Create each venv at most once per run

Author: hugovk

build_docs.py

@@ -641,6 +641,7 @@ class DocBuilder:
     cpython_repo: Repository
     docs_by_version_content: bytes
     switchers_content: bytes
+    built_venvs: set[Path]
     build_root: Path
     www_root: Path
     select_output: Literal["no-html", "only-html", "only-html-en"] | None
@@ -804,22 +805,28 @@ def build(self) -> None:
     def build_venv(self) -> None:
         """Build a venv for the specific Python version.
 
-        The venv is recreated from scratch for every build: pip considers
-        a requirement satisfied when the installed version number matches,
-        even if the requirement is a direct URL now pointing at different
-        code, so a reused venv can silently keep outdated packages.
+        The venv is created at most once per run and reused by later
+        builds of the same version: reusing a venv across runs can
+        silently keep outdated packages, because pip considers a
+        requirement satisfied when the installed version number matches,
+        even if the requirement is a direct URL now pointing at
+        different code.
         """
-        requirements = list(self.build_meta.dependencies)
-        if self.includes_html:
-            # opengraph previews
-            requirements.append("matplotlib>=3")
-
         venv_name = self.build_meta.venv_name
         if self.select_output is not None:
             # Never share a venv with a concurrent differently-selected
             # build, which may recreate it mid-build.
             venv_name += f"-{self.select_output}"
         venv_path = self.build_root / venv_name
+        if venv_path in self.built_venvs:
+            self.venv = venv_path
+            return
+
+        requirements = list(self.build_meta.dependencies)
+        if self.includes_html:
+            # opengraph previews
+            requirements.append("matplotlib>=3")
+
         venv.create(
             venv_path,
             symlinks=os.name != "nt",
@@ -840,6 +847,7 @@ def build_venv(self) -> None:
             cwd=self.checkout / "Doc",
         )
         run((python, "-m", "pip", "freeze", "--all"))
+        self.built_venvs.add(venv_path)
         self.venv = venv_path
 
     def setup_indexsidebar(self) -> None:
@@ -1275,6 +1283,7 @@ def build_docs(args: argparse.Namespace) -> int:
         "https://github.com/python/cpython.git",
         args.build_root / _checkout_name(args.select_output),
     )
+    built_venvs: set[Path] = set()
     while todo:
         build_props = todo.pop()
         logging.root.handlers[0].setFormatter(
@@ -1292,6 +1301,7 @@ def build_docs(args: argparse.Namespace) -> int:
             cpython_repo,
             docs_by_version_content,
             switchers_content,
+            built_venvs,
             **vars(args),
         )
         built_successfully = builder.run(http, force_build=force_build)