Make sql db instance recreate when going private -> public (#3882) (#2400)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/3882.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+sql: fixed an issue where `google_sql_database_instance` would throw an error when removing `private_network`. Removing `private_network` now recreates the resource.
+```

google-beta/resource_sql_database_instance.go

@@ -96,7 +96,8 @@ func resourceSqlDatabaseInstance() *schema.Resource {
 		},
 
 		CustomizeDiff: customdiff.All(
-			customdiff.ForceNewIfChange("settings.0.disk_size", isDiskShrinkage)),
+			customdiff.ForceNewIfChange("settings.0.disk_size", isDiskShrinkage),
+			privateNetworkCustomizeDiff),
 
 		Schema: map[string]*schema.Schema{
 			"region": {
@@ -610,6 +611,18 @@ func isFirstGen(d *schema.ResourceData) bool {
 	return !regexp.MustCompile("db*").Match([]byte(tier))
 }
 
+// Makes private_network ForceNew if it is changing from set to nil. The API returns an error
+// if this change is attempted in-place.
+func privateNetworkCustomizeDiff(d *schema.ResourceDiff, meta interface{}) error {
+	old, new := d.GetChange("settings.0.ip_configuration.0.private_network")
+
+	if old != "" && new == "" {
+		d.ForceNew("settings.0.ip_configuration.0.private_network")
+	}
+
+	return nil
+}
+
 func resourceSqlDatabaseInstanceCreate(d *schema.ResourceData, meta interface{}) error {
 	config := meta.(*Config)