template: warn about interleaved nature of writes

Execute incurs separate writes for each "step", e.g. each
variable that needs to be printed, and the final newline.
While it is correct to state that templates can be executed
concurrently, there is a more subtle nuance that is easily missed:
when writing to the same writer, the writes from concurrent execute
calls can be interleaved, leading to unexpected output.

Change-Id: I0abbd7960d8a8d15e109a8a3eeff3b43b852bbbf
Reviewed-on: https://go-review.googlesource.com/37444Reviewed-by: Rob Pike <r@golang.org>

src/html/template/template.go

@@ -112,7 +112,8 @@ func (t *Template) escape() error {
 // If an error occurs executing the template or writing its output,
 // execution stops, but partial results may already have been written to
 // the output writer.
-// A template may be executed safely in parallel.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
 func (t *Template) Execute(wr io.Writer, data interface{}) error {
 	if err := t.escape(); err != nil {
 		return err
@@ -125,7 +126,8 @@ func (t *Template) Execute(wr io.Writer, data interface{}) error {
 // If an error occurs executing the template or writing its output,
 // execution stops, but partial results may already have been written to
 // the output writer.
-// A template may be executed safely in parallel.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
 func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
 	tmpl, err := t.lookupAndEscapeTemplate(name)
 	if err != nil {

src/text/template/doc.go

@@ -20,7 +20,8 @@ The input text for a template is UTF-8-encoded text in any format.
 "{{" and "}}"; all text outside actions is copied to the output unchanged.
 Except for raw strings, actions may not span newlines, although comments can.
 
-Once parsed, a template may be executed safely in parallel.
+Once parsed, a template may be executed safely in parallel, although if parallel
+executions share a Writer the output may be interleaved.
 
 Here is a trivial example that prints "17 items are made of wool".
 

src/text/template/exec.go

@@ -153,7 +153,8 @@ func errRecover(errp *error) {
 // If an error occurs executing the template or writing its output,
 // execution stops, but partial results may already have been written to
 // the output writer.
-// A template may be executed safely in parallel.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
 func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
 	var tmpl *Template
 	if t.common != nil {
@@ -170,7 +171,8 @@ func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{})
 // If an error occurs executing the template or writing its output,
 // execution stops, but partial results may already have been written to
 // the output writer.
-// A template may be executed safely in parallel.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
 //
 // If data is a reflect.Value, the template applies to the concrete
 // value that the reflect.Value holds, as in fmt.Print.