[gdb/dap] Join JSON writer thread with DAP thread

The DAP interpreter runs in its own thread, and starts a few threads:
- the JSON reader thread,
- the JSON writer thread, and
- the inferior output reader thread.

As part of the DAP shutdown, both the JSON reader thread and the JSON writer
thread, as well as the DAP main thread run to exit, but these exits are not
ordered in any way.

Wait in the main DAP thread for the exit of the JSON writer thread.

This makes sure that the responses are flushed to the DAP client before DAP
shutdown.

An earlier version of this patch used Queue.task_done() to accomplish the
same, but that didn't guarantee writing the "<thread name>: terminating"
log entry from thread_wrapper before DAP shutdown.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR dap/31380
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31380

gdb/python/lib/gdb/dap/io.py

@@ -79,4 +79,4 @@ def start_json_writer(stream, queue):
             stream.write(body_bytes)
             stream.flush()
 
-    start_thread("JSON writer", _json_writer)
+    return start_thread("JSON writer", _json_writer)

gdb/python/lib/gdb/dap/server.py

@@ -212,7 +212,7 @@ class Server:
         # Before looping, start the thread that writes JSON to the
         # client, and the thread that reads output from the inferior.
         start_thread("output reader", self._read_inferior_output)
-        start_json_writer(self.out_stream, self.write_queue)
+        json_writer = start_json_writer(self.out_stream, self.write_queue)
         start_thread("JSON reader", self._reader_thread)
         while not self.done:
             cmd = self.read_queue.get()
@@ -229,6 +229,7 @@ class Server:
         # JSON-writing thread, so that we can ensure that all
         # responses are flushed to the client before exiting.
         self.write_queue.put(None)
+        json_writer.join()
 
     @in_dap_thread
     def send_event_later(self, event, body=None):

gdb/python/lib/gdb/dap/startup.py

@@ -80,6 +80,7 @@ def start_thread(name, target, args=()):
 
     result = gdb.Thread(name=name, target=thread_wrapper, args=args, daemon=True)
     result.start()
+    return result
 
 
 def start_dap(target):