Only chmod files in ActionMetadataHandler if necessary

This ensures that the ctime of the files doesn't change if no permission
change is necessary, thereby improving the hit rate of the digest cache.

src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java

@@ -722,7 +722,8 @@ private static FileArtifactValue fileArtifactValueFromStat(
   }
 
   private void setPathPermissionsIfFile(Path path) throws IOException {
-    if (path.isFile(Symlinks.NOFOLLOW)) {
+    FileStatus stat = path.stat(Symlinks.NOFOLLOW);
+    if (stat.isFile() && stat.getPermissions() != outputPermissions.getPermissionsMode()) {
       setPathPermissions(path);
     }
   }

src/main/java/com/google/devtools/build/lib/unix/UnixFileSystem.java

@@ -120,7 +120,8 @@ public long getNodeId() {
       return status.getInodeNumber();
     }
 
-    int getPermissions() {
+    @Override
+    public int getPermissions() {
       return status.getPermissions();
     }
 

src/main/java/com/google/devtools/build/lib/vfs/FileStatus.java

@@ -85,4 +85,12 @@ public interface FileStatus {
    * ought to cause the node ID of b to change, but appending / modifying b should not.
    */
   long getNodeId() throws IOException;
+
+  /**
+   * Returns the file's permissions in POSIX format (e.g. 0755) if possible without performing
+   * additional IO, otherwise (or if unsupported by the file system) returns -1.
+   */
+  default int getPermissions() {
+    return -1;
+  }
 }

src/main/java/com/google/devtools/build/lib/windows/WindowsFileSystem.java

@@ -202,6 +202,12 @@ public long getNodeId() {
             // TODO(bazel-team): Consider making use of attributes.fileKey().
             return -1;
           }
+
+          @Override
+          public int getPermissions() {
+            // Files on Windows are implicitly readable and executable.
+            return 0555 | (attributes.isReadOnly() ? 0 : 0200);
+          }
         };
 
     return status;