Make *Conn.Prepare idempotent.

fixes #94

conn.go

@@ -68,6 +68,7 @@ type Conn struct {
 
 type PreparedStatement struct {
 	Name              string
+	SQL               string
 	FieldDescriptions []FieldDescription
 	ParameterOids     []Oid
 }
@@ -489,7 +490,15 @@ func configSSL(sslmode string, cc *ConnConfig) error {
 
 // Prepare creates a prepared statement with name and sql. sql can contain placeholders
 // for bound parameters. These placeholders are referenced positional as $1, $2, etc.
+//
+// Prepare is idempotent; i.e. it is safe to call Prepare multiple times with the same
+// name and sql arguments. This allows a code path to Prepare and Query/Exec without
+// concern for if the statement has already been prepared.
 func (c *Conn) Prepare(name, sql string) (ps *PreparedStatement, err error) {
+	if ps, ok := c.preparedStatements[name]; ok && ps.SQL == sql {
+		return ps, nil
+	}
+
 	if c.logLevel >= LogLevelError {
 		defer func() {
 			if err != nil {
@@ -519,7 +528,7 @@ func (c *Conn) Prepare(name, sql string) (ps *PreparedStatement, err error) {
 		return nil, err
 	}
 
-	ps = &PreparedStatement{Name: name}
+	ps = &PreparedStatement{Name: name, SQL: sql}
 
 	var softErr error
 
@@ -549,7 +558,7 @@ func (c *Conn) Prepare(name, sql string) (ps *PreparedStatement, err error) {
 		case readyForQuery:
 			c.rxReadyForQuery(r)
 
-			if softErr == nil {
+			if softErr == nil && name != "" {
 				c.preparedStatements[name] = ps
 			}
 

conn_test.go

@@ -848,6 +848,36 @@ func TestPrepareQueryManyParameters(t *testing.T) {
 	ensureConnValid(t, conn)
 }
 
+func TestPrepareIdempotency(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnect(t, *defaultConnConfig)
+	defer closeConn(t, conn)
+
+	for i := 0; i < 2; i++ {
+		_, err := conn.Prepare("test", "select 42::integer")
+		if err != nil {
+			t.Fatalf("%d. Unable to prepare statement: %v", i, err)
+		}
+
+		var n int32
+		err = conn.QueryRow("test").Scan(&n)
+		if err != nil {
+			t.Errorf("%d. Executing prepared statement failed: %v", i, err)
+		}
+
+		if n != int32(42) {
+			t.Errorf("%d. Prepared statement did not return expected value: %v", i, n)
+		}
+	}
+
+	_, err := conn.Prepare("test", "select 'fail'::varchar")
+	if err == nil {
+		t.Fatalf("Prepare statement with same name but different SQL should have failed but it didn't")
+		return
+	}
+}
+
 func TestListenNotify(t *testing.T) {
 	t.Parallel()