Remove prepared stmnts from pgsql physical backend

Prepared statements prevent the use of connection multiplexing software
such as PGBouncer. Even when PGBouncer is configured for [session mode][1]
there's a possibility that a connection to PostgreSQL can be re-used by
different clients.  This leads to errors when clients use session based
features (like prepared statements).

This change removes prepared statements from the PostgreSQL physical
backend. This will allow vault to successfully work in infrastructures
that employ the use of PGBouncer or other connection multiplexing
software.

[1]: https://pgbouncer.github.io/config.html#poolmode

physical/postgresql.go

@@ -16,7 +16,10 @@ import (
 type PostgreSQLBackend struct {
 	table        string
 	client       *sql.DB
-	statements map[string]*sql.Stmt
+	put_query    string
+	get_query    string
+	delete_query string
+	list_query   string
 	logger     *log.Logger
 }
 
@@ -50,11 +53,11 @@ func newPostgreSQLBackend(conf map[string]string, logger *log.Logger) (Backend,
 
 	// Setup our put strategy based on the presence or absence of a native
 	// upsert.
-	var put_statement string
+	var put_query string
 	if upsert_required {
-		put_statement = "SELECT vault_kv_put($1, $2, $3, $4)"
+		put_query = "SELECT vault_kv_put($1, $2, $3, $4)"
 	} else {
-		put_statement = "INSERT INTO " + quoted_table + " VALUES($1, $2, $3, $4)" +
+		put_query = "INSERT INTO " + quoted_table + " VALUES($1, $2, $3, $4)" +
 			" ON CONFLICT (path, key) DO " +
 			" UPDATE SET (parent_path, path, key, value) = ($1, $2, $3, $4)"
 	}
@@ -63,36 +66,17 @@ func newPostgreSQLBackend(conf map[string]string, logger *log.Logger) (Backend,
 	m := &PostgreSQLBackend{
 		table:        quoted_table,
 		client:       db,
-		statements: make(map[string]*sql.Stmt),
+		put_query:    put_query,
+		get_query:    "SELECT value FROM " + quoted_table + " WHERE path = $1 AND key = $2",
+		delete_query: "DELETE FROM " + quoted_table + " WHERE path = $1 AND key = $2",
+		list_query:  "SELECT key FROM " + quoted_table + " WHERE path = $1" +
+			"UNION SELECT substr(path, length($1)+1) FROM " + quoted_table + "WHERE parent_path = $1",
 		logger:     logger,
 	}
 
-	// Prepare all the statements required
-	statements := map[string]string{
-		"put":    put_statement,
-		"get":    "SELECT value FROM " + quoted_table + " WHERE path = $1 AND key = $2",
-		"delete": "DELETE FROM " + quoted_table + " WHERE path = $1 AND key = $2",
-		"list": "SELECT key FROM " + quoted_table + " WHERE path = $1" +
-			"UNION SELECT substr(path, length($1)+1) FROM " + quoted_table + "WHERE parent_path = $1",
-	}
-	for name, query := range statements {
-		if err := m.prepare(name, query); err != nil {
-			return nil, err
-		}
-	}
 	return m, nil
 }
 
-// prepare is a helper to prepare a query for future execution
-func (m *PostgreSQLBackend) prepare(name, query string) error {
-	stmt, err := m.client.Prepare(query)
-	if err != nil {
-		return fmt.Errorf("failed to prepare '%s': %v", name, err)
-	}
-	m.statements[name] = stmt
-	return nil
-}
-
 // splitKey is a helper to split a full path key into individual
 // parts: parentPath, path, key
 func (m *PostgreSQLBackend) splitKey(fullPath string) (string, string, string) {
@@ -123,7 +107,7 @@ func (m *PostgreSQLBackend) Put(entry *Entry) error {
 
 	parentPath, path, key := m.splitKey(entry.Key)
 
-	_, err := m.statements["put"].Exec(parentPath, path, key, entry.Value)
+	_, err := m.client.Exec(m.put_query, parentPath, path, key, entry.Value)
 	if err != nil {
 		return err
 	}
@@ -137,7 +121,7 @@ func (m *PostgreSQLBackend) Get(fullPath string) (*Entry, error) {
 	_, path, key := m.splitKey(fullPath)
 
 	var result []byte
-	err := m.statements["get"].QueryRow(path, key).Scan(&result)
+	err := m.client.QueryRow(m.get_query, path, key).Scan(&result)
 	if err == sql.ErrNoRows {
 		return nil, nil
 	}
@@ -158,7 +142,7 @@ func (m *PostgreSQLBackend) Delete(fullPath string) error {
 
 	_, path, key := m.splitKey(fullPath)
 
-	_, err := m.statements["delete"].Exec(path, key)
+	_, err := m.client.Exec(m.delete_query, path, key)
 	if err != nil {
 		return err
 	}
@@ -170,7 +154,7 @@ func (m *PostgreSQLBackend) Delete(fullPath string) error {
 func (m *PostgreSQLBackend) List(prefix string) ([]string, error) {
 	defer metrics.MeasureSince([]string{"postgres", "list"}, time.Now())
 
-	rows, err := m.statements["list"].Query("/" + prefix)
+	rows, err := m.client.Query(m.list_query, "/" + prefix)
 	if err != nil {
 		return nil, err
 	}

physical/postgresql_test.go

@@ -32,7 +32,7 @@ func TestPostgreSQLBackend(t *testing.T) {
 
 	defer func() {
 		pg := b.(*PostgreSQLBackend)
-		_, err := pg.client.Exec("DROP TABLE " + pg.table)
+		_, err := pg.client.Exec("TRUNCATE TABLE " + pg.table)
 		if err != nil {
 			t.Fatalf("Failed to drop table: %v", err)
 		}