Attempt to cache all the wheels

.circleci/config.yml

@@ -206,6 +206,11 @@ jobs:
       # "save_cache" step below or caching won't really work right.
       PIP_CACHE_DIR: "pip-cache"
 
+      # And tell pip where it can find out cached wheelhouse for fast wheel
+      # installation, even for projects that don't distribute wheels.  This
+      # must also agree with the "save_cache" step below.
+      PIP_FIND_LINKS: "wheelhouse"
+
     steps:
       - "checkout"
 
@@ -213,7 +218,11 @@ jobs:
       # download all our Python dependencies from PyPI.
       - "restore_cache":
           keys:
-            - "pip-packages-v1-{{ checksum \"setup.py\" }}"
+            # The download cache and/or the wheelhouse may contain Python
+            # version-specific binary packages so include the Python version
+            # in this key, as well as the canonical source of our
+            # dependencies.
+            - "pip-packages-v1-{{ parameters.pythonVersion }}-{{ checksum \"setup.py\" }}"
 
       - "run":
           name: "Fix $env:PATH"
@@ -239,16 +248,34 @@ jobs:
             python misc/build_helpers/show-tool-versions.py
 
       - "run":
-          name: "Install Dependencies"
+          # It's faster to install a wheel than a source package.  If we don't
+          # have a cached wheelhouse then build all of the wheels and dump
+          # them into a directory where they can become a cached wheelhouse.
+          # We would have built these wheels during installation anyway so it
+          # doesn't cost us anything extra and saves us effort next time.
+          name: "(Maybe) Build Wheels"
           command: |
-            python -m pip install .[testenv] .[test]
+            if ((Test-Path .\wheelhouse) -and (Test-Path .\wheelhouse\*)) {
+              echo "Found populated wheelhouse, skipping wheel building."
+            } else {
+              python -m pip wheel --wheel-dir $env:PIP_FIND_LINKS .[testenv] .[test]
+            }
 
       - "save_cache":
           paths:
             # Make sure this agrees with PIP_CACHE_DIR in the environment.
             - "pip-cache"
+            - "wheelhouse"
           key: "pip-packages-v1-{{ checksum \"setup.py\" }}"
 
+      - "run":
+          name: "Install Dependencies"
+          environment:
+            # By this point we should no longer need an index.
+            PIP_NO_INDEX: "1"
+          command: |
+            python -m pip install .[testenv] .[test]
+
       - "run":
           name: "Run Unit Tests"
           environment: