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 golang#31103.

Change-Id: Ia43d4a9a6b0ab814f2dd3471ebaca3e7bb1505cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/321490
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

src/text/template/doc.go

@@ -307,9 +307,10 @@ Predefined global functions are named as follows.
 
 	and
 		Returns the boolean AND of its arguments by returning the
-		first empty argument or the last argument, that is,
-		"and x y" behaves as "if x then y else x". All the
-		arguments are evaluated.
+		first empty argument or the last argument. That is,
+		"and x y" behaves as "if x then y else x."
+		Evaluation proceeds through the arguments left to right
+		and returns when the result is determined.
 	call
 		Returns the result of calling the first argument, which
 		must be a function, with the remaining arguments as parameters.
@@ -344,8 +345,9 @@ Predefined global functions are named as follows.
 	or
 		Returns the boolean OR of its arguments by returning the
 		first non-empty argument or the last argument, that is,
-		"or x y" behaves as "if x then x else y". All the
-		arguments are evaluated.
+		"or x y" behaves as "if x then x else y".
+		Evaluation proceeds through the arguments left to right
+		and returns when the result is determined.
 	print
 		An alias for fmt.Sprint
 	printf

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.

src/text/template/exec_test.go

@@ -481,6 +481,10 @@ var execTests = []execTest{
 	{"not", "{{not true}} {{not false}}", "false true", nil, true},
 	{"and", "{{and false 0}} {{and 1 0}} {{and 0 true}} {{and 1 1}}", "false 0 0 1", nil, true},
 	{"or", "{{or 0 0}} {{or 1 0}} {{or 0 true}} {{or 1 1}}", "0 1 true 1", nil, true},
+	{"or short-circuit", "{{or 0 1 (die)}}", "1", nil, true},
+	{"and short-circuit", "{{and 1 0 (die)}}", "0", nil, true},
+	{"or short-circuit2", "{{or 0 0 (die)}}", "", nil, false},
+	{"and short-circuit2", "{{and 1 1 (die)}}", "", nil, false},
 	{"boolean if", "{{if and true 1 `hi`}}TRUE{{else}}FALSE{{end}}", "TRUE", tVal, true},
 	{"boolean if not", "{{if and true 1 `hi` | not}}TRUE{{else}}FALSE{{end}}", "FALSE", nil, true},
 
@@ -764,6 +768,7 @@ func testExecute(execTests []execTest, template *Template, t *testing.T) {
 		"add":         add,
 		"count":       count,
 		"dddArg":      dddArg,
+		"die":         func() bool { panic("die") },
 		"echo":        echo,
 		"makemap":     makemap,
 		"mapOfThree":  mapOfThree,

src/text/template/funcs.go

@@ -139,18 +139,18 @@ func goodName(name string) bool {
 }
 
 // findFunction looks for a function in the template, and global map.
-func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
+func findFunction(name string, tmpl *Template) (v reflect.Value, isBuiltin, ok bool) {
 	if tmpl != nil && tmpl.common != nil {
 		tmpl.muFuncs.RLock()
 		defer tmpl.muFuncs.RUnlock()
 		if fn := tmpl.execFuncs[name]; fn.IsValid() {
-			return fn, true
+			return fn, false, true
 		}
 	}
 	if fn := builtinFuncs()[name]; fn.IsValid() {
-		return fn, true
+		return fn, true, true
 	}
-	return reflect.Value{}, false
+	return reflect.Value{}, false, false
 }
 
 // prepareArg checks if value can be used as an argument of type argType, and
@@ -382,31 +382,13 @@ func truth(arg reflect.Value) bool {
 // and computes the Boolean AND of its arguments, returning
 // the first false argument it encounters, or the last argument.
 func and(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
-	if !truth(arg0) {
-		return arg0
-	}
-	for i := range args {
-		arg0 = args[i]
-		if !truth(arg0) {
-			break
-		}
-	}
-	return arg0
+	panic("unreachable") // implemented as a special case in evalCall
 }
 
 // or computes the Boolean OR of its arguments, returning
 // the first true argument it encounters, or the last argument.
 func or(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
-	if truth(arg0) {
-		return arg0
-	}
-	for i := range args {
-		arg0 = args[i]
-		if truth(arg0) {
-			break
-		}
-	}
-	return arg0
+	panic("unreachable") // implemented as a special case in evalCall
 }
 
 // not returns the Boolean negation of its argument.