remote-state/pg: skip schema creation instruction if exists

* add `skip_schema_creation` option
* add sanity check to avoid situations where postgres users
  hasn't been granted the "CREATE SCHEMA" right

closes #21604

Signed-off-by: yann degat <yann@2kmail.net>

backend/remote-state/pg/backend.go

@@ -31,6 +31,13 @@ func New() backend.Backend {
 				Description: "Name of the automatically managed Postgres schema to store state",
 				Default:     "terraform_remote_state",
 			},
+
+			"skip_schema_creation": &schema.Schema{
+				Type:        schema.TypeBool,
+				Optional:    true,
+				Description: "If set to `true`, Terraform won't try to create the Postgres schema",
+				Default:     false,
+			},
 		},
 	}
 
@@ -64,9 +71,25 @@ func (b *Backend) configure(ctx context.Context) error {
 
 	// Prepare database schema, tables, & indexes.
 	var query string
-	query = `CREATE SCHEMA IF NOT EXISTS %s`
-	if _, err := db.Exec(fmt.Sprintf(query, b.schemaName)); err != nil {
-		return err
+
+	if !data.Get("skip_schema_creation").(bool) {
+		// list all schemas to see if it exists
+		var count int
+		query = `select count(1) from information_schema.schemata where lower(schema_name) = lower('%s')`
+		if err := db.QueryRow(fmt.Sprintf(query, b.schemaName)).Scan(&count); err != nil {
+			return err
+		}
+
+		// skip schema creation if schema already exists
+		// `CREATE SCHEMA IF NOT EXISTS` is to be avoided if ever
+		// a user hasn't been granted the `CREATE SCHEMA` privilege
+		if count < 1 {
+			// tries to create the schema
+			query = `CREATE SCHEMA %s`
+			if _, err := db.Exec(fmt.Sprintf(query, b.schemaName)); err != nil {
+				return err
+			}
+		}
 	}
 	query = `CREATE TABLE IF NOT EXISTS %s.%s (
 		id SERIAL PRIMARY KEY,

backend/remote-state/pg/backend_test.go

@@ -73,6 +73,50 @@ func TestBackendConfig(t *testing.T) {
 	}
 }
 
+func TestBackendConfigSkipSchema(t *testing.T) {
+	testACC(t)
+	connStr := getDatabaseUrl()
+	schemaName := fmt.Sprintf("terraform_%s", t.Name())
+	db, err := sql.Open("postgres", connStr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// create the schema as a prerequisites
+	db.Query(fmt.Sprintf("CREATE SCHEMA IF NOT EXISTS %s", schemaName))
+	defer db.Query(fmt.Sprintf("DROP SCHEMA IF EXISTS %s CASCADE", schemaName))
+
+	config := backend.TestWrapConfig(map[string]interface{}{
+		"conn_str":             connStr,
+		"schema_name":          schemaName,
+		"skip_schema_creation": true,
+	})
+	b := backend.TestBackendConfig(t, New(), config).(*Backend)
+
+	if b == nil {
+		t.Fatal("Backend could not be configured")
+	}
+
+	_, err = b.db.Query(fmt.Sprintf("SELECT name, data FROM %s.%s LIMIT 1", schemaName, statesTableName))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = b.StateMgr(backend.DefaultStateName)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s, err := b.StateMgr(backend.DefaultStateName)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c := s.(*remote.State).Client.(*RemoteClient)
+	if c.Name != backend.DefaultStateName {
+		t.Fatal("RemoteClient name is not configured")
+	}
+}
+
 func TestBackendStates(t *testing.T) {
 	testACC(t)
 	connStr := getDatabaseUrl()

website/docs/backends/types/pg.html.md

@@ -71,6 +71,7 @@ The following configuration options or environment variables are supported:
 
  * `conn_str` - (Required) Postgres connection string; a `postgres://` URL
  * `schema_name` - Name of the automatically-managed Postgres schema, default `terraform_remote_state`.
+ * `skip_schema_creation` - If set to `true`, Terraform won't try to create the Postgres schema.
 
 ## Technical Design