feat: add cache to resolver in bootstrapper

A long-running bootstrap job may resolve the same requirements
specifier many times, especially for packages like setuptools or
flit-core. This change adds a cache for all requirements based on their
full spec so that if we see the same string more than once we give the
same value without going out to the server to look at available packages
again.

Signed-off-by: Doug Hellmann <dhellmann@redhat.com>

Author: dhellmann

src/fromager/bootstrapper.py

@@ -68,6 +68,9 @@ def __init__(
         # package.
         self._seen_requirements: set[SeenKey] = set()
 
+        # Track requirements we have already resolved so we don't resolve them again.
+        self._resolved_requirements: dict[str, tuple[str, Version]] = {}
+
         self._build_order_filename = self.ctx.work_dir / "build-order.json"
 
     def resolve_version(
@@ -79,6 +82,11 @@ def resolve_version(
 
         Returns the source URL and the version of the requirement.
         """
+        req_str = str(req)
+        if req_str in self._resolved_requirements:
+            logger.debug(f"resolved {req_str} from cache")
+            return self._resolved_requirements[req_str]
+
         pbi = self.ctx.package_build_info(req)
         if pbi.pre_built:
             source_url, resolved_version = self._resolve_prebuilt_with_history(
@@ -90,6 +98,8 @@ def resolve_version(
                 req=req,
                 req_type=req_type,
             )
+
+        self._resolved_requirements[req_str] = (source_url, resolved_version)
         return source_url, resolved_version
 
     def _processing_build_requirement(self, current_req_type: RequirementType) -> bool: