&
>  	echo "commit: ok" >expected &&
> -	read line <out &&
> +	read line <&8 &&
>  	echo "$line" >actual &&
>  	test_cmp expected actual
>  '
> -- 
> 2.33.0.917.g33ebf6a5f6
>