Do not merge when origin/dest. match

Do not attempt to merge when upstream and downstream files are the same.

diff3 will not merge when the same edit happens to upstream and downstream at the same time. When the rest of the file is the same across the repos, it is safe to skip the merge.

BUG=392575734
FIXED=392575734
PiperOrigin-RevId: 723533220
Change-Id: I4bea76cdc7fb05ecd2d361c1079d3901b25e1631

java/com/google/copybara/util/MergeImportTool.java

@@ -132,6 +132,16 @@ public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
                   baselineFile, destinationFile);
               return FileVisitResult.CONTINUE;
             }
+            try {
+              // same changes to both upstream and downstream, no need to merge
+              if (compareFileContents(destinationFile, file)) {
+                return FileVisitResult.CONTINUE;
+              }
+            } catch (IOException e) {
+              logger.atWarning().withCause(e).log(
+                  "Cannot read one of (%s, %s) - will not attempt to merge", file, destinationFile);
+              return FileVisitResult.CONTINUE;
+            }
             filesToProcess.add(
                 FilePathInformation.create(file, relativeFile, baselineFile, destinationFile));
 

javatests/com/google/copybara/util/MergeImportToolTest.java

@@ -19,6 +19,7 @@
 import static com.google.common.truth.Truth.assertThat;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
@@ -243,6 +244,25 @@ public void testNoDestinationEdits_noMergeImport() throws Exception {
         .merge(any(Path.class), any(Path.class), any(Path.class), any(Path.class));
   }
 
+  @Test
+  public void testOriginEqualsDestination_noMergeImport() throws Exception {
+    commandLineDiffUtil = Mockito.mock(MergeRunner.class);
+    String fileName = "foo.txt";
+    String commonFileContents = "a\nb\nc\n";
+    String additionalFileContents = "foo\n";
+    writeFile(baselineWorkdir, fileName, commonFileContents);
+    writeFile(originWorkdir, fileName, commonFileContents.concat(additionalFileContents));
+    writeFile(destinationWorkdir, fileName, commonFileContents.concat(additionalFileContents));
+    underTest = new MergeImportTool(console, commandLineDiffUtil, 10, null);
+
+    var unused =
+        underTest.mergeImport(
+            originWorkdir, destinationWorkdir, baselineWorkdir, diffToolWorkdir, glob, packagePath);
+
+    verify(commandLineDiffUtil, never())
+        .merge(any(Path.class), any(Path.class), any(Path.class), any(Path.class));
+  }
+
   private Path createDir(Path parent, String name) throws IOException {
     Path path = parent.resolve(name);
     Files.createDirectories(path);