oracle: referrers: fix crash when loading nonexistent files

Files with //line directives may not exist.
Print a helpful error instead of panicking when a file fails to load.

+ Test.

Fixes issue #13459

Change-Id: I305d1380e66e64d20ea84a27c284f53c019fe5e6
Reviewed-on: https://go-review.googlesource.com/18209
Reviewed-by: Michael Matloob <matloob@golang.org>

oracle/referrers.go

@@ -169,28 +169,32 @@ func (r *referrersResult) display(printf printfFunc) {
 	type fileinfo struct {
 		refs     []*ast.Ident
 		linenums []int            // line number of refs[i]
-		data     chan []byte // file contents
+		data     chan interface{} // file contents or error
 	}
 	var fileinfos []*fileinfo
 	fileinfosByName := make(map[string]*fileinfo)
 
 	// First pass: start the file reads concurrently.
+	sema := make(chan struct{}, 20) // counting semaphore to limit I/O concurrency
 	for _, ref := range r.refs {
 		posn := r.qpos.fset.Position(ref.Pos())
 		fi := fileinfosByName[posn.Filename]
 		if fi == nil {
-			fi = &fileinfo{data: make(chan []byte)}
+			fi = &fileinfo{data: make(chan interface{})}
 			fileinfosByName[posn.Filename] = fi
 			fileinfos = append(fileinfos, fi)
 
 			// First request for this file:
 			// start asynchronous read.
 			go func() {
+				sema <- struct{}{} // acquire token
 				content, err := ioutil.ReadFile(posn.Filename)
+				<-sema // release token
 				if err != nil {
-					content = []byte(fmt.Sprintf("error: %v", err))
+					fi.data <- err
-				}
+				} else {
 					fi.data <- content
+				}
 			}()
 		}
 		fi.refs = append(fi.refs, ref)
@@ -200,8 +204,20 @@ func (r *referrersResult) display(printf printfFunc) {
 	// Second pass: print refs in original order.
 	// One line may have several refs at different columns.
 	for _, fi := range fileinfos {
-		content := <-fi.data // wait for I/O completion
+		v := <-fi.data // wait for I/O completion
-		lines := bytes.Split(content, []byte("\n"))
+
+		// Print one item for all refs in a file that could not
+		// be loaded (perhaps due to //line directives).
+		if err, ok := v.(error); ok {
+			var suffix string
+			if more := len(fi.refs) - 1; more > 0 {
+				suffix = fmt.Sprintf(" (+ %d more refs in this file)", more)
+			}
+			printf(fi.refs[0], "%v%s", err, suffix)
+			continue
+		}
+
+		lines := bytes.Split(v.([]byte), []byte("\n"))
 		for i, ref := range fi.refs {
 			printf(ref, "%s", lines[fi.linenums[i]-1])
 		}

oracle/testdata/src/referrers/main.go

@@ -24,3 +24,11 @@ func main() {
 	var s2 s
 	s2.f = 1
 }
+
+// Test //line directives:
+
+type U int // @referrers ref-type-U "U"
+
+//line nosuchfile.y:123
+var u1 U
+var u2 U

oracle/testdata/src/referrers/main.golden

@@ -36,3 +36,7 @@
 	_ = s{}.f // @referrers ref-field "f"
 	s2.f = 1
 
+-------- @referrers ref-type-U --------
+2 references to type U int
+open testdata/src/referrers/nosuchfile.y: no such file or directory (+ 1 more refs in this file)
+