ci: single-source the llvm tag so key and URL can't drift

The git-clang-format cache key and fetch URL each hardcoded llvmorg-19.1.7.
A future one-sided version bump would leave the cache serving the old driver
under a stale key. Pull the tag into an LLVM_TAG job env, mirroring how the
lint job already single-sources SHFMT_VERSION.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

.github/workflows/ci.yml

@@ -320,20 +320,37 @@ jobs:
   lint:
     name: lint (shellcheck, shfmt)
     runs-on: ubuntu-24.04
+    env:
+      SHFMT_VERSION: v3.8.0
     steps:
       - uses: actions/checkout@v6
 
+      # shfmt is a pinned release binary, so it never changes: cache it keyed on
+      # the version. Same rationale as the git-clang-format driver below -- avoid
+      # re-downloading an unchanging file from github.com on every run.
+      - name: Cache shfmt binary
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/shfmt/shfmt
+          key: shfmt-${{ env.SHFMT_VERSION }}-${{ runner.arch }}
+
       - name: Install linters
-        env:
-          SHFMT_VERSION: v3.8.0
         run: |
           set -euo pipefail
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends shellcheck
-          # shfmt is not packaged in apt; fetch a pinned release binary.
-          curl -fsSL -o /tmp/shfmt \
-            "https://github.com/mvdan/sh/releases/download/${SHFMT_VERSION}/shfmt_${SHFMT_VERSION}_linux_$(dpkg --print-architecture)"
-          sudo install -m 0755 /tmp/shfmt /usr/local/bin/shfmt
+          # shfmt is not packaged in apt; fetch a pinned release binary (cold
+          # cache only), retrying through transient errors.
+          shfmt="$HOME/.cache/shfmt/shfmt"
+          if [ ! -s "$shfmt" ]; then
+            echo "shfmt cache MISS: fetching ${SHFMT_VERSION} from github.com"
+            mkdir -p "$(dirname "$shfmt")"
+            curl --retry 5 --retry-all-errors -fsSL -o "$shfmt" \
+              "https://github.com/mvdan/sh/releases/download/${SHFMT_VERSION}/shfmt_${SHFMT_VERSION}_linux_$(dpkg --print-architecture)"
+          else
+            echo "shfmt cache HIT: using cached ${SHFMT_VERSION}"
+          fi
+          sudo install -m 0755 "$shfmt" /usr/local/bin/shfmt
 
       # Lint the scripts we maintain; the legacy scripts are a separate cleanup.
       - name: shellcheck
@@ -349,11 +366,24 @@ jobs:
     name: format (clang-format-19, changed lines)
     if: github.event_name == 'pull_request'
     runs-on: ubuntu-24.04
+    env:
+      # Single-source the tag so the cache key and the fetch URL can never drift.
+      LLVM_TAG: llvmorg-19.1.7
     steps:
       - uses: actions/checkout@v6
         with:
           fetch-depth: 0
 
+      # The git-clang-format driver is pinned to an immutable release tag, so the
+      # fetched file never changes: cache it keyed on the tag. raw.githubusercontent.com
+      # 429-rate-limits the shared runner egress IPs, and re-downloading an unchanging
+      # file every run was the only thing that could (and did) hit that limit.
+      - name: Cache git-clang-format driver
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/git-clang-format/git-clang-format
+          key: git-clang-format-${{ env.LLVM_TAG }}
+
       - name: Install clang-format 19 (pinned, from apt.llvm.org)
         run: |
           set -euo pipefail
@@ -364,11 +394,17 @@ jobs:
             | sudo tee /etc/apt/sources.list.d/llvm-19.list >/dev/null
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends clang-format-19
-          # git-clang-format driver, pinned to an immutable release tag (not a
-          # moving branch) since we curl and then execute it.
-          sudo curl -fsSL -o /usr/local/bin/git-clang-format \
-            https://raw.githubusercontent.com/llvm/llvm-project/llvmorg-19.1.7/clang/tools/clang-format/git-clang-format
-          sudo chmod 0755 /usr/local/bin/git-clang-format
+          # Cold cache only: fetch the driver, retrying through transient 429s.
+          driver="$HOME/.cache/git-clang-format/git-clang-format"
+          if [ ! -s "$driver" ]; then
+            echo "git-clang-format cache MISS: fetching ${LLVM_TAG} from raw.githubusercontent.com"
+            mkdir -p "$(dirname "$driver")"
+            curl --retry 5 --retry-all-errors -fsSL -o "$driver" \
+              "https://raw.githubusercontent.com/llvm/llvm-project/${LLVM_TAG}/clang/tools/clang-format/git-clang-format"
+          else
+            echo "git-clang-format cache HIT: using cached ${LLVM_TAG}"
+          fi
+          sudo install -m 0755 "$driver" /usr/local/bin/git-clang-format
           clang-format-19 --version
 
       - name: Check formatting of changed lines