Actually fix setting the mtime on a shared disk cache.

The earlier attempt in e9a18ce was not the correct fix, due to the requirements spelled out in the utimensat(2) man page: if the current user has write permission but is not the owner of the file, it may set both atime and mtime to UTIME_NOW, but any other combination is forbidden. Although in theory we could still leave the atime alone in other cases, it's not worth the complexity: atime is often unreliable and Bazel doesn't care about it, anyway.

Unfortunately, it's difficult to write an integration test for this, because we can't arrange for the existence of multiple users in our CI environment.

Fixes #23512.

PiperOrigin-RevId: 729042375
Change-Id: I7ca5e8bc0858a9796f448fca554d0c9465579e19

Author: tjgq

src/main/native/unix_jni.cc

@@ -411,15 +411,17 @@ extern "C" JNIEXPORT void JNICALL
 Java_com_google_devtools_build_lib_unix_NativePosixFiles_utimensat(
     JNIEnv *env, jclass clazz, jstring path, jboolean now, jlong millis) {
   const char *path_chars = GetStringLatin1Chars(env, path);
+  // If `now` is true use the current time, otherwise use `millis`.
+  // On Linux, if the current user has write permission but isn't the owner of
+  // the file, atime and mtime may be simultaneously set to UTIME_NOW, but any
+  // other combination is forbidden. For simplicity, always set both.
   int64_t sec = millis / 1000;
   int32_t nsec = (millis % 1000) * 1000000;
-  struct timespec spec[2] = {
-      // Do not set atime.
-      {0, UTIME_OMIT},
-      // Set mtime to now if `now` is true, otherwise to the specified time.
+  struct timespec times[2] = {
+      {sec, now ? UTIME_NOW : nsec},
       {sec, now ? UTIME_NOW : nsec},
   };
-  if (::utimensat(AT_FDCWD, path_chars, spec, 0) == -1) {
+  if (::utimensat(AT_FDCWD, path_chars, times, 0) == -1) {
     PostException(env, errno, path_chars);
   }
   ReleaseStringLatin1Chars(path_chars);