text/template: implement short-circuit and, or

Making the builtin and and or functions use short-circuit
evaluation was accepted as a proposal in April 2019,
but we never got around to implementing it. Do that.

Fixes #31103.

Change-Id: Ia43d4a9a6b0ab814f2dd3471ebaca3e7bb1505cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/321490
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 18 insertions, 4 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 5ad3b4ec58..6e005b57d7 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -572,11 +572,11 @@ func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ide
 func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
 	s.at(node)
 	name := node.Ident
-	function, ok := findFunction(name, s.tmpl)
+	function, isBuiltin, ok := findFunction(name, s.tmpl)
 	if !ok {
 		s.errorf("%q is not a defined function", name)
 	}
-	return s.evalCall(dot, function, cmd, name, args, final)
+	return s.evalCall(dot, function, isBuiltin, cmd, name, args, final)
 }
 
 // evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
@@ -605,7 +605,7 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 		ptr = ptr.Addr()
 	}
 	if method := ptr.MethodByName(fieldName); method.IsValid() {
-		return s.evalCall(dot, method, node, fieldName, args, final)
+		return s.evalCall(dot, method, false, node, fieldName, args, final)
 	}
 	hasArgs := len(args) > 1 || final != missingVal
 	// It's not a method; must be a field of a struct or an element of a map.
@@ -669,7 +669,7 @@ var (
 // evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
 // it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
 // as the function itself.
-func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
+func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
 	if args != nil {
 		args = args[1:] // Zeroth arg is function name/node; not passed to function.
 	}
@@ -691,6 +691,20 @@ func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, a
 		// TODO: This could still be a confusing error; maybe goodFunc should provide info.
 		s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
 	}
+
+	// Special case for builtin and/or, which short-circuit.
+	if isBuiltin && (name == "and" || name == "or") {
+		argType := typ.In(0)
+		var v reflect.Value
+		for _, arg := range args {
+			v = s.evalArg(dot, argType, arg).Interface().(reflect.Value)
+			if truth(v) == (name == "or") {
+				break
+			}
+		}
+		return v
+	}
+
 	// Build the arg list.
 	argv := make([]reflect.Value, numIn)
 	// Args must be evaluated. Fixed args first.