CI: Run multi-threading test job with execution context

.github/workflows/linux.yml

@@ -67,7 +67,7 @@ jobs:
       - name: Test
         run: bin/ci build
 
-  x86_64-gnu-test-preview_mt:
+  x86_64-gnu-test-execution-context-parallel:
     env:
       ARCH: x86_64
       ARCH_CMD: linux64
@@ -88,7 +88,30 @@ jobs:
         run: bin/ci with_build_env 'make crystal'
 
       - name: Test
-        run: bin/ci with_build_env 'CRYSTAL_WORKERS=4 make std_spec threads=1 FLAGS="-D preview_mt"'
+        run: bin/ci with_build_env 'CRYSTAL_WORKERS=4 make std_spec threads=1 FLAGS="-Dexecution_context -Dpreview_mt"'
+
+  x86_64-gnu-test-execution-context-concurrent:
+    env:
+      ARCH: x86_64
+      ARCH_CMD: linux64
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download Crystal source
+        uses: actions/checkout@v5
+        with:
+          persist-credentials: false
+
+      - name: Prepare System
+        run: bin/ci prepare_system
+
+      - name: Prepare Build
+        run: bin/ci prepare_build
+
+      - name: Make Crystal
+        run: bin/ci with_build_env 'make crystal'
+
+      - name: Test
+        run: bin/ci with_build_env 'CRYSTAL_WORKERS=1 make std_spec threads=1 FLAGS="-Dexecution_context -Dpreview_mt"'
 
   check_format:
     env:

src/spec.cr

@@ -130,6 +130,12 @@ module Spec
         Process.on_terminate { abort! }
       {% end %}
 
+      {% if Fiber.has_constant?(:ExecutionContext) && Fiber::ExecutionContext.class.has_method?(:current) %}
+        count = ENV["CRYSTAL_WORKERS"]?.try(&.to_i?) || 1
+        # FIXME: ExecutionContext.current should return ExecutionContext::Parallel
+        Fiber::ExecutionContext.current.as(Fiber::ExecutionContext::Parallel).resize(count)
+      {% end %}
+
       run
     end
   end