Address comments and fix renaming check

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleFileFunction.java

@@ -642,14 +642,24 @@ private GetModuleFileResult getModuleFile(
     throw errorf(Code.MODULE_NOT_FOUND, "module not found in registries: %s", key);
   }
 
+  /**
+   * Applies any patches specified in registry overrides.
+   *
+   * <p>This allows users to modify MODULE.bazel files and thus influence resolution and visibility
+   * for modules via patches without having to replace the entire module via a non-registry
+   * override.
+   *
+   * <p>Note: Only patch files from the main repo are applied, all other patches are ignored. This
+   * is necessary as we can't load other repos during resolution (unless they are subject to a
+   * non-registry override). Patch commands are also not supported as they cannot be selectively
+   * applied to the module file only.
+   */
   private ModuleFile maybePatchModuleFile(
       ModuleFile moduleFile, ModuleOverride override, Environment env)
       throws InterruptedException, ModuleFileFunctionException {
     if (!(override instanceof SingleVersionOverride singleVersionOverride)) {
       return moduleFile;
     }
-    // Apply any patches specified in the override that come from the main repo. Non-main repo
-    // patches as well as patch commands are not supported.
     var patchesInMainRepo =
         singleVersionOverride.getPatches().stream()
             .filter(label -> label.getRepository().isMain())
@@ -716,7 +726,7 @@ private ModuleFile maybePatchModuleFile(
       FileSystemUtils.writeContent(moduleFilePath, moduleFile.getContent());
       for (var patchPath : patchPaths) {
         try {
-          PatchUtil.apply(
+          PatchUtil.applyToSingleFile(
               patchPath.asPath(),
               singleVersionOverride.getPatchStrip(),
               moduleRoot,

src/main/java/com/google/devtools/build/lib/bazel/repository/PatchUtil.java

@@ -421,19 +421,26 @@ private static void checkFilesStatusForPatching(
    */
   public static void apply(Path patchFile, int strip, Path outputDirectory)
       throws IOException, PatchFailedException {
-    apply(patchFile, strip, outputDirectory, /* singleFile= */ null);
+    applyInternal(patchFile, strip, outputDirectory, /* singleFile= */ null);
   }
 
   /**
-   * Apply a patch file under a directory
+   * Apply a patch file under a directory, skipping all parts of the patch file that do not apply to
+   * the given single file.
    *
    * @param patchFile the patch file to apply
    * @param strip the number of leading components to strip from file path in the patch file
    * @param outputDirectory the directory to apply the patch file to
-   * @param singleFile if not null, only apply the parts of the patch file that apply to this file.
-   *     Renaming the file is not supported in this case.
+   * @param singleFile only apply the parts of the patch file that apply to this file. Renaming the
+   *     file is not supported in this case.
    */
-  public static void apply(
+  public static void applyToSingleFile(
+      Path patchFile, int strip, Path outputDirectory, Path singleFile)
+      throws IOException, PatchFailedException {
+    applyInternal(patchFile, strip, outputDirectory, singleFile);
+  }
+
+  private static void applyInternal(
       Path patchFile, int strip, Path outputDirectory, @Nullable Path singleFile)
       throws IOException, PatchFailedException {
     if (!patchFile.exists()) {
@@ -577,12 +584,15 @@ public static void apply(
 
             if (isRenaming) {
               if (singleFile != null) {
-                throw new PatchFailedException(
-                    "Renaming is not supported when applying a patch to the single file %s."
-                        .formatted(singleFile));
+                if (singleFile.equals(newFile) || singleFile.equals(oldFile)) {
+                  throw new PatchFailedException(
+                      "Renaming %s while applying patches to it as a single file is not supported."
+                          .formatted(singleFile));
+                }
+              } else {
+                checkFilesStatusForRenaming(
+                    oldFile, newFile, oldFileStr, newFileStr, patchStartLocation);
               }
-              checkFilesStatusForRenaming(
-                  oldFile, newFile, oldFileStr, newFileStr, patchStartLocation);
             }
 
             if (singleFile == null || (singleFile.equals(newFile) && singleFile.equals(oldFile))) {

src/test/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleFileFunctionTest.java

@@ -1734,18 +1734,23 @@ public void testSingleVersionOverridePatches() throws Exception {
         @@ -1,1 +1,2 @@
          module(name='bbb',version='1.0',bazel_compatibility=[">=7.0.0"])
         +bazel_dep(name='ccc',version='3.0')
+        diff --git a/also/not/MODULE.bazel b/also/not/MODULE.bazel.bak
+        similarity index 55%
+        rename from also/not/MODULE.bazel
+        rename to also/not/MODULE.bazel.bak
+        index 3f855b5..949dd15 100644
         """);
     scratch.file("other/pkg/BUILD");
     Path otherPatch =
         scratch.file(
             "other/pkg/other_patch.diff",
             """
-        --- a/MODULE.bazel
-        +++ b/MODULE.bazel
-        @@ -1,1 +1,2 @@
-         module(name='bbb',version='1.0',bazel_compatibility=[">=7.0.0"])
-        +bazel_dep(name='ccc',version='3.0')
-        """);
+            --- a/MODULE.bazel
+            +++ b/MODULE.bazel
+            @@ -1,1 +1,2 @@
+             module(name='bbb',version='1.0',bazel_compatibility=[">=7.0.0"])
+            +bazel_dep(name='ccc',version='3.0')
+            """);
 
     var moduleFileKey =
         ModuleFileValue.key(
@@ -1829,6 +1834,8 @@ public void testSingleVersionOverridePatches_failsOnRename() throws Exception {
     assertThat(result.getError().getException())
         .hasMessageThat()
         .isEqualTo(
-            "error applying single_version_override patch /workspace/patch.diff to module file: Renaming is not supported when applying a patch to the single file /module/MODULE.bazel.");
+            "error applying single_version_override patch /workspace/patch.diff to module file:"
+                + " Renaming /module/MODULE.bazel while applying patches to it as a single file is"
+                + " not supported.");
   }
 }