[Bug 3020] New: Mac OS X/Darwin incorrectly shows read sockets ready when they aren't

[samba-bugs at samba.org]

https://bugzilla.samba.org/show_bug.cgi?id=3020

           Summary: Mac OS X/Darwin incorrectly shows read sockets ready
                    when they aren't
           Product: rsync
           Version: 2.6.5
          Platform: PPC
        OS/Version: Mac OS X
            Status: NEW
          Severity: critical
          Priority: P3
         Component: core
        AssignedTo: wayned at samba.org
        ReportedBy: toasty at dragondata.com
         QAContact: rsync-qa at samba.org


Occasionally (especially when the parent process has alarm signals running, which get inherited to the 
child rsync process, but even when no signal activity is happening) Mac OS X will return that a read 
socket is ready to be read from, but 0 bytes are returned from a read() call.

Rsync assumes this means the connection is closed, which tears the session down. On OS X this seems 
not to be true. This only seems to occur on very busy systems, it's difficult to reproduce on a 
development machine, but nearly impossible to work around on a busy server. This bug also appears 
only to be present in OS X 10.4, reverting to 10.3 is fine.

This patch has worked fine for me for a few weeks now, which changed things from "unusable on OS X" 
to "works flawlessly". I think it's safe to commit, I don't know of any OS that returns 0 on a read without 
setting errno when something really is wrong.


--- io.c.orig   2005-08-20 18:44:34.000000000 +0000
+++ io.c        2005-08-20 19:02:37.000000000 +0000
@@ -570,7 +570,7 @@
 
                n = read(fd, buf, len);
 
-               if (n <= 0) {
+               if ((n <= 0) && errno) {
                        if (n == 0)
                                whine_about_eof(fd); /* Doesn't return. */
                        if (errno == EINTR || errno == EWOULDBLOCK

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