bitbake: cooker: properly fix bitbake.lock handling

If the PR server or indeed any other child process takes some time to
exit (which it sometimes does when saving its database), it can end up
holding bitbake.lock after the UI exits, which led to errors if you ran
bitbake commands successively - we saw this when running the PR server
oe-selftest tests in OE-Core. The recent attempt to fix this wasn't
quite right and ended up breaking memory resident bitbake. This time we
close the lock file when cooker shuts down (inside the UI process)
instead of unlocking it, and this is done in the cooker code rather than
the actual UI code so it doesn't matter which UI is in use. Additionally
we report that we're waiting for the lock to be released, using lsof or
fuser if available to list the processes with the lock open.

The 'magic' in the locking is due to all spawned subprocesses of bitbake
holding an open file descriptor to the bitbake.lock. It is automatically
unlocked when all those fds close the file (as all the processes terminate).
We close the UI copy of the lock explicitly, then close the server process
copy, any remaining open copy is therefore some proess exiting.

(The reproducer for the problem is to set PRSERV_HOST = "localhost:0"
and add a call to time.sleep(20) after self.server_close() in
lib/prserv/serv.py, then run "bitbake -p; bitbake -p" ).

Cleanup work done by Paul Eggleton <paul.eggleton@linux.intel.com>.

This reverts bitbake commit 69ecd15aece54753154950c55d7af42f85ad8606 and
e97a9f1528d77503b5c93e48e3de9933fbb9f3cd.

(Bitbake rev: a29780bd43f74b7326fe788dbd65177b86806fcf)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cooker.py

@@ -38,6 +38,8 @@ import bb, bb.exceptions, bb.command
 from bb import utils, data, parse, event, cache, providers, taskdata, runqueue, build
 import Queue
 import signal
+import subprocess
+import errno
 import prserv.serv
 import pyinotify
 
@@ -1532,6 +1534,34 @@ class BBCooker:
     def post_serve(self):
         prserv.serv.auto_shutdown(self.data)
         bb.event.fire(CookerExit(), self.expanded_data)
+        lockfile = self.lock.name
+        self.lock.close()
+        self.lock = None
+
+        while not self.lock:
+            with bb.utils.timeout(3):
+                self.lock = bb.utils.lockfile(lockfile, shared=False, retry=False, block=True)
+                if not self.lock:
+                    # Some systems may not have lsof available
+                    procs = None
+                    try:
+                        procs = subprocess.check_output(["lsof", '-w', lockfile], stderr=subprocess.STDOUT)
+                    except OSError as e:
+                        if e.errno != errno.ENOENT:
+                            raise
+                    if procs is None:
+                        # Fall back to fuser if lsof is unavailable
+                        try:
+                            procs = subprocess.check_output(["fuser", '-v', lockfile], stderr=subprocess.STDOUT)
+                        except OSError as e:
+                            if e.errno != errno.ENOENT:
+                                raise
+
+                    msg = "Delaying shutdown due to active processes which appear to be holding bitbake.lock"
+                    if procs:
+                        msg += ":\n%s" % str(procs)
+                    print(msg)
+
 
     def shutdown(self, force = False):
         if force: