tgc-revival: allow the fields with empty string to show up in HCL (GoogleCloudPlatform#16507)

Author: zli82016

mmv1/products/compute/Subnetwork.yaml

@@ -223,6 +223,7 @@ properties:
     validation:
       function: verify.ValidateIpCidrRange
     default_from_api: true
+    custom_tgc_flatten: 'templates/tgc_next/custom_flatten/compute_subnetwork_ip_cidr_range.go.tmpl'
   - name: reservedInternalRange
     type: ResourceRef
     description: |

mmv1/products/datastream/ConnectionProfile.yaml

@@ -208,6 +208,7 @@ properties:
           Password for the Oracle connection.
         sensitive: true
         custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_oracle_profile_password.go.tmpl'
+        tgc_ignore_terraform_custom_flatten: true
       - name: 'secretManagerStoredPassword'
         type: String
         description: |
@@ -280,6 +281,7 @@ properties:
           Password for the MySQL connection.
         sensitive: true
         custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_mysql_profile_password.go.tmpl'
+        tgc_ignore_terraform_custom_flatten: true
         is_missing_in_cai: true
       - name: 'secretManagerStoredPassword'
         type: String
@@ -299,6 +301,7 @@ properties:
             immutable: true
             sensitive: true
             custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_mysql_profile_ssl_config_client_key.go.tmpl'
+            tgc_ignore_terraform_custom_flatten: true
           - name: 'clientKeySet'
             type: Boolean
             description: |
@@ -314,6 +317,7 @@ properties:
             immutable: true
             sensitive: true
             custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_mysql_profile_ssl_config_client_certificate.go.tmpl'
+            tgc_ignore_terraform_custom_flatten: true
           - name: 'clientCertificateSet'
             type: Boolean
             description: |
@@ -327,6 +331,7 @@ properties:
             immutable: true
             sensitive: true
             custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_mysql_profile_ssl_config_ca_certificate.go.tmpl'
+            tgc_ignore_terraform_custom_flatten: true
           - name: 'caCertificateSet'
             type: Boolean
             description: |
@@ -387,6 +392,7 @@ properties:
         sensitive: true
         is_missing_in_cai: true
         custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_postgresql_profile_password.go.tmpl'
+        tgc_ignore_terraform_custom_flatten: true
       - name: 'secretManagerStoredPassword'
         type: String
         description: |
@@ -590,6 +596,7 @@ properties:
           Password for the SQL Server connection.
         sensitive: true
         custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_sql_server_profile_password.go.tmpl'
+        tgc_ignore_terraform_custom_flatten: true
       - name: 'secretManagerStoredPassword'
         type: String
         description: |
@@ -774,6 +781,7 @@ properties:
         conflicts:
           - forward_ssh_connectivity.0.private_key
         custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_forward_ssh_connectivity_password.go.tmpl'
+        tgc_ignore_terraform_custom_flatten: true
         is_missing_in_cai: true
       - name: 'privateKey'
         type: String
@@ -783,6 +791,7 @@ properties:
         conflicts:
           - forward_ssh_connectivity.0.password
         custom_flatten: 'templates/terraform/custom_flatten/datastream_connection_profile_forward_ssh_connectivity_private_key.go.tmpl'
+        tgc_ignore_terraform_custom_flatten: true
         is_missing_in_cai: true
   - name: 'privateConnectivity'
     type: NestedObject

mmv1/templates/tgc_next/custom_flatten/compute_subnetwork_ip_cidr_range.go.tmpl

@@ -0,0 +1,11 @@
+// flatten{{$.GetPrefix}}{{$.TitlelizeProperty}} handles empty strings for IpCidrRange which occur in IPV6_ONLY subnets.
+// Terraform validation fails on empty string, so we convert it to nil.
+func flatten{{$.GetPrefix}}{{$.TitlelizeProperty}}(v interface{}, d *schema.ResourceData, config *transport_tpg.Config) interface{} {
+	if v == nil {
+		return nil
+	}
+	if s, ok := v.(string); ok && s == "" {
+		return nil
+	}
+	return v
+}

mmv1/third_party/tgc_next/pkg/cai2hcl/models/hcl_block.go

@@ -65,9 +65,6 @@ func hclWriteBlock(val cty.Value, body *hclwrite.Body) error {
 			}
 			fallthrough
 		default:
-			if objValType.FriendlyName() == "string" && objVal.AsString() == "" {
-				continue
-			}
 			body.SetAttributeValue(objKey.AsString(), objVal)
 		}
 	}