Avoid pip install --dry-run downloading full wheels

 * Removed "more preparation" (downloading) from the resolver to prevent downloading before dry-run validation
 * Added distribution caching to `InstallRequirement` with `set_dist()` and `get_dist()` methods to preserve metadata-only distributions
 * Set `download_info` during metadata-only fetching to ensure it's available for commands like `pip lock` and `--report` without requiring full downloads

Closes #12603.

Author: pelson

news/12603.feature.rst

@@ -0,0 +1 @@
+When PEP-658 metadata is available, full distribution download no longer occurs when using dry-run mode on install.

src/pip/_internal/commands/download.py

@@ -130,15 +130,15 @@ def run(self, options: Values, args: list[str]) -> int:
 
         requirement_set = resolver.resolve(reqs, check_supported_wheels=True)
 
+        preparer.prepare_linked_requirements_more(requirement_set.requirements.values())
+
         downloaded: list[str] = []
         for req in requirement_set.requirements.values():
             if req.satisfied_by is None:
                 assert req.name is not None
                 preparer.save_linked_requirement(req)
                 downloaded.append(req.name)
 
-        preparer.prepare_linked_requirements_more(requirement_set.requirements.values())
-
         if downloaded:
             write_output("Successfully downloaded %s", " ".join(downloaded))
 

src/pip/_internal/commands/install.py

@@ -413,6 +413,13 @@ def run(self, options: Values, args: list[str]) -> int:
                     )
                 return SUCCESS
 
+            # If there is any more preparation to do for the actual installation, do
+            # so now. This includes actually downloading the files in the case that
+            # we have been using PEP-658 metadata so far.
+            preparer.prepare_linked_requirements_more(
+                requirement_set.requirements.values()
+            )
+
             try:
                 pip_req = requirement_set.get_requirement("pip")
             except KeyError:

src/pip/_internal/operations/prepare.py

@@ -527,6 +527,12 @@ def prepare_linked_requirement(
                 metadata_dist = self._fetch_metadata_only(req)
                 if metadata_dist is not None:
                     req.needs_more_preparation = True
+                    req.set_dist(metadata_dist)
+                    # Ensure download_info is available even in dry-run mode
+                    if req.download_info is None:
+                        req.download_info = direct_url_from_link(
+                            req.link, req.source_dir
+                        )
                     return metadata_dist
 
             # None of the optimizations worked, fully prepare the requirement

src/pip/_internal/req/req_install.py

@@ -168,6 +168,10 @@ def __init__(
         # details).
         self.metadata_directory: str | None = None
 
+        # The cached metadata distribution that this requirement represents.
+        # See get_dist / set_dist.
+        self._distribution: BaseDistribution | None = None
+
         # The static build requirements (from pyproject.toml)
         self.pyproject_requires: list[str] | None = None
 
@@ -604,8 +608,13 @@ def metadata(self) -> Any:
 
         return self._metadata
 
+    def set_dist(self, distribution: BaseDistribution) -> None:
+        self._distribution = distribution
+
     def get_dist(self) -> BaseDistribution:
-        if self.metadata_directory:
+        if self._distribution is not None:
+            return self._distribution
+        elif self.metadata_directory:
             return get_directory_distribution(self.metadata_directory)
         elif self.local_file_path and self.is_wheel:
             assert self.req is not None

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -180,11 +180,6 @@ def resolve(
 
             req_set.add_named_requirement(ireq)
 
-        reqs = req_set.all_requirements
-        self.factory.preparer.prepare_linked_requirements_more(reqs)
-        for req in reqs:
-            req.prepared = True
-            req.needs_more_preparation = False
         return req_set
 
     def get_installation_order(