Bug 856

Summary: scp hangs on FIFOs rather than erroring
Product: Portable OpenSSH Reporter: Colin Watson <cjwatson>
Component: scpAssignee: OpenSSH Bugzilla mailing list <openssh-bugs>
Status: CLOSED FIXED    
Severity: normal CC: djm
Priority: P2    
Version: 3.8p1   
Hardware: Other   
OS: Linux   
URL: http://bugs.debian.org/246774
Bug Depends on:    
Bug Blocks: 1289    
Attachments:
Description Flags
Don't block on FIFOs none

Description Colin Watson 2004-05-01 19:31:23 AEST 
scp hangs if you ask it to copy a named pipe whose other end is closed, rather
than printing the "not a regular file" error the way it's supposed to.

strace shows that this is because it does open() then fstat(), but doing open()
on a closed FIFO hangs. Perhaps it should open with O_NONBLOCK? My open(2)
manual page says: "When possible, the file is opened in non-blocking mode.
Neither the open nor any subsequent operations on the file descriptor which is
returned will cause the calling process to wait."

See http://www.opengroup.org/onlinepubs/009695399/functions/open.html for
SuSv3/POSIX's description of O_NONBLOCK, which explicitly mentions its behaviour
on FIFOs, block specials, and character specials.
Comment 1 Darren Tucker 2004-05-02 15:25:24 AEST 
The last bit of the description of O_NONBLOCK (after the description of the
behaviour with FIFOs and block/char specials) is:

"Otherwise, the behavior of O_NONBLOCK is unspecified."

Can we rely on all platforms to do something reasonable with O_NONBLOCK on
normal files?  On the other hand, stat'ing it to find out if it's a FIFO or
device first is racy...
Comment 2 Ricky Ng-Adam 2005-02-18 09:36:01 AEDT 
Just thought I'd drop in to ask for this bug to be solved and add another vote
to solving this bug... 

I was trying to copy my CrossOver Office config (about 470M worth of data) using
scp to another machine configured as follows:

The source: kernel 2.6.10 (compiled manually), full-duplex 100Mbps
The target: kernel 2.6.8 (from Debian), half-duplex 10 Mbps

scp just hang after transferring data for a while.

I eventually isolated the problem to a specific directory:

$ strace scp -v -r ~/.cxoffice/dotwine/fake_windows/Windows/Temp 142.225.125.203:
debug1: Sending command: scp -v -r -t .
"\0", 1)                        = 1
open(".cxoffice/dotwine/fake_windows/Windows/Temp", O_RDONLY|O_LARGEFILE) = 3
fstat64(3, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open(".cxoffice/dotwine/fake_windows/Windows/Temp",
O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY) = 4
fstat64(4, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
fcntl64(4, F_SETFD, FD_CLOEXEC)         = 0
write(2, "Entering directory: D0755 0 Temp"..., 33Entering directory: D0755 0 Temp
) = 33
write(6, "D0755 0 Temp\n", 13)          = 13
read(7, "\0", 1)                        = 1
getdents64(4, /* 50 entries */, 4096)   = 1928
open(".cxoffice/dotwine/fake_windows/Windows/Temp/IExplorer6.log.fifo",
O_RDONLY|O_LARGEFILE   
[CTRL-C]
$ file ~/.cxoffice/dotwine/fake_windows/Windows/Temp/IExplorer6.log.fifo
.cxoffice/dotwine/fake_windows/Windows/Temp/IExplorer6.log.fifo: fifo (named pipe)

It took me a while to figure this out, find that there was an existing Debian
bug #246774 that referred to this one.  I think it is rather important from a
usability POV as the cause of this hang is entirely non-obvious.
Comment 3 Damien Miller 2005-04-21 18:45:37 AEST 
Created attachment 892 [details]
Don't block on FIFOs

This works on OpenBSD, and shouldn't hurt on other platforms
Comment 4 Damien Miller 2007-06-12 18:24:41 AEST 
fix applied - will be in 4.7, thanks!
Comment 5 Damien Miller 2008-04-04 09:54:50 AEDT 
Close resolved bugs after release.