Use ccache for emscripten library build (#25392)

Use ccache to cache the output of clang when building the emscripten
libraries.
When building on the main branch, save the ccache directory to the
CircleCI cache affter
building, using the clang version as the key. When building from other
branches (including PRs,
where the branch will appear as 'pull/12345') restore the cache before
building.

Author: dschuff

.circleci/config.yml

@@ -142,6 +142,10 @@ commands:
             cd ~/emsdk
             ./emsdk install tot
             ./emsdk activate tot
+            # Write the version of clang into a file for use in the ccache key
+            ./upstream/bin/clang --version > clang_version.txt
+            echo "clang version:"
+            cat ~/emsdk/clang_version.txt
             # Remove the emsdk version of emscripten to save space in the
             # persistent workspace and to avoid any confusion with the version
             # we are trying to test.
@@ -171,7 +175,7 @@ commands:
           command: |
             ./emcc --clear-cache
       - pip-install
-      - run: apt-get install -q -y ninja-build
+      - run: apt-get install -q -y ninja-build ccache
       - run:
           name: embuilder build ALL
           command: |
@@ -196,6 +200,12 @@ commands:
           command: |
             ./embuilder build MINIMAL --pic --lto
             ./test/runner test_hello_world
+      - run:
+          name: "Ccache stats"
+          command: |
+            ccache --print-stats
+            ccache --zero-stats
+
   persist:
     description: "Persist the emsdk, libraries, and engines"
     steps:
@@ -209,6 +219,19 @@ commands:
             - vms/
             - wasi-sdk/
             - .jsvu/
+      - when:
+          # Only the main branch will store its cache results, and only the non-main branches
+          # will use cached results. This prevents untrusted PRs from polluting caches used
+          # by other branches.
+          condition:
+            equal: [ "main", << pipeline.git.branch >> ]
+          steps:
+            - save_cache:
+                name: "Save Ccache cache"
+                paths:
+                  - ~/.ccache
+                key: clang-{{ checksum "~/emsdk/clang_version.txt" }}
+
   prepare-for-tests:
     description: "Setup emscripten tests"
     steps:
@@ -543,6 +566,7 @@ jobs:
     environment:
       EMCC_CORES: 16
       EMCC_USE_NINJA: 1
+      EM_COMPILER_WRAPPER: "ccache"
     steps:
       - checkout
       - run:
@@ -570,6 +594,14 @@ jobs:
             tar xvf wasi-sysroot-11.0.tar.gz -C ~/wasi-sdk/
       - install-v8
       - install-emsdk
+      - when:
+          condition:
+            not:
+              equal: [ "main", << pipeline.git.branch >> ]
+          steps:
+            - restore_cache:
+                name: "Restore Ccache cache"
+                key: clang-{{ checksum "~/emsdk/clang_version.txt" }}
       - build-libs
       - run:
           name: Clean build directory