sysemu/os-win32: Test for and use _lock_file/_unlock_file

The bug referenced in os-win32.h was fixed in mingw-w64 v6.

According to repology, version 5 used by ubuntu 18, which is
not yet out of support, so provide a meson link test for it.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-27-richard.henderson@linaro.org>

include/sysemu/os-win32.h

@@ -109,20 +109,22 @@ static inline char *realpath(const char *path, char *resolved_path)
     return resolved_path;
 }
 
-/* ??? Mingw appears to export _lock_file and _unlock_file as the functions
- * with which to lock a stdio handle.  But something is wrong in the markup,
- * either in the header or the library, such that we get undefined references
- * to "_imp___lock_file" etc when linking.  Since we seem to have no other
- * alternative, and the usage within the logging functions isn't critical,
- * ignore FILE locking.
+/*
+ * Older versions of MinGW do not import _lock_file and _unlock_file properly.
+ * This was fixed for v6.0.0 with commit b48e3ac8969d.
  */
-
 static inline void qemu_flockfile(FILE *f)
 {
+#ifdef HAVE__LOCK_FILE
+    _lock_file(f);
+#endif
 }
 
 static inline void qemu_funlockfile(FILE *f)
 {
+#ifdef HAVE__LOCK_FILE
+    _unlock_file(f);
+#endif
 }
 
 /* We wrap all the sockets functions so that we can

meson.build

@@ -2030,6 +2030,18 @@ foreach k, v: config_host
   endif
 endforeach
 
+# Older versions of MinGW do not import _lock_file and _unlock_file properly.
+# This was fixed for v6.0.0 with commit b48e3ac8969d.
+if targetos == 'windows'
+  config_host_data.set('HAVE__LOCK_FILE', cc.links('''
+    #include <stdio.h>
+    int main(void) {
+      _lock_file(NULL);
+      _unlock_file(NULL);
+      return 0;
+    }''', name: '_lock_file and _unlock_file'))
+endif
+
 ########################
 # Target configuration #
 ########################