runtime: add an opt-in sharded spawn_blocking queue (#8337)

Re-lands the sharded queue from #7757 (reverted due to #8056), disabled by
default. Opt in via the unstable `Builder::enable_sharded_blocking_queue` or
the `TOKIO_UNSTABLE_SHARDED_BLOCKING_QUEUE` environment variable.

.github/workflows/ci.yml

@@ -315,6 +315,30 @@ jobs:
           # the unstable cfg to RustDoc
           RUSTDOCFLAGS: --cfg tokio_unstable
 
+  # Run the test suite with the sharded `spawn_blocking` queue.
+  test-sharded-blocking-queue:
+    name: test tokio full with sharded blocking queue
+    needs: basics
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@stable
+        with:
+            toolchain: ${{ env.rust_stable }}
+
+      - name: Install cargo-nextest
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+
+      - uses: Swatinem/rust-cache@v2
+      - name: test tokio full with sharded blocking queue
+        run: cargo nextest run --features full
+        working-directory: tokio
+        env:
+          TOKIO_UNSTABLE_SHARDED_BLOCKING_QUEUE: "1"
+
   test-unstable-taskdump:
     name: test tokio full --unstable --taskdump
     needs: basics

.github/workflows/loom.yml

@@ -28,6 +28,13 @@ jobs:
     # base_ref is null when it's not a pull request
     if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-blocking') || (github.base_ref == null))
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        # Run the blocking pool loom tests against both `spawn_blocking`
+        # queue implementations.
+        include:
+          - sharded_blocking_queue: "0"
+          - sharded_blocking_queue: "1"
     steps:
       - uses: actions/checkout@v7
       - name: Install Rust ${{ env.rust_stable }}
@@ -38,6 +45,8 @@ jobs:
       - name: run tests
         run: cargo test --lib --release --features full -- --nocapture loom_blocking
         working-directory: tokio
+        env:
+          TOKIO_UNSTABLE_SHARDED_BLOCKING_QUEUE: ${{ matrix.sharded_blocking_queue }}
 
   loom-sync:
     name: loom tokio::sync