Fix: don't panic when both source and content of file provisioner are null (#1376)

Signed-off-by: Zejun Chen <tibazq@gmail.com>

internal/builtin/provisioners/file/resource_provisioner.go

@@ -149,7 +149,7 @@ func getSrc(v cty.Value) (string, bool, error) {
 		return expansion, false, err
 
 	default:
-		panic("source and content cannot both be null")
+		return "", false, errors.New("source and content cannot both be null")
 	}
 }
 

internal/builtin/provisioners/file/resource_provisioner_test.go

@@ -121,3 +121,28 @@ func TestResourceProvisioner_connectionRequired(t *testing.T) {
 		t.Fatalf("expected 'Missing connection' error: got %q", got)
 	}
 }
+
+func TestResourceProvisioner_nullSrcVars(t *testing.T) {
+	conn := cty.ObjectVal(map[string]cty.Value{
+		"type": cty.StringVal(""),
+		"host": cty.StringVal("localhost"),
+	})
+	config := cty.ObjectVal(map[string]cty.Value{
+		"source":      cty.NilVal,
+		"content":     cty.NilVal,
+		"destination": cty.StringVal("/tmp/bar"),
+	})
+	p := New()
+	resp := p.ProvisionResource(provisioners.ProvisionResourceRequest{
+		Connection: conn,
+		Config:     config,
+	})
+	if !resp.Diagnostics.HasErrors() {
+		t.Fatal("expected error")
+	}
+
+	got := resp.Diagnostics.Err().Error()
+	if !strings.Contains(got, "file provisioner error: source and content cannot both be null") {
+		t.Fatalf("file provisioner error: source and content cannot both be null' error: got %q", got)
+	}
+}