Check for errors before calling each syscall

We should not fflush a file that has not been written to if it fails.

Author: AZero13

Python/sysmodule.c

@@ -2753,20 +2753,31 @@ PyAPI_FUNC(int) PyUnstable_CopyPerfMapFile(const char* parent_filename) {
     }
     char buf[4096];
     PyThread_acquire_lock(perf_map_state.map_lock, 1);
-    int fflush_result = 0, result = 0;
+    int result = 0;
     while (1) {
         size_t bytes_read = fread(buf, 1, sizeof(buf), from);
+        if (bytes_read == 0) {
+            if (ferror(from)) {
+                result = -1;
+            }
+            break;
+        }
+
         size_t bytes_written = fwrite(buf, 1, bytes_read, perf_map_state.perf_map);
-        fflush_result = fflush(perf_map_state.perf_map);
-        if (fflush_result != 0 || bytes_read == 0 || bytes_written < bytes_read) {
+        if (bytes_written < bytes_read) {
             result = -1;
-            goto close_and_release;
+            break;
         }
+
+        if (fflush(perf_map_state.perf_map) != 0) {
+            result = -1;
+            break;
+        }
+
         if (bytes_read < sizeof(buf) && feof(from)) {
-            goto close_and_release;
+            break;
         }
     }
-close_and_release:
     fclose(from);
     PyThread_release_lock(perf_map_state.map_lock);
     return result;