Add bazel mirror as a backup URL

distdir.bzl

@@ -83,21 +83,22 @@ def _repo_cache_tar_impl(ctx):
     archive_files = []
     readme_content = "This directory contains repository cache artifacts for the following URLs:\n\n"
     for artifact in http_artifacts:
+        urls = artifact["urls"]
         if "integrity" in artifact:
             # ./tempfile could be a hard link if --experimental_repository_cache_hardlinks is used,
             # therefore we must delete it before creating or writing it again.
             ctx.delete("./tempfile")
-            checksum = ctx.download(artifact["url"], "./tempfile", executable = False, integrity = artifact["integrity"])
+            checksum = ctx.download(urls, "./tempfile", executable = False, integrity = artifact["integrity"])
             artifact["sha256"] = checksum.sha256
 
         if "sha256" in artifact:
             sha256 = artifact["sha256"]
             output_file = "content_addressable/sha256/%s/file" % sha256
-            ctx.download(artifact["url"], output_file, sha256, executable = False)
+            ctx.download(urls, output_file, sha256, executable = False)
             archive_files.append(output_file)
-            readme_content += "- %s (SHA256: %s)\n" % (artifact["url"], sha256)
+            readme_content += "- %s (SHA256: %s)\n" % (urls[0], sha256)
         else:
-            fail("Could not find integrity or sha256 hash for artifact %s" % artifact["url"])
+            fail("Could not find integrity or sha256 hash for artifact %s" % urls[0])
 
     ctx.file("WORKSPACE", "")
     ctx.file("README.md", readme_content)

extensions.bzl

@@ -71,7 +71,7 @@ def _test_repo_extension_impl(ctx):
         # Define one http_file for each artifact so that we can fetch them in parallel.
         http_file(
             name = name + str(cnt),
-            url = artifact["url"],
+            urls = artifact["urls"],
             sha256 = artifact["sha256"] if "sha256" in artifact else None,
             integrity = artifact["integrity"] if "integrity" in artifact else None,
         )

src/tools/bzlmod/utils.bzl

@@ -20,6 +20,10 @@ def get_canonical_repo_name(apparent_repo_name):
         apparent_repo_name = "@" + apparent_repo_name
     return Label(apparent_repo_name).workspace_name
 
+def with_bazel_mirror(url):
+    """Adds the bazel mirror URL as a backup."""
+    return [url, "https://mirror.bazel.build/" + url.split("://")[1]]
+
 def extract_url(attributes):
     """Extracts the url from the given attributes."""
     if "urls" in attributes:
@@ -59,13 +63,13 @@ def extract_http_artifacts(ctx, lockfile_path, required_repos):
 
             http_artifacts.append({
                 "integrity": attributes["integrity"][2:],
-                "url": extract_url(attributes),
+                "urls": with_bazel_mirror(extract_url(attributes)),
             })
             if "remote_patches" in attributes:
                 for patch, integrity in attributes["remote_patches"].items():
                     http_artifacts.append({
                         "integrity": integrity[2:],
-                        "url": patch[2:],
+                        "urls": with_bazel_mirror(patch[2:]),
                     })
 
     for _, extension in lockfile["moduleExtensions"].items():
@@ -81,7 +85,7 @@ def extract_http_artifacts(ctx, lockfile_path, required_repos):
 
                 http_artifacts.append({
                     "sha256": attributes["sha256"][2:],
-                    "url": extract_url(attributes),
+                    "urls": with_bazel_mirror(extract_url(attributes)),
                 })
 
     missing_repos = [repo for repo in required_repos if repo not in found_repos]