feat: use lazy connections

Running an import action (or any action for that matter) which is not
touching posgres resources should not trigger the initialisation of the
postgres provider.

Same behaviour as terraform-provider-mysql.

Fixes cyrilgdn#140

Signed-off-by: Enrico Stahn <enrico.stahn@gmail.com>

postgresql/config.go

@@ -119,43 +119,9 @@ func (c *Config) NewClient(database string) (*Client, error) {
 	dbRegistryLock.Lock()
 	defer dbRegistryLock.Unlock()
 
-	dsn := c.connStr(database)
-	dbEntry, found := dbRegistry[dsn]
-	if !found {
-		db, err := sql.Open("postgres", dsn)
-		if err != nil {
-			return nil, fmt.Errorf("Error connecting to PostgreSQL server: %w", err)
-		}
-
-		// We don't want to retain connection
-		// So when we connect on a specific database which might be managed by terraform,
-		// we don't keep opened connection in case of the db has to be dopped in the plan.
-		db.SetMaxIdleConns(0)
-		db.SetMaxOpenConns(c.MaxConns)
-
-		defaultVersion, _ := semver.Parse(defaultExpectedPostgreSQLVersion)
-		version := &c.ExpectedVersion
-		if defaultVersion.Equals(c.ExpectedVersion) {
-			// Version hint not set by user, need to fingerprint
-			version, err = fingerprintCapabilities(db)
-			if err != nil {
-				db.Close()
-				return nil, fmt.Errorf("error detecting capabilities: %w", err)
-			}
-		}
-
-		dbEntry = dbRegistryEntry{
-			db:      db,
-			version: *version,
-		}
-		dbRegistry[dsn] = dbEntry
-	}
-
 	client := Client{
 		config:       *c,
 		databaseName: database,
-		db:           dbEntry.db,
-		version:      dbEntry.version,
 	}
 
 	return &client, nil
@@ -305,9 +271,52 @@ func (c *Config) getDatabaseUsername() string {
 // return their database resources.  Use of QueryRow() or Exec() is encouraged.
 // Query() must have their rows.Close()'ed.
 func (c *Client) DB() *sql.DB {
+	c.connectDB()
 	return c.db
 }
 
+func (c *Client) connectDB() (*Client, error) {
+	dbRegistryLock.Lock()
+	defer dbRegistryLock.Unlock()
+
+	dsn := c.config.connStr(c.databaseName)
+	dbEntry, found := dbRegistry[dsn]
+	if !found {
+		db, err := sql.Open("postgres", dsn)
+		if err != nil {
+			return nil, fmt.Errorf("Error connecting to PostgreSQL server: %w", err)
+		}
+
+		// We don't want to retain connection
+		// So when we connect on a specific database which might be managed by terraform,
+		// we don't keep opened connection in case of the db has to be dopped in the plan.
+		db.SetMaxIdleConns(0)
+		db.SetMaxOpenConns(c.config.MaxConns)
+
+		defaultVersion, _ := semver.Parse(defaultExpectedPostgreSQLVersion)
+		version := &c.config.ExpectedVersion
+		if defaultVersion.Equals(c.config.ExpectedVersion) {
+			// Version hint not set by user, need to fingerprint
+			version, err = fingerprintCapabilities(db)
+			if err != nil {
+				db.Close()
+				return nil, fmt.Errorf("error detecting capabilities: %w", err)
+			}
+		}
+
+		dbEntry = dbRegistryEntry{
+			db:      db,
+			version: *version,
+		}
+		dbRegistry[dsn] = dbEntry
+	}
+
+	c.db = dbEntry.db
+	c.version = dbEntry.version
+
+	return nil, nil
+}
+
 // fingerprintCapabilities queries PostgreSQL to populate a local catalog of
 // capabilities.  This is only run once per Client.
 func fingerprintCapabilities(db *sql.DB) (*semver.Version, error) {

postgresql/helpers.go

@@ -258,6 +258,7 @@ func startTransaction(client *Client, database string) (*sql.Tx, error) {
 			return nil, err
 		}
 	}
+	// spew.Dump(client)
 	db := client.DB()
 	txn, err := db.Begin()
 	if err != nil {