fix: make CORS reconciliation recoverable

scripts/deploy.py

@@ -174,21 +174,28 @@ def configure_cors(bucket, origins, endpoint, aws_env):
     """Reconcile read-only browser access without exposing publication credentials."""
     if origins is None:
         return
-    if not origins:
+    config = None
+    if origins:
+        config = {
+            "CORSRules": [{
+                "AllowedOrigins": origins,
+                "AllowedMethods": ["GET", "HEAD"],
+                "AllowedHeaders": ["*"],
+                "ExposeHeaders": ["ETag"],
+                "MaxAgeSeconds": 3600,
+            }],
+        }
+    set_cors_configuration(bucket, config, endpoint, aws_env)
+
+
+def set_cors_configuration(bucket, config, endpoint, aws_env):
+    """Apply an exact bucket CORS configuration, or remove it when absent."""
+    if config is None:
         run([
             "aws", "--endpoint-url", endpoint, "s3api", "delete-bucket-cors",
             "--bucket", bucket,
         ], env=aws_env)
         return
-    config = {
-        "CORSRules": [{
-            "AllowedOrigins": origins,
-            "AllowedMethods": ["GET", "HEAD"],
-            "AllowedHeaders": ["*"],
-            "ExposeHeaders": ["ETag"],
-            "MaxAgeSeconds": 3600,
-        }],
-    }
     with tempfile.NamedTemporaryFile("w", suffix=".json", encoding="utf-8") as handle:
         json.dump(config, handle)
         handle.flush()
@@ -198,11 +205,61 @@ def configure_cors(bucket, origins, endpoint, aws_env):
         ], env=aws_env)
 
 
-def configure_artifact_cors(artifact, credential_env_names=None):
-    configure_cors(
-        artifact["bucket"], artifact["cors_origins"], artifact["s3_endpoint"],
-        publication_aws_env(artifact, credential_env_names),
-    )
+def get_cors_configuration(bucket, endpoint, aws_env):
+    """Read the exact bucket CORS configuration for rollback."""
+    result = _aws_capture([
+        "aws", "--endpoint-url", endpoint, "s3api", "get-bucket-cors",
+        "--bucket", bucket, "--output", "json",
+    ], aws_env)
+    if result.returncode == 0:
+        try:
+            config = json.loads(result.stdout)
+        except json.JSONDecodeError as error:
+            raise RuntimeError(f"get-bucket-cors returned invalid JSON for {bucket}") from error
+        if not isinstance(config, dict) or not isinstance(config.get("CORSRules"), list):
+            raise RuntimeError(f"get-bucket-cors returned an invalid policy for {bucket}")
+        return config
+    error = f"{result.stdout}\n{result.stderr}"
+    if "NoSuchCORSConfiguration" in error:
+        return None
+    raise RuntimeError(f"get-bucket-cors failed for {bucket}: {error.strip()}")
+
+
+def reconcile_artifact_cors(artifacts, credential_env_names=None):
+    """Reconcile all policies, restoring the prior set if any write fails."""
+    snapshots = []
+    for artifact in artifacts:
+        if artifact["cors_origins"] is None:
+            continue
+        aws_env = publication_aws_env(artifact, credential_env_names)
+        snapshots.append((
+            artifact,
+            aws_env,
+            get_cors_configuration(artifact["bucket"], artifact["s3_endpoint"], aws_env),
+        ))
+
+    attempted = []
+    try:
+        for artifact, aws_env, previous in snapshots:
+            attempted.append((artifact, aws_env, previous))
+            configure_cors(
+                artifact["bucket"], artifact["cors_origins"], artifact["s3_endpoint"], aws_env,
+            )
+    except Exception as error:
+        rollback_errors = []
+        for artifact, aws_env, previous in reversed(attempted):
+            try:
+                set_cors_configuration(
+                    artifact["bucket"], previous, artifact["s3_endpoint"], aws_env,
+                )
+            except Exception as rollback_error:
+                rollback_errors.append(f"{artifact['bucket']}: {rollback_error}")
+        if rollback_errors:
+            raise RuntimeError(
+                f"CORS reconciliation failed ({error}); rollback also failed for "
+                f"{'; '.join(rollback_errors)}"
+            ) from error
+        raise
 
 
 def publish_route_immutables(artifact, route, site_dir, credential_env_names=None):
@@ -499,8 +556,7 @@ def deploy_static(site_name, site_dir, action_dir, token, cfg):
     # Reconcile every browser-read policy before publishing mutable content.
     # A CORS failure therefore cannot leave a new channel pointing at a release
     # whose cross-origin assets browsers cannot consume.
-    for artifact in cfg["artifacts"]:
-        configure_artifact_cors(artifact, credential_env_names)
+    reconcile_artifact_cors(cfg["artifacts"], credential_env_names)
     for route in cfg["routes"]:
         s3_sync(
             artifact_by_name[route["artifact"]], route, site_dir, credential_env_names,