[gdb/dap] Factor out thread_log

In thread_wrapper I used a style where a message is prefixed with the thread
name.

Factor this out into a new function thread_log.

Also treat the GDB main thread special, because it's usual name is MainThread:
...
MainThread: <msg>
...
which is the default name assigned by python, so instead use the more
explicit:
...
GDB main: <msg>
...

Tested on aarch64-linux.

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

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

@@ -64,7 +64,6 @@ def start_thread(name, target, args=()):
     correctly blocked."""
 
     def thread_wrapper(*args):
-        thread_name = threading.current_thread().name
         # Catch any exception, and log it.  If we let it escape here, it'll be
         # printed in gdb_stderr, which is not safe to access from anywhere but
         # gdb's main thread.
@@ -72,11 +71,11 @@ def start_thread(name, target, args=()):
             target(*args)
         except Exception as err:
             err_string = "%s, %s" % (err, type(err))
-            log(thread_name + ": caught exception: " + err_string)
+            thread_log("caught exception: " + err_string)
             log_stack()
         finally:
             # Log when a thread terminates.
-            log(thread_name + ": terminating")
+            thread_log("terminating")
 
     result = gdb.Thread(name=name, target=thread_wrapper, args=args, daemon=True)
     result.start()
@@ -178,6 +177,16 @@ def log(something, level=LogLevel.DEFAULT):
             dap_log.log_file.flush()
 
 
+def thread_log(something, level=LogLevel.DEFAULT):
+    """Log SOMETHING to the log file, if logging is enabled, and prefix
+    the thread name."""
+    if threading.current_thread() is _gdb_thread:
+        thread_name = "GDB main"
+    else:
+        thread_name = threading.current_thread().name
+    log(thread_name + ": " + something, level)
+
+
 def log_stack(level=LogLevel.DEFAULT):
     """Log a stack trace to the log file, if logging is enabled."""
     with dap_log.lock: