Fix goroutine leak, expose global stop ctx

It is common to want a global stop context passed to the provider configure
function. Unfortunately the grpc ctx is request scoped and cancels after
the function returns. This makes saving/resusing a global ctx impossible.
This hack will attach a global stop synchronized context... onto the ctx
passed into the provider config function. The reason this is done is to
avoid an API signature breaking change, this feature should also be hidden
as we would prefer providers use the request scoped and deadlines contexts
passed into the CRUD functions over a global one.

internal/helper/plugin/grpc_provider.go

@@ -37,9 +37,16 @@ type GRPCProviderServer struct {
 	stopMu   sync.Mutex
 }
 
-func mergeStop(stopCh chan struct{}, cancel context.CancelFunc) {
-	<-stopCh
-	cancel()
+// mergeStop is called in a goroutine and waits for the global stop signal
+// and propagates cancellation to the passed in ctx/cancel func. The ctx is
+// also passed to this function and waited upon so no goroutine leak is caused.
+func mergeStop(ctx context.Context, cancel context.CancelFunc, stopCh chan struct{}) {
+	select {
+	case <-ctx.Done():
+		return
+	case <-stopCh:
+		cancel()
+	}
 }
 
 // StopContext derives a new context from the passed in grpc context.
@@ -50,10 +57,7 @@ func (s *GRPCProviderServer) StopContext(ctx context.Context) context.Context {
 	defer s.stopMu.Unlock()
 
 	stoppable, cancel := context.WithCancel(ctx)
-	// It's important to pass the reference to the current stopCh.
-	// Closing over with an anonymous function and referencing s.stopCh
-	// across a goroutine is unsafe, given a new stopCh is set in Stop()
-	go mergeStop(s.stopCh, cancel)
+	go mergeStop(stoppable, cancel, s.stopCh)
 	return stoppable
 }
 
@@ -506,7 +510,10 @@ func (s *GRPCProviderServer) Configure(ctx context.Context, req *proto.Configure
 	}
 
 	config := terraform.NewResourceConfigShimmed(configVal, schemaBlock)
-	diags := s.provider.Configure(ctx, config)
+	// TODO: hack, pass a context with global cancel hooked up.. on the request
+	// scoped context...
+	ctxHack := context.WithValue(ctx, "StopContext", s.StopContext(context.Background()))
+	diags := s.provider.Configure(ctxHack, config)
 	resp.Diagnostics = convert.AppendProtoDiag(resp.Diagnostics, diags)
 
 	return resp, nil