text/template: check for malformed pipelines

Catch some malformed pipelines at parsing time.
The current code accepts pipelines such as:

{{12|.}}
{{"hello"|print|false}}
{{.|"blah blah"}}

Such pipelines generate panic in html/template at execution time.

Add an extra check to verify all the commands of the pipeline are executable
(except for the first one).

Fixes #10610

Change-Id: Id72236ba8f76a59fa284fe3d4c2cb073e50b51f1
Reviewed-on: https://go-review.googlesource.com/9626Reviewed-by: Rob Pike <r@golang.org>

src/text/template/parse/parse.go

@@ -413,9 +413,8 @@ func (t *Tree) pipeline(context string) (pipe *PipeNode) {
 	for {
 		switch token := t.nextNonSpace(); token.typ {
 		case itemRightDelim, itemRightParen:
-			if len(pipe.Cmds) == 0 {
-				t.errorf("missing value for %s", context)
-			}
+			// At this point, the pipeline is complete
+			t.checkPipeline(pipe, context)
 			if token.typ == itemRightParen {
 				t.backup()
 			}
@@ -430,6 +429,21 @@ func (t *Tree) pipeline(context string) (pipe *PipeNode) {
 	}
 }
 
+func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
+	// Reject empty pipelines
+	if len(pipe.Cmds) == 0 {
+		t.errorf("missing value for %s", context)
+	}
+	// Only the first command of a pipeline can start with a non executable operand
+	for i, c := range pipe.Cmds[1:] {
+		switch c.Args[0].Type() {
+		case NodeBool, NodeDot, NodeNil, NodeNumber, NodeString:
+			// With A|B|C, pipeline stage 2 is B
+			t.errorf("non executable command in pipeline stage %d", i+2)
+		}
+	}
+}
+
 func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
 	defer t.popVars(len(t.vars))
 	line = t.lex.lineNumber()

src/text/template/parse/parse_test.go

@@ -269,6 +269,14 @@ var parseTests = []parseTest{
 	{"dot after string", `{{"hello".guys}}`, hasError, ""},
 	{"dot after dot", "{{..E}}", hasError, ""},
 	{"dot after nil", "{{nil.E}}", hasError, ""},
+	// Wrong pipeline
+	{"wrong pipeline dot", "{{12|.}}", hasError, ""},
+	{"wrong pipeline number", "{{.|12|printf}}", hasError, ""},
+	{"wrong pipeline string", "{{.|print|\"error\"}}", hasError, ""},
+	{"wrong pipeline char", "{{12|print|html|'e'}}", hasError, ""},
+	{"wrong pipeline boolean", "{{.|true}}", hasError, ""},
+	{"wrong pipeline nil", "{{'c'|nil}}", hasError, ""},
+	{"empty pipeline", `{{printf "%d" ( ) }}`, hasError, ""},
 }
 
 var builtins = map[string]interface{}{
@@ -422,6 +430,12 @@ var errorTests = []parseTest{
 	{"wrongdot",
 		"{{true.any}}",
 		hasError, `unexpected . after term`},
+	{"wrongpipeline",
+		"{{12|false}}",
+		hasError, `non executable command in pipeline`},
+	{"emptypipeline",
+		`{{ ( ) }}`,
+		hasError, `missing value for parenthesized pipeline`},
 }
 
 func TestErrors(t *testing.T) {