[PATCH] Fix race in DAP startup

[Tom Tromey <tromey@adacore.com>]

Internal AdaCore DAP testing on Windows has had occasional failures
that show:

    assert threading.current_thread() is _dap_thread

I think this is a race in DAP startup: the _dap_thread global is only
set on return from start_thread, but it seems possible that the thread
itself could already run and encounter a @in_dap_thread decorator.

This patch fixes the problem by setting the global before running any
of the code in the new thread.  This also lets us remove a FIXME.
---
 gdb/python/lib/gdb/dap/server.py  |  4 +---
 gdb/python/lib/gdb/dap/startup.py | 12 +++++++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/gdb/python/lib/gdb/dap/server.py b/gdb/python/lib/gdb/dap/server.py
index 92b4eee1c5e..ff88282049f 100644
--- a/gdb/python/lib/gdb/dap/server.py
+++ b/gdb/python/lib/gdb/dap/server.py
@@ -100,9 +100,7 @@ class Server:
         log("WROTE: <<<" + json.dumps(obj) + ">>>")
         self.write_queue.put(obj)
 
-    # This must be run in the DAP thread, but we can't use
-    # @in_dap_thread here because the global isn't set until after
-    # this starts running.  FIXME.
+    @in_dap_thread
     def main_loop(self):
         """The main loop of the DAP server."""
         # Before looping, start the thread that writes JSON to the
diff --git a/gdb/python/lib/gdb/dap/startup.py b/gdb/python/lib/gdb/dap/startup.py
index 22246a937ce..aa834cdb14c 100644
--- a/gdb/python/lib/gdb/dap/startup.py
+++ b/gdb/python/lib/gdb/dap/startup.py
@@ -59,14 +59,20 @@ def start_thread(name, target, args=()):
     with blocked_signals():
         result = threading.Thread(target=target, args=args, daemon=True)
         result.start()
-        return result
 
 
 def start_dap(target):
     """Start the DAP thread and invoke TARGET there."""
-    global _dap_thread
     exec_and_log("set breakpoint pending on")
-    _dap_thread = start_thread("DAP", target)
+
+    # Functions in this thread contain assertions that check for this
+    # global, so we must set it before letting these functions run.
+    def really_start_dap():
+        global _dap_thread
+        _dap_thread = threading.current_thread()
+        target()
+
+    start_thread("DAP", really_start_dap)
 
 
 def in_gdb_thread(func):
-- 
2.39.1