geo-rep: Fix sync hang with tarssh

Problem:
Geo-rep sync hangs when tarssh is used as sync
engine at heavy workload.

Analysis and Root cause:
It's found out that the tar process was hung.
When debugged further, it's found out that stderr
buffer of tar process on master was full i.e., 64k.
When the buffer was copied to a file from /proc/pid/fd/2,
the hang is resolved.

This can happen when files picked by tar process
to sync doesn't exist on master anymore. If this count
increases around 1k, the stderr buffer is filled up.

Fix:
The tar process is executed using Popen with stderr as PIPE.
The final execution is something like below.

tar | ssh <args> root@slave tar --overwrite -xf - -C <path>

It was waiting on ssh process first using communicate() and then tar.
Note that communicate() reads stdout and stderr. So when stderr of tar
process is filled up, there is no one to read until untar via ssh is
completed. This can't happen and leads to deadlock.
Hence we should be waiting on both process parallely, so that stderr is
read on both processes.

Change-Id: I609c7cc5c07e210c504771115b4d551a2e891adf
fixes: bz#1707728
Signed-off-by: Kotresh HR <khiremat@redhat.com>

1 files changed, 18 insertions, 4 deletions

diff --git a/geo-replication/syncdaemon/resource.py b/geo-replication/syncdaemon/resource.py
index 522279bb7e1..b16db607967 100644
--- a/geo-replication/syncdaemon/resource.py
+++ b/geo-replication/syncdaemon/resource.py
@@ -1540,15 +1540,29 @@ class SSH(object):
 
         p0.stdin.close()
         p0.stdout.close()  # Allow p0 to receive a SIGPIPE if p1 exits.
-        # wait for tar to terminate, collecting any errors, further
-        # waiting for transfer to complete
-        _, stderr1 = p1.communicate()
 
         # stdin and stdout of p0 is already closed, Reset to None and
         # wait for child process to complete
         p0.stdin = None
         p0.stdout = None
-        p0.communicate()
+
+        def wait_for_tar(p0):
+            _, stderr = p0.communicate()
+            if log_err:
+                for errline in stderr.strip().split("\n")[:-1]:
+                    if "No such file or directory" not in errline:
+                        logging.error(lf("SYNC Error",
+                                         sync_engine="Tarssh",
+                                         error=errline))
+
+        t = syncdutils.Thread(target=wait_for_tar, args=(p0, ))
+        # wait for tar to terminate, collecting any errors, further
+        # waiting for transfer to complete
+        t.start()
+
+        # wait for ssh process
+        _, stderr1 = p1.communicate()
+        t.join()
 
         if log_err:
             for errline in stderr1.strip().split("\n")[:-1]: