internal/lsp: allow end of file byte offsets

Change-Id: I46d7e07a4603f19f615fed2e37e733573ea0fe08
Reviewed-on: https://go-review.googlesource.com/c/tools/+/166880
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/lsp_test.go

@@ -458,13 +458,15 @@ func TestBytesOffset(t *testing.T) {
 		{text: `a𐐀b`, pos: protocol.Position{Line: 0, Character: 1}, want: 1},
 		{text: `a𐐀b`, pos: protocol.Position{Line: 0, Character: 2}, want: 1},
 		{text: `a𐐀b`, pos: protocol.Position{Line: 0, Character: 3}, want: 5},
-		{text: `a𐐀b`, pos: protocol.Position{Line: 0, Character: 4}, want: -1},
+		{text: `a𐐀b`, pos: protocol.Position{Line: 0, Character: 4}, want: 6},
+		{text: `a𐐀b`, pos: protocol.Position{Line: 0, Character: 5}, want: -1},
 		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 0, Character: 3}, want: 3},
 		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 0, Character: 4}, want: -1},
 		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 1, Character: 0}, want: 4},
 		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 1, Character: 3}, want: 7},
 		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 1, Character: 4}, want: -1},
-		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 2, Character: 0}, want: -1},
+		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 2, Character: 0}, want: 8},
+		{text: "aaa\nbbb\n", pos: protocol.Position{Line: 2, Character: 1}, want: -1},
 		{text: "aaa\nbbb\n\n", pos: protocol.Position{Line: 2, Character: 0}, want: 8},
 	}
 

internal/lsp/server.go

@@ -188,10 +188,13 @@ func (s *server) DidOpen(ctx context.Context, params *protocol.DidOpenTextDocume
 func bytesOffset(content []byte, pos protocol.Position) int {
 	var line, char, offset int
 
-	for len(content) > 0 {
+	for {
 		if line == int(pos.Line) && char == int(pos.Character) {
 			return offset
 		}
+		if len(content) == 0 {
+			return -1
+		}
 
 		r, size := utf8.DecodeRune(content)
 		char++
@@ -210,7 +213,6 @@ func bytesOffset(content []byte, pos protocol.Position) int {
 			char = 0
 		}
 	}
-	return -1
 }
 
 func (s *server) applyChanges(ctx context.Context, params *protocol.DidChangeTextDocumentParams) (string, error) {