stdlib: fix data race compiling a shared ModuleImpl

stdlib/moduleinit_test.go

@@ -0,0 +1,58 @@
+// Copyright 2026 The go-python Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package stdlib_test
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/go-python/gpython/py"
+	_ "github.com/go-python/gpython/stdlib"
+)
+
+// A registered ModuleImpl is shared by every Context, so importing a CodeSrc
+// module from several contexts at once must not race (run with -race).
+func TestModuleInitConcurrentCodeSrc(t *testing.T) {
+	py.RegisterModule(&py.ModuleImpl{
+		Info: py.ModuleInfo{
+			Name:     "codesrc_test",
+			FileDesc: "<codesrc_test>",
+		},
+		CodeSrc: "X = 42\ndef get():\n    return X\n",
+	})
+
+	const numContexts = 8
+	start := make(chan struct{})
+	var wg sync.WaitGroup
+	for i := 0; i < numContexts; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			ctx := py.NewContext(py.DefaultContextOpts())
+			defer ctx.Close()
+			<-start
+			err := py.Import(ctx, "codesrc_test")
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			module, err := ctx.GetModule("codesrc_test")
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			res, err := module.Call("get", nil, nil)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			if res != py.Int(42) {
+				t.Errorf("get() = %v, want 42", res)
+			}
+		}()
+	}
+	close(start)
+	wg.Wait()
+}

stdlib/stdlib.go

@@ -76,6 +76,37 @@ func (ctx *context) ModuleInit(impl *py.ModuleImpl) (*py.Module, error) {
 		return nil, err
 	}
 
+	code, err := moduleCode(impl)
+	if err != nil {
+		return nil, err
+	}
+
+	module, err := ctx.Store().NewModule(ctx, impl)
+	if err != nil {
+		return nil, err
+	}
+
+	if code != nil {
+		_, err = ctx.RunCode(code, module.Globals, module.Globals, nil)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return module, nil
+}
+
+// moduleCodeMu guards the compiling of ModuleImpl.CodeSrc or CodeBuf into
+// ModuleImpl.Code, since a registered ModuleImpl is shared by every Context.
+var moduleCodeMu sync.Mutex
+
+// moduleCode returns impl.Code, setting it from impl.CodeSrc or impl.CodeBuf
+// the first time it is needed.
+func moduleCode(impl *py.ModuleImpl) (*py.Code, error) {
+	moduleCodeMu.Lock()
+	defer moduleCodeMu.Unlock()
+
+	var err error
 	if impl.Code == nil && len(impl.CodeSrc) > 0 {
 		impl.Code, err = py.Compile(string(impl.CodeSrc), impl.Info.FileDesc, py.ExecMode, 0, true)
 		if err != nil {
@@ -95,19 +126,7 @@ func (ctx *context) ModuleInit(impl *py.ModuleImpl) (*py.Module, error) {
 		}
 	}
 
-	module, err := ctx.Store().NewModule(ctx, impl)
-	if err != nil {
-		return nil, err
-	}
-
-	if impl.Code != nil {
-		_, err = ctx.RunCode(impl.Code, module.Globals, module.Globals, nil)
-		if err != nil {
-			return nil, err
-		}
-	}
-
-	return module, nil
+	return impl.Code, nil
 }
 
 // See interface py.Context defined in py/run.go