[2410] Attempt to fix race condition in test

Attempts to fix a process/subprocess race condition by having
the parent process loop waiting for output instead of assuming
that the subprocess has started and has written something by the
time it gets round to reading it.

Also closes a few unclosed file descriptors at the end of a test.

src/bin/dhcp6/tests/dhcp6_test.py

@@ -89,45 +89,66 @@ class TestDhcpv6Daemon(unittest.TestCase):
         fl = fcntl.fcntl(fd, fcntl.F_GETFL)
         fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
 
-        # There's potential problem if b10-dhcp4 prints out more
-        # than 16k of text
-        try:
-            output = os.read(self.stdout_pipes[0], 16384)
-        except OSError:
-            print("No data available from stdout")
-            output = ""
-
-        # read can return None. Make sure we have a string
-        if (output is None):
-            output = ""
-
-        try:
-            error = os.read(self.stderr_pipes[0], 16384)
-        except OSError:
-            print("No data available on stderr")
-            error = ""
-
-        # read can return None. Make sure we have a string
-        if (error is None):
-            error = ""
-
-        try:
-            if (not pi.process.poll()):
-                # let's be nice at first...
+        # As we don't know how long the subprocess will take to start and
+        # produce output, we'll loop and sleep for 250ms between each
+        # iteration.  To avoid an infinite loop, we'll loop for a maximum
+        # of five seconds: that should be enough.
+        count = 20
+        output = ""
+        error = ""
+        while count > 0:
+            try:
+                new_output = os.read(self.stdout_pipes[0], 16384)
+            except OSError:
+                new_output = ""
+
+            # read can return None. Make sure we have a string
+            if (new_output is not None):
+                output = output + str(new_output)
+
+            try:
+                new_error = os.read(self.stderr_pipes[0], 16384)
+            except OSError:
+                new_error = ""
+
+            # read can return None. Make sure we have a string
+            if (new_error is not None):
+                error = error + str(new_error)
+
+            # If the process has already exited, or if it has output something,
+            # quit the loop now.
+            if pi.process.poll() is not None or len(error) > 0 or len(output) > 0:
+                break
+
+            # Process still running, try again in 250 ms.
+            count = count - 1
+            time.sleep(0.25)
+
+        # Exited loop, kill the process if it is still running
+        if pi.process.poll() is None:
+            try:
                 pi.process.terminate()
-        except OSError:
-            print("Ignoring failed kill attempt. Process is dead already.")
+            except OSError:
+                print("Ignoring failed kill attempt. Process is dead already.")
 
         # call this to get returncode, process should be dead by now
         rc = pi.process.wait()
 
         # Clean up our stdout/stderr munging.
         os.dup2(self.stdout_old, sys.stdout.fileno())
+        os.close(self.stdout_old)
         os.close(self.stdout_pipes[0])
 
         os.dup2(self.stderr_old, sys.stderr.fileno())
+        os.close(self.stderr_old)
         os.close(self.stderr_pipes[0])
 
+        # Free up resources (file descriptors) from the ProcessInfo object
+        # TODO: For some reason, this gives an error if the process has ended,
+        #       although it does cause all descriptors still allocated to the
+        #       object to be freed.
+        pi = None
+
         print ("Process finished, return code=%d, stdout=%d bytes, stderr=%d bytes"
                % (rc, len(output), len(error)) )