Fix bug that sometimes made --spin fail with "fatal: Couldn't find remote ref HEAD"

Tricky stdin buffering problem.

An easier fix would have been:

	hSetBuffering stdin NoBuffering

But that approach is less robust; even with NoBuffering, anything that
uses hLookAhead causes 1 byte of buffering. And, any reads from stdin
before hSetBuffering would still cause the problem. Instead, I used a
bigger hammer that will always work. It involves a bit more CPU work,
but this is data that is already being fed through ssh; copying it one
more time won't cause a measurable performance impact.

This commit was sponsored by Jack Hill on Patreon.

1 files changed, 30 insertions, 3 deletions

diff --git a/doc/todo/spin_failure_HEAD.mdwn b/doc/todo/spin_failure_HEAD.mdwn
index af525f61..f838e469 100644
--- a/doc/todo/spin_failure_HEAD.mdwn
+++ b/doc/todo/spin_failure_HEAD.mdwn
@@ -51,7 +51,6 @@ Sending privdata (73139 bytes) to kite.kitenet.net ... done
 [2017-06-18 16:27:13 EDT] received marked GITPUSH
 [2017-06-18 16:27:13 EDT] command line:  GitPush 11 12
 16:27:13.953638 pkt-line.c:80           packet:        fetch< 3a3c8a731d169a2768dd243581803dcb7b275049 HEAD\0multi_ack thin-pack side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed symref=HEAD:refs/heads/joeyconfig agent=git/2.11.0
-16:27:13.953638 pkt-line.c:80           packet:        fetch< 3a3c8a731d169a2768dd243581803dcb7b275049 HEAD\0multi_ack thin-pack side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed symref=HEAD:refs/heads/joeyconfig agent=git/2.11.0
 16:27:13.953781 pkt-line.c:80           packet:        fetch< 86b077b7a21efd5484dfaeee3c31fc5f3c151f6c refs/heads/confpairs
 16:27:13.953789 pkt-line.c:80           packet:        fetch< e03e4bf0f1e557f87d1fe7e01a6de7866296fce6 refs/heads/d-i
 16:27:13.953795 pkt-line.c:80           packet:        fetch< 3a3c8a731d169a2768dd243581803dcb7b275049 refs/heads/joeyconfig
@@ -94,7 +93,35 @@ Sending privdata (73139 bytes) to kite.kitenet.net ... done
 > > * Could be in gitPushHelper, perhaps it's failing to write
 > >   some of the first lines somehow?
 > > * Could be something on the remote side is consuming stdin
-> >   that is not supposed to, and eats some of the protocol.a
+> >   that is not supposed to, and eats some of the protocol.
+> > 
 > > 
 > > I added debug dumping to gitPushHelper, and it seems to be
-> > reading the same truncated data.
+> > reading the same truncated data, so it seems the problem is not there.
+> > 
+> > Aha! The problem comes from stdin/stdInput confusion here:
+
+		req NeedGitPush gitPushMarker $ \_ -> do
+			hin <- dup stdInput
+			hout <- dup stdOutput
+			hClose stdin
+			hClose stdout
+
+> > A line read from stdin just before the dup gets the first line of the protocol
+> > as expected. But reading from stdInput starts with a later line. 
+> > Apparently data is being buffered in the stdin Handle, so gitPushHelper,
+> > which reads from the Fd, does not see it.
+> >
+> > Here's a simple test case. Feeding this 2 lines on stdin will
+> > print the first and then fail with "hGetLine: end of file".
+> > The second line is lost in the buffer. This test case behaves
+> > like that reliably, so I'm surprised propellor only fails sometimes.
+
+	main = do
+	        l <- hGetLine stdin
+	        print l
+	        bob <- fdToHandle stdInput
+	        l2 <- hGetLine bob
+	        print l2
+
+> > [[fixed|done]] --[[Joey]]