fix(pypi): do not fail on indexes without root index (bazel-contrib#3799)

With this PR we are changing the strategy of assuming that
all of the packages that we have here will be available
through the index and we stop calling the root index if
only a single index is specified. This will improve the
performance for public-only hubs (like the twine deps)
and should in generally reflect better the pre-2.0 behaviour.

Fixes bazel-contrib#3769
Closes bazel-contrib#3770

Author: aignas

CHANGELOG.md

@@ -96,6 +96,11 @@ END_UNRELEASED_TEMPLATE
 * Fix the forwarding of `target_compatible_with` from `compile_pip_requirements`
   towards the underlying `*.update` target.
   ([#3787](https://github.com/bazel-contrib/rules_python/pull/3787))
+* (pypi) Assume that all of the packages are available on a particular hub if
+  there is only a single PyPI compatible index to be used. This saves us an expensive
+  PyPI download and supports PyPI mirror implementations that do not support the root
+  index functionality. Fixes
+  ([#3769](https://github.com/bazel-contrib/rules_python/pull/3769)).
 
 {#v0-0-0-added}
 ### Added

python/private/pypi/simpleapi_download.bzl

@@ -131,9 +131,15 @@ 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:
+    # Ensure the value is not frozen
+    index_urls = [] + (index_urls or [])
+    if default_index not in index_urls:
+        index_urls.append(default_index)
+
+    index_url_overrides = index_url_overrides or {}
+    if index_url_overrides or len(index_urls) == 1:
         # Let's not call the index at all and just assume that all of the overrides have been
-        # specified.
+        # specified or there is only a single index and there is no need to download anything
         return {
             pkg: _normalize_url("{}/{}/".format(
                 index_url_overrides.get(pkg, default_index),
@@ -145,11 +151,6 @@ def _get_dist_urls(ctx, *, default_index, index_urls, index_url_overrides, sourc
     downloads = {}
     results = {}
 
-    # Ensure the value is not frozen
-    index_urls = [] + (index_urls or [])
-    if default_index not in index_urls:
-        index_urls.append(default_index)
-
     for index_url in index_urls:
         download = read_simpleapi(
             ctx = ctx,

tests/integration/bzlmod_lockfile/MODULE.bazel.lock

@@ -169,12 +169,6 @@ _tests.append(_test_index_overrides)
 def _test_download_url(env):
     downloads = {}
     reads = [
-        # The first read is the index which seeds the downloads later
-        """
-        <a href="/main/simple/bar/">bar</a>
-        <a href="/main/simple/baz/">baz</a>
-        <a href="/main/simple/foo/">foo</a>
-        """,
         "",
         "",
         "",
@@ -208,7 +202,6 @@ def _test_download_url(env):
     )
 
     env.expect.that_dict(downloads).contains_exactly({
-        "https://example.com/main/simple/": "path/for/https___example_com_main_simple.html",
         "https://example.com/main/simple/bar/": "path/for/https___example_com_main_simple_bar.html",
         "https://example.com/main/simple/baz/": "path/for/https___example_com_main_simple_baz.html",
         "https://example.com/main/simple/foo/": "path/for/https___example_com_main_simple_foo.html",
@@ -316,12 +309,6 @@ _tests.append(_test_download_url_parallel_with_overrides)
 def _test_download_envsubst_url(env):
     downloads = {}
     reads = [
-        # The first read is the index which seeds the downloads later
-        """
-        <a href="/main/simple/bar/">bar</a>
-        <a href="/main/simple/baz/">baz</a>
-        <a href="/main/simple/foo/">foo</a>
-        """,
         "",
         "",
         "",
@@ -355,7 +342,6 @@ def _test_download_envsubst_url(env):
     )
 
     env.expect.that_dict(downloads).contains_exactly({
-        "https://example.com/main/simple/": "path/for/~index_url~.html",
         "https://example.com/main/simple/bar/": "path/for/~index_url~_bar.html",
         "https://example.com/main/simple/baz/": "path/for/~index_url~_baz.html",
         "https://example.com/main/simple/foo/": "path/for/~index_url~_foo.html",