[7.1.0] Avoid a superfluous stat() in DigestUtil. (#21400)

Similar in spirit to
bb80319.
DigestUtil wraps DigestUtils, trading in Digests instead of byte arrays.

There are probably other callsites that would benefit from the
optimization, but I'm focused on UploadManifest at the moment.

PiperOrigin-RevId: 607695005
Change-Id: Iaa9b1f07b26f05482cb6f3fe8dfaacf50e0c07f5

src/main/java/com/google/devtools/build/lib/remote/UploadManifest.java

@@ -240,7 +240,7 @@ void addFiles(Collection<Path> files) throws ExecException, IOException, Interru
         continue;
       }
       if (statNoFollow.isFile() && !statNoFollow.isSpecialFile()) {
-        Digest digest = digestUtil.compute(file, statNoFollow.getSize());
+        Digest digest = digestUtil.compute(file, statNoFollow);
         addFile(digest, file);
         continue;
       }
@@ -264,7 +264,7 @@ void addFiles(Collection<Path> files) throws ExecException, IOException, Interru
         if (statFollow.isFile() && !statFollow.isSpecialFile()) {
           if (followSymlinks || target.isAbsolute()) {
             // Symlink to file uploaded as a file.
-            addFile(digestUtil.compute(file), file);
+            addFile(digestUtil.compute(file, statFollow), file);
           } else {
             // Symlink to file uploaded as a symlink.
             if (target.isAbsolute()) {

src/main/java/com/google/devtools/build/lib/remote/util/DigestUtil.java

@@ -26,6 +26,7 @@
 import com.google.devtools.build.lib.remote.common.RemoteCacheClient.ActionKey;
 import com.google.devtools.build.lib.vfs.DigestHashFunction;
 import com.google.devtools.build.lib.vfs.DigestUtils;
+import com.google.devtools.build.lib.vfs.FileStatus;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.XattrProvider;
 import com.google.protobuf.Message;
@@ -67,12 +68,27 @@ public Digest compute(byte[] blob) {
     return buildDigest(hashFn.getHashFunction().hashBytes(blob).toString(), blob.length);
   }
 
-  public Digest compute(Path file) throws IOException {
-    return compute(file, file.getFileSize());
+  /**
+   * Computes a digest for a file.
+   *
+   * <p>Prefer calling {@link #compute(Path, FileStatus)} when a recently obtained {@link
+   * FileStatus} is available.
+   *
+   * @param path the file path
+   */
+  public Digest compute(Path path) throws IOException {
+    return compute(path, path.stat());
   }
 
-  public Digest compute(Path file, long fileSize) throws IOException {
-    return buildDigest(DigestUtils.getDigestWithManualFallback(file, xattrProvider), fileSize);
+  /**
+   * Computes a digest for a file.
+   *
+   * @param path the file path
+   * @param status a recently obtained file status, if available
+   */
+  public Digest compute(Path path, FileStatus status) throws IOException {
+    return buildDigest(
+        DigestUtils.getDigestWithManualFallback(path, xattrProvider, status), status.getSize());
   }
 
   public Digest compute(VirtualActionInput input) throws IOException {