feat(bzlmod): add a flag to control if the parallel downloading is used (#1854)

I have found this to be useful when debugging auth issues when using a
private
repo and I thought that having it configurable from the user's
MODULE.bazel
is a better user experience.

Ammending #1827.

Author: aignas

CHANGELOG.md

@@ -70,7 +70,9 @@ A brief description of the categories of changes:
   `experimental_index_url_overrides` to `pip.parse` for using the bazel
   downloader. If you see any issues, report in
   [#1357](https://github.com/bazelbuild/rules_python/issues/1357). The URLs for
-  the whl and sdist files will be written to the lock file.
+  the whl and sdist files will be written to the lock file. Controlling whether
+  the downloading of metadata is done in parallel can be done using
+  `parallel_download` attribute.
 
 [0.XX.0]: https://github.com/bazelbuild/rules_python/releases/tag/0.XX.0
 [python_default_visibility]: gazelle/README.md#directive-python_default_visibility

python/private/bzlmod/pip.bzl

@@ -199,6 +199,7 @@ def _create_whl_repos(module_ctx, pip_attr, whl_map, whl_overrides, simpleapi_ca
                 auth_patterns = pip_attr.auth_patterns,
             ),
             cache = simpleapi_cache,
+            parallel_download = pip_attr.parallel_download,
         )
 
     major_minor = _major_minor_version(pip_attr.python_version)
@@ -539,6 +540,23 @@ hubs can be created, and each program can use its respective hub's targets.
 Targets from different hubs should not be used together.
 """,
         ),
+        "parallel_download": attr.bool(
+            doc = """\
+The flag allows to make use of parallel downloading feature in bazel 7.1 and above
+when the bazel downloader is used. This is by default enabled as it improves the
+performance by a lot, but in case the queries to the simple API are very expensive
+or when debugging authentication issues one may want to disable this feature.
+
+NOTE, This will download (potentially duplicate) data for multiple packages if
+there is more than one index available, but in general this should be negligible
+because the simple API calls are very cheap and the user should not notice any
+extra overhead.
+
+If we are in synchronous mode, then we will use the first result that we
+find in case extra indexes are specified.
+""",
+            default = True,
+        ),
         "python_version": attr.string(
             mandatory = True,
             doc = """

python/private/pypi_index.bzl

@@ -23,7 +23,7 @@ load(":auth.bzl", "get_auth")
 load(":envsubst.bzl", "envsubst")
 load(":normalize_name.bzl", "normalize_name")
 
-def simpleapi_download(ctx, *, attr, cache):
+def simpleapi_download(ctx, *, attr, cache, parallel_download = True):
     """Download Simple API HTML.
 
     Args:
@@ -49,6 +49,7 @@ def simpleapi_download(ctx, *, attr, cache):
             undesirable because additions to the PyPI index would not be
             reflected when re-evaluating the extension unless we do
             `bazel clean --expunge`.
+        parallel_download: A boolean to enable usage of bazel 7.1 non-blocking downloads.
 
     Returns:
         dict of pkg name to the parsed HTML contents - a list of structs.
@@ -60,17 +61,8 @@ def simpleapi_download(ctx, *, attr, cache):
 
     download_kwargs = {}
     if bazel_features.external_deps.download_has_block_param:
-        download_kwargs["block"] = False
-
-    # Download in parallel if possible. This will download (potentially
-    # duplicate) data for multiple packages if there is more than one index
-    # available, but that is the price of convenience. However, that price
-    # should be mostly negligible because the simple API calls are very cheap
-    # and the user should not notice any extra overhead.
-    #
-    # If we are in synchronous mode, then we will use the first result that we
-    # find.
-    #
+        download_kwargs["block"] = not parallel_download
+
     # NOTE @aignas 2024-03-31: we are not merging results from multiple indexes
     # to replicate how `pip` would handle this case.
     async_downloads = {}