Restore last modification times to commit used to build cache

.github/workflows/build.yml

@@ -53,6 +53,35 @@ jobs:
         key: buildCache-${{ runner.os }}-${{ matrix.platform }}-${{ github.sha }}
         restore-keys: buildCache-${{ runner.os }}-${{ matrix.platform }}-
 
+    - name: Set modification times
+      if: ${{ hashFiles('.build/lastBuildSha.txt') != '' }}
+      shell: bash
+      run: |
+          # Determine SHA of last cached build (ignore cache if not marked)
+          lastBuildSha=$(<.build/lastBuildSha.txt) || exit 0
+          echo "Last build SHA: ${lastBuildSha}"
+
+          # Fetch source code for last build
+          # (exit successfully if not found, cache will be ignored)
+          git fetch --depth=1 origin "${lastBuildSha}" || exit 0
+          git switch --detach "${lastBuildSha}"
+
+          # Set last modification times to that of last build's committer time
+          # The committer time should pre-date any cached output modification time
+          commitTime=$(git log -1 --format="%cI")
+          echo "commitTime=${commitTime}"
+          find . -type f -exec touch -d "${commitTime}" '{}' +
+
+          # Re-check out current branch, which updates last modification times of modified files
+          git switch -
+
+    - name: Set build SHA
+      shell: bash
+      run: |
+          # Save copy of current SHA to be cached, and used for reference in future builds
+          mkdir --parents .build/
+          echo "${{ github.sha }}" > .build/lastBuildSha.txt
+
     - name: Build
       # Add additional options to the MSBuild command line here (like platform or verbosity level).
       # See https://docs.microsoft.com/visualstudio/msbuild/msbuild-command-line-reference