Subject: [PATCH 1/2] run-parallel: rename set_nonblocking to set_nonblocking_or_die

From: Stefan Beller <[email-masked]>
Date: Wed, 4 Nov 2015 14:43:48 -0800

Discussion turned out a warning is not enough, but we need to die in case
of blocking output as we can lockup the child processes.

Junio wrote:
> Imagine that we are running two things A and B at the same time.  We
> ask poll(2) and it says both A and B have some data ready to be
> read, and we try to read from A.  strbuf_read_once() would try to
> read up to 8K, relying on the fact that you earlier set the IO to be
> nonblock.  It will get stuck reading from A without allowing output
> from B to drain.  B's write may get stuck because we are not reading
> from it, and would cause B to stop making progress.

> What if the other sides of the connection from A and B are talking
> with each other, and B's non-progress caused the processing for A on
> the other side of the connection to block, causing it not to produce
> more output to allow us to make progress reading from A (so that
> eventually we can give B a chance to drain its output)?  Imagine A
> and B are pushes to the same remote, B may be pushing a change to a
> submodule while A may be pushing a matching change to its
> superproject, and the server may be trying to make sure that the
> submodule update completes and updates the ref before making the
> superproject's tree that binds that updated submodule's commit
> availble, for example?  Can we make any progress from that point?

Signed-off-by: Stefan Beller <[email-masked]>
---
 run-command.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/run-command.c b/run-command.c
index 0a3c24e..86fbe50 100644
--- a/run-command.c
+++ b/run-command.c
@@ -1006,15 +1006,13 @@ static void pp_cleanup(struct parallel_processes *pp)
 	sigchain_pop_common();
 }
 
-static void set_nonblocking(int fd)
+static void set_nonblocking_or_die(int fd)
 {
 	int flags = fcntl(fd, F_GETFL);
 	if (flags < 0)
-		warning("Could not get file status flags, "
-			"output will be degraded");
+		die("Could not get file status flags");
 	else if (fcntl(fd, F_SETFL, flags | O_NONBLOCK))
-		warning("Could not set file status flags, "
-			"output will be degraded");
+		die("Could not set file status flags");
 }
 
 /* returns
@@ -1052,7 +1050,7 @@ static int pp_start_one(struct parallel_processes *pp)
 		return code ? -1 : 1;
 	}
 
-	set_nonblocking(pp->children[i].process.err);
+	set_nonblocking_or_die(pp->children[i].process.err);
 
 	pp->nr_processes++;
 	pp->children[i].in_use = 1;
-- 
2.6.1.247.ge8f2a41.dirty