Tied the pipeline's Rust caches to the exact dependencies (#1135)

.github/workflows/build-and-release.yml

@@ -843,10 +843,26 @@ jobs:
 
           rustup toolchain list
 
-      # The key of this cache includes the version of rustc and Cargo.lock, so the cache has to come
-      # after the setup of Rust. A new toolchain or a change to the dependencies leads to a new cache;
-      # until then, the latest cache of the same toolchain serves as the starting point. Before the
-      # cache is saved, the action removes everything which is no dependency, our own crate among it.
+      # Without a key of our own, the cache below would start from the latest cache of the same
+      # toolchain whenever the dependencies change. The action then keeps the old version of each
+      # updated crate next to its new one: a single update of Tauri let the caches grow by a fifth to
+      # a third. Also, GitHub dropped the new cache of this job right away, since build_main still
+      # read its old caches afterwards. Therefore, a hash of Cargo.toml and Cargo.lock goes into the
+      # key, so that only the very same dependencies restore a cache. The version of our own crate
+      # stays out of the hash, since every release changes it; in both files, it directly follows the
+      # name of the crate:
+      - name: Hash the Rust dependencies
+        id: rust_dependencies
+        shell: bash
+        run: |
+          hash=$(sed '/^name = "mindwork-ai-studio"/{n;d;}' runtime/Cargo.toml runtime/Cargo.lock | git hash-object --stdin)
+          echo "hash=${hash}" >> "$GITHUB_OUTPUT"
+          echo "Hash of the Rust dependencies: '${hash}'"
+
+      # The key of this cache includes the version of rustc and the hash from above, so the cache has
+      # to come after both steps. A new toolchain or a change to the dependencies leads to a new
+      # cache, which starts empty. Before the cache is saved, the action removes everything which is
+      # no dependency, our own crate among it.
       # Only pushes to main save a cache: pull requests and releases read it alone, so they cannot
       # crowd out the other caches, since GitHub keeps at most 10 GB of them per repository.
       #
@@ -858,6 +874,7 @@ jobs:
         uses: Swatinem/rust-cache@d1527c8f67ef9af357290475d4af4b0fab8f12e0 # v2.9.3
         with:
           workspaces: runtime -> target
+          key: ${{ steps.rust_dependencies.outputs.hash }}
           cache-bin: false
           save-if: ${{ github.ref == 'refs/heads/main' }}
 
@@ -1295,15 +1312,24 @@ jobs:
 
           rustup toolchain list
 
+      # The same step of the verify job explains why the cache needs this hash:
+      - name: Hash the Rust dependencies
+        id: rust_dependencies
+        shell: bash
+        run: |
+          hash=$(sed '/^name = "mindwork-ai-studio"/{n;d;}' runtime/Cargo.toml runtime/Cargo.lock | git hash-object --stdin)
+          echo "hash=${hash}" >> "$GITHUB_OUTPUT"
+          echo "Hash of the Rust dependencies: '${hash}'"
+
       # The same step of the verify job explains how this cache works, and why the two jobs keep
       # separate caches. The action tells the caches apart by the job name and the runner. Both are
-      # the same for the two macOS builds, and for the two Windows builds as well, so the runtime
-      # identifier gives each platform a cache of its own:
+      # the same for the two macOS builds, and for the two Windows builds as well, so the key holds
+      # the runtime identifier, which gives each platform a cache of its own, next to the hash:
       - name: Cache Rust
         uses: Swatinem/rust-cache@d1527c8f67ef9af357290475d4af4b0fab8f12e0 # v2.9.3
         with:
           workspaces: runtime -> target
-          key: ${{ matrix.dotnet_runtime }}
+          key: ${{ matrix.dotnet_runtime }}-${{ steps.rust_dependencies.outputs.hash }}
           cache-bin: false
           save-if: ${{ github.ref == 'refs/heads/main' }}