allow updating (R)IGM target pools to empty

.changelog/3581.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+compute: fixed issue where removing all target pools from `google_compute_instance_group_manager` or `google_compute_region_instance_group_manager` had no effect
+```

google/resource_compute_instance_group_manager.go

@@ -470,6 +470,7 @@ func resourceComputeInstanceGroupManagerUpdate(d *schema.ResourceData, meta inte
 
 	if d.HasChange("target_pools") {
 		updatedManager.TargetPools = convertStringSet(d.Get("target_pools").(*schema.Set))
+		updatedManager.ForceSendFields = append(updatedManager.ForceSendFields, "TargetPools")
 		change = true
 	}
 

google/resource_compute_instance_group_manager_test.go

@@ -91,6 +91,14 @@ func TestAccInstanceGroupManager_update(t *testing.T) {
 				ImportState:       true,
 				ImportStateVerify: true,
 			},
+			{
+				Config: testAccInstanceGroupManager_update3(template1, target1, target2, template2, igm),
+			},
+			{
+				ResourceName:      "google_compute_instance_group_manager.igm-update",
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
 		},
 	})
 }
@@ -537,6 +545,92 @@ resource "google_compute_instance_group_manager" "igm-update" {
 `, template1, target1, target2, template2, igm)
 }
 
+// Remove target pools
+func testAccInstanceGroupManager_update3(template1, target1, target2, template2, igm string) string {
+	return fmt.Sprintf(`
+data "google_compute_image" "my_image" {
+  family  = "debian-9"
+  project = "debian-cloud"
+}
+
+resource "google_compute_instance_template" "igm-update" {
+  name           = "%s"
+  machine_type   = "n1-standard-1"
+  can_ip_forward = false
+  tags           = ["foo", "bar"]
+
+  disk {
+    source_image = data.google_compute_image.my_image.self_link
+    auto_delete  = true
+    boot         = true
+  }
+
+  network_interface {
+    network = "default"
+  }
+
+  service_account {
+    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
+  }
+}
+
+resource "google_compute_target_pool" "igm-update" {
+  description      = "Resource created for Terraform acceptance testing"
+  name             = "%s"
+  session_affinity = "CLIENT_IP_PROTO"
+}
+
+resource "google_compute_target_pool" "igm-update2" {
+  description      = "Resource created for Terraform acceptance testing"
+  name             = "%s"
+  session_affinity = "CLIENT_IP_PROTO"
+}
+
+resource "google_compute_instance_template" "igm-update2" {
+  name           = "%s"
+  machine_type   = "n1-standard-1"
+  can_ip_forward = false
+  tags           = ["foo", "bar"]
+
+  disk {
+    source_image = data.google_compute_image.my_image.self_link
+    auto_delete  = true
+    boot         = true
+  }
+
+  network_interface {
+    network = "default"
+  }
+
+  service_account {
+    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
+  }
+}
+
+resource "google_compute_instance_group_manager" "igm-update" {
+  description = "Terraform test instance group manager"
+  name        = "%s"
+
+  version {
+    name              = "prod"
+    instance_template = google_compute_instance_template.igm-update2.self_link
+  }
+
+  base_instance_name = "igm-update"
+  zone               = "us-central1-c"
+  target_size        = 3
+  named_port {
+    name = "customhttp"
+    port = 8080
+  }
+  named_port {
+    name = "customhttps"
+    port = 8443
+  }
+}
+`, template1, target1, target2, template2, igm)
+}
+
 func testAccInstanceGroupManager_updateLifecycle(tag, igm string) string {
 	return fmt.Sprintf(`
 data "google_compute_image" "my_image" {

google/resource_compute_region_instance_group_manager.go

@@ -436,6 +436,7 @@ func resourceComputeRegionInstanceGroupManagerUpdate(d *schema.ResourceData, met
 
 	if d.HasChange("target_pools") {
 		updatedManager.TargetPools = convertStringSet(d.Get("target_pools").(*schema.Set))
+		updatedManager.ForceSendFields = append(updatedManager.ForceSendFields, "TargetPools")
 		change = true
 	}
 

google/resource_compute_region_instance_group_manager_test.go

@@ -92,6 +92,14 @@ func TestAccRegionInstanceGroupManager_update(t *testing.T) {
 				ImportState:       true,
 				ImportStateVerify: true,
 			},
+			{
+				Config: testAccRegionInstanceGroupManager_update3(template1, target1, target2, template2, igm),
+			},
+			{
+				ResourceName:      "google_compute_region_instance_group_manager.igm-update",
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
 		},
 	})
 }
@@ -549,6 +557,92 @@ resource "google_compute_region_instance_group_manager" "igm-update" {
 `, template1, target1, target2, template2, igm)
 }
 
+// Remove target pools
+func testAccRegionInstanceGroupManager_update3(template1, target1, target2, template2, igm string) string {
+	return fmt.Sprintf(`
+data "google_compute_image" "my_image" {
+  family  = "debian-9"
+  project = "debian-cloud"
+}
+
+resource "google_compute_instance_template" "igm-update" {
+  name           = "%s"
+  machine_type   = "n1-standard-1"
+  can_ip_forward = false
+  tags           = ["foo", "bar"]
+
+  disk {
+    source_image = data.google_compute_image.my_image.self_link
+    auto_delete  = true
+    boot         = true
+  }
+
+  network_interface {
+    network = "default"
+  }
+
+  service_account {
+    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
+  }
+}
+
+resource "google_compute_target_pool" "igm-update" {
+  description      = "Resource created for Terraform acceptance testing"
+  name             = "%s"
+  session_affinity = "CLIENT_IP_PROTO"
+}
+
+resource "google_compute_target_pool" "igm-update2" {
+  description      = "Resource created for Terraform acceptance testing"
+  name             = "%s"
+  session_affinity = "CLIENT_IP_PROTO"
+}
+
+resource "google_compute_instance_template" "igm-update2" {
+  name           = "%s"
+  machine_type   = "n1-standard-1"
+  can_ip_forward = false
+  tags           = ["foo", "bar"]
+
+  disk {
+    source_image = data.google_compute_image.my_image.self_link
+    auto_delete  = true
+    boot         = true
+  }
+
+  network_interface {
+    network = "default"
+  }
+
+  service_account {
+    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
+  }
+}
+
+resource "google_compute_region_instance_group_manager" "igm-update" {
+  description = "Terraform test instance group manager"
+  name        = "%s"
+
+  version {
+    instance_template = google_compute_instance_template.igm-update2.self_link
+    name              = "primary"
+  }
+
+  base_instance_name = "igm-update"
+  region             = "us-central1"
+  target_size        = 3
+  named_port {
+    name = "customhttp"
+    port = 8080
+  }
+  named_port {
+    name = "customhttps"
+    port = 8443
+  }
+}
+`, template1, target1, target2, template2, igm)
+}
+
 func testAccRegionInstanceGroupManager_updateLifecycle(tag, igm string) string {
 	return fmt.Sprintf(`
 data "google_compute_image" "my_image" {