fix(pypi): skip index lookups when all package overrides are specified (bazel-contrib#3710)

When index_url_overrides is provided for all packages, we no longer need
to
call the index at all. This improves performance and aligns with the
expected
behavior where overrides should be sufficient.

Just to note, currently Pytorch, PyPI, Artifactory have the root index
pages, whilst GAR does not.

Fixes bazel-contrib#3709

(cherry picked from commit 208ca88)

Author: aignas

CHANGELOG.md

@@ -81,7 +81,8 @@ Other changes:
   we will from now on fetch the lists of available packages on each index. The
   used package mappings will be written as facts to the `MODULE.bazel.lock` file
   on supported bazel versions and it should be done at most once. As a result,
-  per-package {obj}`experimental_index_url_overrides` is no longer needed . What
+  per-package {obj}`experimental_index_url_overrides` is no longer needed, but 
+  if specified, it needs to be provided for all packages not on the default index. What
   is more, the flags for `--index_url` and `--extra-index-url` now behave in the
   same way as in `uv` or `pip`, i.e. we default to `--index-url` if the package
   is not found in `--extra-index-url`.  Fixes

python/private/pypi/simpleapi_download.bzl

@@ -43,8 +43,7 @@ def simpleapi_download(
         attr: Contains the parameters for the download. They are grouped into a
           struct for better clarity. It must have attributes:
            * index_url: str, the index, or if `extra_index_urls` are passed, the default index.
-           * index_url_overrides: dict[str, str], the index overrides for
-             separate packages.
+           * index_url_overrides: dict[str, str], the index overrides for separate packages.
            * extra_index_urls: Will be looked at in the order they are defined and the first match
                 wins. This is similar to what uv does, see
                 https://docs.astral.sh/uv/concepts/indexes/#searching-across-multiple-indexes.
@@ -132,16 +131,22 @@ def simpleapi_download(
     return contents
 
 def _get_dist_urls(ctx, *, default_index, index_urls, index_url_overrides, sources, read_simpleapi, attr, block, _fail = fail, **kwargs):
+    if index_url_overrides:
+        # Let's not call the index at all and just assume that all of the overrides have been
+        # specified.
+        return {
+            pkg: _normalize_url("{}/{}/".format(
+                index_url_overrides.get(pkg, default_index),
+                pkg.replace("_", "-"),  # Use the official normalization for URLs
+            ))
+            for pkg in sources
+        }
+
     downloads = {}
     results = {}
 
     # Ensure the value is not frozen
     index_urls = [] + (index_urls or [])
-    for extra in index_url_overrides.values():
-        if extra not in index_urls:
-            index_urls.append(extra)
-
-    index_urls = index_urls or []
     if default_index not in index_urls:
         index_urls.append(default_index)
 
@@ -174,10 +179,6 @@ def _get_dist_urls(ctx, *, default_index, index_urls, index_url_overrides, sourc
                 # and in the outer function process merging of the results.
                 continue
 
-            if index_url_overrides.get(pkg, index_url) != index_url:
-                # we should not use this index for the package
-                continue
-
             found = result.output.get(pkg)
             if not found:
                 continue

tests/pypi/simpleapi_download/simpleapi_download_tests.bzl

@@ -273,6 +273,51 @@ def _test_download_url_parallel(env):
 
 _tests.append(_test_download_url_parallel)
 
+def _test_download_url_parallel_with_overrides(env):
+    downloads = {}
+    reads = [
+        "",
+        "",
+        "",
+    ]
+
+    def download(url, output, **kwargs):
+        _ = kwargs  # buildifier: disable=unused-variable
+        downloads[url[0]] = output
+        return struct(wait = lambda: struct(success = True))
+
+    simpleapi_download(
+        ctx = struct(
+            getenv = {}.get,
+            download = download,
+            report_progress = lambda _: None,
+            # We will first add a download to the list, so this is a poor man's `next(foo)`
+            # implementation. We use 2 because we will enqueue 2 downloads in parallel.
+            read = lambda i: reads[len(downloads) - 2],
+            path = lambda i: "path/for/" + i,
+        ),
+        attr = struct(
+            index_url_overrides = {
+                "bar": "https://example.com/extra/simple/",
+            },
+            index_url = "https://example.com/default/simple/",
+            extra_index_urls = [],
+            sources = {"bar": None, "baz": None, "foo": None},
+            envsubst = [],
+        ),
+        cache = pypi_cache(),
+        parallel_download = True,
+        get_auth = lambda ctx, urls, ctx_attr: struct(),
+    )
+
+    env.expect.that_dict(downloads).contains_exactly({
+        "https://example.com/default/simple/baz/": "path/for/https___example_com_default_simple_baz.html",
+        "https://example.com/default/simple/foo/": "path/for/https___example_com_default_simple_foo.html",
+        "https://example.com/extra/simple/bar/": "path/for/https___example_com_extra_simple_bar.html",
+    })
+
+_tests.append(_test_download_url_parallel_with_overrides)
+
 def _test_download_envsubst_url(env):
     downloads = {}
     reads = [