profiling(Gecko): Properly obtain main thread identifier

Since running a profiler via CLI (python -m profiling.sampling run)
spawns a new subprocess where the actual user-specified code will run,
a call to threading.main_thread() in the collector's process will not
return the profiled process's main thread.

To combat this, we rely on the fact that thread objects are inserted
in such a way that the first object in the list represents the oldest
ThreadState object [1], which corresponds to a ThreadState associated
with the main thread.

[1] - https://github.com/python/cpython/blob/1b118353bb0a9d816de6ef673f3b11775de5bec5/Include/internal/pycore_interp_structs.h#L831

Signed-off-by: Sofia Donato Ferreira <flowlnlnln@gmail.com>

Author: flowln

Lib/profiling/sampling/gecko_collector.py

@@ -170,14 +170,19 @@ def collect(self, stack_frames, timestamps_us=None):
         self.last_sample_time = times[-1]
 
         # Process threads
+        main_tid = None
         for interpreter_info in stack_frames:
             for thread_info in interpreter_info.threads:
                 frames = filter_internal_frames(thread_info.frame_info)
                 tid = thread_info.thread_id
 
                 # Initialize thread if needed
                 if tid not in self.threads:
-                    self.threads[tid] = self._create_thread(tid)
+                    # Since 'threads' is in order from oldest to newest,
+                    # we know the first thread must be the main thread.
+                    if len(self.threads) == 0:
+                        main_tid = tid
+                    self.threads[tid] = self._create_thread(tid, main_tid)
 
                 thread_data = self.threads[tid]
 
@@ -288,14 +293,10 @@ def collect(self, stack_frames, timestamps_us=None):
 
         self.sample_count += len(times)
 
-    def _create_thread(self, tid):
+    def _create_thread(self, tid, main_tid):
         """Create a new thread structure with processed profile format."""
 
-        # Determine if this is the main thread
-        try:
-            is_main = tid == threading.main_thread().ident
-        except (RuntimeError, AttributeError):
-            is_main = False
+        is_main = tid == main_tid
 
         thread = {
             "name": f"Thread-{tid}",