do not sample stack if the frame provided is NULL,

cancel buffer if compare and swap of current code failed (impressed me, how can that error be in the code base that long)

Author: planrich

src/_vmprof.c

@@ -79,7 +79,7 @@ static int emit_code_object(PyCodeObject *co)
     if (sz > MAX_FUNC_NAME / 2) sz = MAX_FUNC_NAME / 2;
     snprintf(buf + sz, MAX_FUNC_NAME / 2, ":%d:%s", co_firstlineno,
              co_filename);
-    return vmprof_register_virtual_function(buf, CODE_ADDR_TO_UID(co), 500000);
+    return vmprof_register_virtual_function(buf, CODE_ADDR_TO_UID(co), 500);
 }
 
 static int _look_for_code_object(PyObject *o, void *all_codes)

src/vmp_stack.c

@@ -196,6 +196,9 @@ int vmp_walk_and_record_stack(PY_STACK_FRAME_T *frame, void ** result,
     // PyPy saves the information of an address in the same way as line information
     // is saved in CPython. _write_python_stack_entry for details.
     //
+    if (frame == NULL) {
+        return 0;
+    }
 #ifdef VMP_SUPPORTS_NATIVE_PROFILING
     intptr_t func_addr;
     unw_cursor_t cursor;

src/vmprof_main.h

@@ -470,6 +470,8 @@ int vmprof_register_virtual_function(char *code_name, intptr_t code_uid,
     if (!__sync_bool_compare_and_swap(&current_codes, NULL, p)) {
         /* failed, flush it */
         commit_buffer(vmp_profile_fileno(), p);
+    } else {
+        cancel_buffer(p);
     }
     return 0;
 }