Merge branch '40-when-migrating-qemu-discs-from-one-tail-to-another-there-s-an-edge-case-where-flexnbd-freezes-and-the-vm-eventually-disconnects-and-becomes-unstartable' into 'develop'

Resolve "When migrating QEmu discs from one tail to another there's an edge case where flexnbd freezes and the VM eventually disconnects and becomes unstartable."

Closes #40

See merge request open-source/flexnbd-c!58

debian/changelog

@@ -1,3 +1,10 @@
+flexnbd (0.4.0) UNRELEASED; urgency=medium
+
+  * Explicitly close the server control socket, and wait for it to close, to
+    prevent deadlocks during the server clean-up process (#40 !58)
+
+ -- Patrick J Cherry <patrick@bytemark.co.uk>  Fri, 07 Dec 2018 16:38:56 +0000
+
 flexnbd (0.4.0) stable; urgency=medium
 
   * Ensure proxy state is completely reset before upstream init is read,

src/server/control.c

@@ -78,6 +78,14 @@ void control_destroy(struct control *control)
     free(control);
 }
 
+void control_wait_for_close(struct control *control)
+{
+    NULLCHECK(control);
+    while (!fd_is_closed(control->control_fd)) {
+       usleep(10000);
+    }
+}
+
 struct control_client *control_client_create(struct flexnbd *flexnbd,
 					     int client_fd,
 					     struct mbox *state_mbox)

src/server/control.h

@@ -47,6 +47,7 @@ struct control_client {
 struct control *control_create(struct flexnbd *,
 			       const char *control_socket_name);
 void control_signal_close(struct control *);
+void control_wait_for_close(struct control *control);
 void control_destroy(struct control *);
 
 void *control_runner(void *);

src/server/serve.c

@@ -812,8 +812,6 @@ void server_control_arrived(struct server *serve)
 }
 
 
-void flexnbd_stop_control(struct flexnbd *flexnbd);
-
 /** Closes sockets, frees memory and waits for all client threads to finish */
 void serve_cleanup(struct server *params,
 		   int fatal __attribute__ ((unused)))
@@ -822,8 +820,20 @@ void serve_cleanup(struct server *params,
     void *status;
 
     info("cleaning up");
+    
+    /* Close the control socket, and wait for it to close before proceeding.
+     * If we do not wait, we risk a race condition with the tail supervisor
+     * sending a status command, and deadlocking the mirroring.  */
+    if (params->flexnbd && params->flexnbd->control) {
+	debug("closing control socket");
+        control_signal_close(params->flexnbd->control);
 
+	debug("waiting for control socket to close");
+	control_wait_for_close(params->flexnbd->control);
+    }
+    
     if (params->server_fd) {
+	debug("closing server_fd");
 	close(params->server_fd);
     }
 
@@ -861,15 +871,6 @@ void serve_cleanup(struct server *params,
 	server_unlock_acl(params);
     }
 
-    /* if( params->flexnbd ) { */
-    /*      if ( params->flexnbd->control ) { */
-    /*              flexnbd_stop_control( params->flexnbd ); */
-    /*      } */
-    /*      flexnbd_destroy( params->flexnbd ); */
-    /* } */
-
-    /* server_destroy( params ); */
-
     debug("Cleanup done");
 }
 

tests/acceptance/test_write_during_migration.rb

@@ -82,15 +82,14 @@ class TestWriteDuringMigration < Test::Unit::TestCase
     UNIXSocket.open(@source_sock) do |sock|
       sock.write(['mirror', '127.0.0.1', @dest_port.to_s, 'exit'].join("\x0A") + "\x0A\x0A")
       sock.flush
-      rsp = sock.readline
+      sock.readline
     end
   end
 
   def wait_for_quit
     Timeout.timeout(10) do
-      start_time = Time.now
-      dst_result = Process.waitpid2(@dst_proc)
-      src_result = Process.waitpid2(@src_proc)
+      Process.waitpid2(@dst_proc)
+      Process.waitpid2(@src_proc)
     end
   end
 
@@ -100,13 +99,28 @@ class TestWriteDuringMigration < Test::Unit::TestCase
     loop do
       begin
         client.write(offsets[rand(offsets.size)] * 4096, @write_data)
-      rescue StandardError => err
+      rescue StandardError
         # We expect a broken write at some point, so ignore it
         break
       end
     end
   end
 
+  def bombard_with_status
+    loop do
+      begin
+        UNIXSocket.open(@source_sock) do |sock|
+          sock.write("status\x0A\x0A")
+          sock.flush
+          sock.readline
+        end
+      rescue StandardError
+        # If the socket disappears, that's OK.
+        break
+      end
+    end
+  end
+
   def assert_both_sides_identical
     # puts `md5sum #{@source_file} #{@dest_file}`
 
@@ -160,5 +174,25 @@ class TestWriteDuringMigration < Test::Unit::TestCase
         (src_writers_1 + src_writers_2).each(&:join)
         assert_both_sides_identical
       end
-    end end
+    end
+  end
+
+
+  def test_status_call_after_cleanup
+    Dir.mktmpdir do |tmpdir|
+      Dir.chdir(tmpdir) do
+        make_files
+
+        launch_servers
+
+        status_poker = Thread.new { bombard_with_status }
+
+        start_mirror
+
+        wait_for_quit
+        status_poker.join
+        assert_both_sides_identical
+      end
+    end
+  end
 end