Disallow GET_LOCK() calls in vttablet.

Named locks are unsafe with server-side connection pooling.

See #3631 for background.

Signed-off-by: David Weitzman <dweitzman@pinterest.com>

data/test/tabletserver/exec_cases.txt

@@ -2151,3 +2151,7 @@ options:PassthroughDMLs
 # syntax error
 "syntax error"
 "syntax error at position 7 near 'syntax'"
+
+# named locks are unsafe with server-side connection pooling
+"select get_lock('foo') from dual"
+"get_lock() not allowed"

data/test/tabletserver/stream_cases.txt

@@ -52,3 +52,7 @@
 # syntax error
 "syntax error"
 "syntax error at position 7 near 'syntax'"
+
+# named locks are unsafe with server-side connection pooling
+"select get_lock('foo') from dual"
+"get_lock() not allowed"

go/vt/vttablet/tabletserver/planbuilder/plan.go

@@ -267,6 +267,12 @@ func (plan *Plan) setTable(tableName sqlparser.TableIdent, tables map[string]*sc
 func Build(statement sqlparser.Statement, tables map[string]*schema.Table) (*Plan, error) {
 	var plan *Plan
 	var err error
+
+	err = checkForPoolingUnsafeConstructs(statement)
+	if err != nil {
+		return nil, err
+	}
+
 	switch stmt := statement.(type) {
 	case *sqlparser.Union:
 		plan, err = &Plan{
@@ -309,6 +315,11 @@ func BuildStreaming(sql string, tables map[string]*schema.Table) (*Plan, error)
 		return nil, err
 	}
 
+	err = checkForPoolingUnsafeConstructs(statement)
+	if err != nil {
+		return nil, err
+	}
+
 	plan := &Plan{
 		PlanID:      PlanSelectStream,
 		FullQuery:   GenerateFullQuery(statement),
@@ -350,3 +361,20 @@ func BuildMessageStreaming(name string, tables map[string]*schema.Table) (*Plan,
 	}}
 	return plan, nil
 }
+
+// checkForPoolingUnsafeConstructs returns an error if the SQL expression contains
+// a call to GET_LOCK(), which is unsafe with server-side connection pooling.
+// For more background, see https://github.com/vitessio/vitess/issues/3631.
+func checkForPoolingUnsafeConstructs(expr sqlparser.SQLNode) error {
+	return sqlparser.Walk(func(node sqlparser.SQLNode) (kontinue bool, err error) {
+		if f, ok := node.(*sqlparser.FuncExpr); ok {
+			if f.Name.Lowered() == "get_lock" {
+				return false, vterrors.New(vtrpcpb.Code_FAILED_PRECONDITION, "get_lock() not allowed")
+			}
+		}
+
+		// TODO: This could be smarter about not walking down parts of the AST that can't contain
+		// function calls.
+		return true, nil
+	}, expr)
+}