[GitHub] [qpid-dispatch] ganeshmurthy commented on a diff in pull request #1567: DISPATCH-2324 - Coordinate port assignment between all parallel tests spawned by ctest -j (#123)

[GitBox <gi...@apache.org>]

ganeshmurthy commented on code in PR #1567:
URL: https://github.com/apache/qpid-dispatch/pull/1567#discussion_r885650797


##########
tests/system_test.py:
##########
@@ -765,6 +772,46 @@ def setup(self):
             os.makedirs(self.directory)
             os.chdir(self.directory)
 
+    def _next_port(self) -> int:
+        """Reads and increments value stored in self.port_file, under an exclusive file lock.
+
+        When a lock cannot be acquired immediately, fcntl.lockf blocks.
+
+        Failure possibilities:
+            File locks may not work correctly on network filesystems. We still should be no worse off than we were.
+
+            This method always unlocks the lock file, so it should not ever deadlock other tests running in parallel.
+            Even if that happened, the lock is unlocked by the OS when the file is closed, which happens automatically
+            when the process that opened and locked it ends.
+
+            Invalid content in the self.port_file will break this method. Manual intervention is then required.
+        """
+        try:
+            fcntl.flock(self.port_file, fcntl.LOCK_EX)
+
+            # read old value

Review Comment:
   Minor suggestion - 
   Does `next_port = port + 1` need to be done if the file is empty. 
   ```
               # read old value
               self.port_file.seek(0, os.SEEK_END)
               if self.port_file.tell() != 0:
                   # file is not empty, read the old port and increment it to get next possible available port.
                   self.port_file.seek(0)
                   port = int(self.port_file.read())
                   next_port = port + 1
               else:
                   # file is empty, so just start with a randint between 20000 and 30000
                   port = random.randint(self.port_range[0], self.port_range[1])
   ```



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