Add schema.Provider.TestReset method

Provider.TestReset resets the internal state of the Provider at the
start of a test. This also adds a MetaReset function field to
schema.Provider, which is called by TestReset and can be used to reset
any other tsated stored in the provider metadata.

This is currently used to reset the internal Context returned by
StopContext between tests, and should  be implemented by a provider if
it stores a Context from a previous test.

helper/schema/provider.go

@@ -50,8 +50,15 @@ type Provider struct {
 	// See the ConfigureFunc documentation for more information.
 	ConfigureFunc ConfigureFunc
 
+	// MetaReset is called by TestReset to reset any state stored in the meta
+	// interface.  This is especially important if the StopContext is stored by
+	// the provider.
+	MetaReset func() error
+
 	meta interface{}
 
+	// a mutex is required because TestReset can directly repalce the stopCtx
+	stopMu        sync.Mutex
 	stopCtx       context.Context
 	stopCtxCancel context.CancelFunc
 	stopOnce      sync.Once
@@ -124,20 +131,43 @@ func (p *Provider) Stopped() bool {
 // StopCh returns a channel that is closed once the provider is stopped.
 func (p *Provider) StopContext() context.Context {
 	p.stopOnce.Do(p.stopInit)
+
+	p.stopMu.Lock()
+	defer p.stopMu.Unlock()
+
 	return p.stopCtx
 }
 
 func (p *Provider) stopInit() {
+	p.stopMu.Lock()
+	defer p.stopMu.Unlock()
+
 	p.stopCtx, p.stopCtxCancel = context.WithCancel(context.Background())
 }
 
 // Stop implementation of terraform.ResourceProvider interface.
 func (p *Provider) Stop() error {
 	p.stopOnce.Do(p.stopInit)
+
+	p.stopMu.Lock()
+	defer p.stopMu.Unlock()
+
 	p.stopCtxCancel()
 	return nil
 }
 
+// TestReset resets any state stored in the Provider, and will call TestReset
+// on Meta if it implements the TestProvider interface.
+// This may be used to reset the schema.Provider at the start of a test, and is
+// automatically called by resource.Test.
+func (p *Provider) TestReset() error {
+	p.stopInit()
+	if p.MetaReset != nil {
+		return p.MetaReset()
+	}
+	return nil
+}
+
 // Input implementation of terraform.ResourceProvider interface.
 func (p *Provider) Input(
 	input terraform.UIInput,

helper/schema/provider_test.go

@@ -381,3 +381,29 @@ func TestProviderStop_stopFirst(t *testing.T) {
 		t.Fatal("should be stopped")
 	}
 }
+
+func TestProviderReset(t *testing.T) {
+	var p Provider
+	stopCtx := p.StopContext()
+	p.MetaReset = func() error {
+		stopCtx = p.StopContext()
+		return nil
+	}
+
+	// cancel the current context
+	p.Stop()
+
+	if err := p.TestReset(); err != nil {
+		t.Fatal(err)
+	}
+
+	// the first context should have been replaced
+	if err := stopCtx.Err(); err != nil {
+		t.Fatal(err)
+	}
+
+	// we should not get a canceled context here either
+	if err := p.StopContext().Err(); err != nil {
+		t.Fatal(err)
+	}
+}