Fix _tracemalloc start/stop

Issue #26588: Fix _tracemalloc start/stop: don't play with the reentrant flag.

set_reentrant(1) fails with an assertion error if tracemalloc_init() is called
first in a thread A and tracemalloc_start() is called second in a thread B. The
tracemalloc is imported in a thread A. Importing the module calls
tracemalloc_init(). tracemalloc.start() is called in a thread B.

Modules/_tracemalloc.c

@@ -740,10 +740,6 @@ tracemalloc_clear_traces(void)
     assert(PyGILState_Check());
 #endif
 
-    /* Disable also reentrant calls to tracemalloc_malloc() to not add a new
-       trace while we are clearing traces */
-    assert(get_reentrant());
-
     TABLES_LOCK();
     _Py_hashtable_clear(tracemalloc_traces);
     tracemalloc_traced_memory = 0;
@@ -823,11 +819,6 @@ tracemalloc_init(void)
     tracemalloc_empty_traceback.frames[0].lineno = 0;
     tracemalloc_empty_traceback.hash = traceback_hash(&tracemalloc_empty_traceback);
 
-    /* Disable tracing allocations until hooks are installed. Set
-       also the reentrant flag to detect bugs: fail with an assertion error
-       if set_reentrant(1) is called while tracing is disabled. */
-    set_reentrant(1);
-
     tracemalloc_config.initialized = TRACEMALLOC_INITIALIZED;
     return 0;
 }
@@ -912,7 +903,6 @@ tracemalloc_start(int max_nframe)
 
     /* everything is ready: start tracing Python memory allocations */
     tracemalloc_config.tracing = 1;
-    set_reentrant(0);
 
     return 0;
 }
@@ -926,10 +916,6 @@ tracemalloc_stop(void)
     /* stop tracing Python memory allocations */
     tracemalloc_config.tracing = 0;
 
-    /* set the reentrant flag to detect bugs: fail with an assertion error if
-       set_reentrant(1) is called while tracing is disabled. */
-    set_reentrant(1);
-
     /* unregister the hook on memory allocators */
 #ifdef TRACE_RAW_MALLOC
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &allocators.raw);