Add ratchet coverage action

.github/actions/ratchet-coverage/CHANGELOG.md

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+# Changelog
+
+## v1.0.0
+
+- Initial version with caching and baseline ratcheting.

.github/actions/ratchet-coverage/README.md

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+# Ratchet coverage
+
+Generate code coverage using `cargo tarpaulin` and fail the workflow if the
+coverage percentage falls below a stored baseline.
+
+## Inputs
+
+| Name | Description | Required | Default |
+| --- | --- | --- | --- |
+| baseline-file | File used to persist the baseline coverage percentage between runs | no | `.coverage-baseline` |
+| args | Additional arguments passed to `cargo tarpaulin` | no | *(none)* |
+
+## Outputs
+
+| Name | Description |
+| --- | --- |
+| percent | Coverage percentage reported by `cargo tarpaulin` |
+
+## Example
+
+```yaml
+- uses: ./.github/actions/setup-rust@v1
+- uses: ./.github/actions/ratchet-coverage@v1
+  with:
+    baseline-file: .cache/coverage-baseline
+    args: --workspace
+```
+
+`cargo tarpaulin` only runs on Linux hosts, so use this action on
+`ubuntu-latest` runners.
+
+### How it works
+
+The action restores the previous coverage baseline using `actions/cache` and
+installs `cargo-tarpaulin` if necessary. After running the coverage command, it
+compares the new percentage with the stored baseline. The job fails if coverage
+has dropped. On success, the baseline file is updated and saved back to the
+cache using a branch-specific key for future runs.
+
+Release history is available in [CHANGELOG](CHANGELOG.md).

.github/actions/ratchet-coverage/action.yml

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+name: Ratchet coverage
+description: Run cargo tarpaulin and fail if coverage decreases
+inputs:
+  baseline-file:
+    description: Path to store the coverage baseline
+    required: false
+    default: .coverage-baseline
+  args:
+    description: Additional arguments passed to cargo tarpaulin
+    required: false
+    default: ''
+outputs:
+  percent:
+    description: Coverage percentage reported by cargo tarpaulin
+    value: ${{ steps.cov.outputs.percent }}
+runs:
+  using: composite
+  steps:
+    - name: Restore baseline
+      uses: actions/cache@v4
+      with:
+        path: ${{ inputs.baseline-file }}
+        key: ratchet-baseline-${{ runner.os }}-${{ github.ref_name }}
+    - name: Cache cargo artifacts
+      uses: actions/cache@v4
+      with:
+        path: |
+          ~/.cargo/bin/cargo-tarpaulin
+          ~/.cargo/registry
+          ~/.cargo/git
+          target
+        key: ${{ runner.os }}-tarpaulin-${{ hashFiles('**/Cargo.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-tarpaulin-
+    - name: Install cargo-tarpaulin
+      run: cargo install cargo-tarpaulin
+      shell: bash
+    - name: Run coverage
+      id: cov
+      run: |
+        set -euo pipefail
+        output=$(cargo tarpaulin ${{ inputs.args }} --out lcov 2>&1)
+        echo "$output"
+        percent=$(echo "$output" | grep -oP '[0-9]+(?:\.[0-9]+)?(?=%)' | head -n1)
+        if [ -z "$percent" ]; then
+          echo "Unable to extract coverage percentage" >&2
+          exit 1
+        fi
+        echo "percent=$percent" >> "$GITHUB_OUTPUT"
+      shell: bash
+    - name: Ratchet coverage
+      run: |
+        set -euo pipefail
+        file="${{ inputs.baseline-file }}"
+        current="${{ steps.cov.outputs.percent }}"
+        baseline=0
+        if [ -f "$file" ]; then
+          baseline=$(cat "$file")
+        fi
+        echo "Current coverage: $current%"
+        echo "Baseline coverage: $baseline%"
+        if ! [[ "$current" =~ ^[0-9]+(\.[0-9]+)?$ ]] || \
+           ! [[ "$baseline" =~ ^[0-9]+(\.[0-9]+)?$ ]]; then
+          echo "Invalid coverage values" >&2
+          exit 1
+        fi
+        # Ensure we're on Linux where `bc` is guaranteed to be installed
+        if [[ "$RUNNER_OS" != "Linux" ]]; then
+          echo "This action only supports Linux runners for float comparisons." >&2
+          exit 1
+        fi
+
+        if [ "$(echo "$current < $baseline" | bc -l)" = "1" ]; then
+          echo "Coverage decreased" >&2
+          exit 1
+        fi
+        printf '%.2f' "$current" > "$file"
+      shell: bash
+    - name: Save baseline
+      if: success()
+      uses: actions/cache@v4
+      with:
+        path: ${{ inputs.baseline-file }}
+        key: ratchet-baseline-${{ runner.os }}-${{ github.ref_name }}

CODEOWNERS

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+.github/actions/export-postgres-url/ @leynos
+.github/actions/generate-coverage/ @leynos
+.github/actions/setup-rust/ @leynos
+.github/actions/upload-codescene-coverage/ @leynos
+.github/actions/ratchet-coverage/ @leynos

README.md

@@ -9,3 +9,4 @@ GitHub Actions
 | Generate coverage | `.github/actions/generate-coverage` | v1 |
 | Setup Rust | `.github/actions/setup-rust` | v1 |
 | Upload CodeScene Coverage | `.github/actions/upload-codescene-coverage` | v1 |
+| Ratchet coverage | `.github/actions/ratchet-coverage` | v1 |