Add package caching

To speed up Cygwin installations, allow users to store the downloaded
packages using GitHub's caching, rather than needing the packages to be
downloaded from the Cygwin mirrors on every run.

This speeds up the installation when there's a good cache hit, at the
expense of adding some additional complexity, and using the limited
cache space (although with 10GB assigned to every repository on GitHub,
I can't imagine most folk are getting close to the limit!).

To avoid unnecessary cache churn, the new steps check if the cache has
actually meaningfully changed before storing a new cache.  This uses
b2sum for speed, since we're just checking for unexpected changes in the
cache, and we don't need cryptographic security.

.github/workflows/test.yml

@@ -228,3 +228,34 @@ jobs:
       shell: bash
       env:
         SHELLOPTS: igncr
+
+  caching:
+    runs-on: windows-latest
+
+    name: 'Test use of the cache'
+
+    steps:
+      - run: git config --global core.autocrlf input
+      - uses: actions/checkout@v2
+      - name: Install Cygwin and bash-completion
+        uses: ./
+        with:
+          packages: bash-completion
+          package-cache-key: testing-cache
+          add-to-path: false
+      - name: Delete Cygwin installation and cache
+        run: |
+          Remove-Item -Force -Recurse C:\cygwin
+          Remove-Item -Force -Recurse C:\cygwin-packages
+      - name: Reinstall Cygwin only
+        uses: ./
+        with:
+          package-cache-key: testing-cache
+          add-to-path: false
+      - name: Find bash-completion in the package cache
+        shell: C:\cygwin\bin\bash.exe --noprofile --norc -e -o pipefail -o igncr {0}
+        run: |
+          for file in /cygdrive/c/cygwin-packages/*/*/*/bash-completion/bash-completion-*.tar.*; do
+            [[ -f "$file" ]] && exit 0  # File actually exists
+          done
+          exit 1  # No such downloaded file

README.md

@@ -26,6 +26,7 @@ Parameters
 | site        | http://mirrors.kernel.org/sourceware/cygwin/ | Mirror site to install from
 | check-sig   | true                                         | Whether to check the setup.ini signature
 | add-to-path | true                                         | Whether to add Cygwin's `/bin` directory to the system `PATH`
+| package-cache-key | ''                                     | The key to use for caching downloaded packages.
 
 Line endings
 ------------
@@ -85,6 +86,22 @@ those executables directly in a `run:` in your workflow. Execute them via
 Alternatively, putting e.g. `CYGWIN=winsymlinks:native` into the workflow's
 environment works, since setup now honours that.
 
+Caching
+-------
+
+If you're likely to do regular builds, you might want to store the packages
+locally rather than needing to download them from the Cygwin mirrors on every
+build.  Set `package-cache-key` to some string (e.g. `cygwin-package-cache`),
+and the action will use [GitHub's dependency caching][0] to store downloaded
+package files between runs.
+
+[0]: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows
+
+This has the effect of speeding up the run of the installation itself, at the
+expense of taking slightly longer before and after the installation to check
+and potentially update the cache.  The installer will still check for updated
+packages, and will download new packages if the cached ones are out of date
+
 Mirrors and signatures
 ----------------------
 

action.yml

@@ -26,10 +26,39 @@ inputs:
     description: Should Cygwin's bin directory be added to the system PATH?
     required: false
     default: true
+  package-cache-key:
+    description: Key prefix to use for package caches
+    required: false
+    default: ''
 
 runs:
   using: "composite"
   steps:
+    # Get the path to Git Bash, so we can confidently run that later
+    # regardless of whether Cygwin Bash is in the PATH.
+    - if: inputs.cache != 'disabled'
+      shell: pwsh
+      run: Write-Output "GIT_BASH_PATH=$($(Get-Command git).Source)" >> $Env:GITHUB_ENV
+
+    - uses: actions/cache/restore@v3
+      if: inputs.package-cache-key != ''
+      with:
+        key: ${{ inputs.package-cache-key }}-${{ github.run_id }}-${{ github.run_attempt }}
+        path: C:\cygwin-packages
+        restore-keys:
+          ${{ inputs.package-cache-key }}-${{ github.run_id }}-
+          ${{ inputs.package-cache-key }}-
+
+    - if: inputs.package-cache-key != ''
+      working-directory: C:\
+      shell: ${{ env.GIT_BASH_PATH }} --noprofile --norc -eo pipefail {0}
+      run: |
+        if [[ -d cygwin-packages ]]; then
+          find cygwin-packages -type f ! -name setup.ini -print0 |
+            sort -z |
+            xargs -0 b2sum >cygwin-packages-checksum.old
+        fi
+
     - run: |
         $platform = '${{ inputs.platform }}'
         $platform = $platform -replace '^(x64|amd64)$', 'x86_64'
@@ -82,6 +111,26 @@ runs:
         & C:\setup.exe $args | Out-Default
       shell: powershell
 
+    - if: inputs.package-cache-key != ''
+      id: refresh-cache
+      working-directory: C:\
+      shell: ${{ env.GIT_BASH_PATH }} --noprofile --norc -eo pipefail {0}
+      run: |
+        if [[ -d cygwin-packages ]]; then
+          find cygwin-packages -type f ! -name setup.ini -print0 |
+            sort -z |
+            xargs -0 b2sum >cygwin-packages-checksum.new
+          if ! diff cygwin-packages-checksum.old cygwin-packages-checksum.new; then
+            printf 'update_package_cache=YesPlease\n' >>"$GITHUB_OUTPUT"
+          fi
+        fi
+
+    - if: steps.refresh-cache.outputs.update_package_cache != ''
+      uses: actions/cache/save@v3
+      with:
+        key: ${{ inputs.package-cache-key }}-${{ github.run_id }}-${{ github.run_attempt }}
+        path: C:\cygwin-packages
+
     - if: ${{ inputs.add-to-path  == 'true' }}
       run: echo "${{ inputs.install-dir }}\bin" >> $env:GITHUB_PATH
       shell: powershell