gopls/internal/template: fix completion token boundary conditions

The template completion logic checked to see if the cursor position was
inside a logical {{ }} block, but the boundary conditions in the
relevant checks was wrong: in one case, a panic would occur if the
cursor was in between the pair of left braces. In another, it would
panic even if the cursor was at the start of a pair of left braces.

Fix the boundary conditions, with a test.

Fixes golang/go#57621

Change-Id: I826349906ee1ae67b2c5378e1b59d56e94c14fb2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/622875
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/template/completion.go

@@ -84,13 +84,15 @@ func inTemplate(fc *Parsed, pos protocol.Position) int {
 	offset := fc.FromPosition(pos)
 	// this could be a binary search, as the tokens are ordered
 	for _, tk := range fc.tokens {
-		if tk.Start < offset && offset <= tk.End {
+		if tk.Start+len(Left) <= offset && offset+len(Right) <= tk.End {
 			return tk.Start
 		}
 	}
 	for _, x := range fc.elided {
-		if x > offset {
-			// fc.elided is sorted
+		if x+len(Left) > offset {
+			// fc.elided is sorted, and x is the position where a '{{' was replaced
+			// by '  '. We consider only cases where the replaced {{ is to the left
+			// of the cursor.
 			break
 		}
 		// If the interval [x,offset] does not contain Left or Right

gopls/internal/test/integration/template/template_test.go

@@ -228,4 +228,30 @@ func shorten(fn protocol.DocumentURI) string {
 	return pieces[j-2] + "/" + pieces[j-1]
 }
 
+func TestCompletionPanic_Issue57621(t *testing.T) {
+	const src = `
+-- go.mod --
+module mod.com
+
+go 1.12
+-- hello.tmpl --
+{{range .Planets}}
+Hello {{
+{{end}}
+`
+	Run(t, src, func(t *testing.T, env *Env) {
+		env.OpenFile("hello.tmpl")
+		// None of these should panic.
+		env.Completion(env.RegexpSearch("hello.tmpl", `Hello ()\{\{`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `Hello \{()\{`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `Hello \{\{()`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `()\{\{range`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `\{()\{range`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `\{\{()range`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `Planets()}}`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `Planets}()}`))
+		env.Completion(env.RegexpSearch("hello.tmpl", `Planets}}()`))
+	})
+}
+
 // Hover needs tests