runtime/pprof: handle empty stack traces in Profile.Add

If the caller passes a large number to Profile.Add,
the list of pcs is empty, which results in junk
(a nil pc) being recorded. Check for that explicitly,
and replace such stack traces with a lostProfileEvent.

Fixes #18836.

Change-Id: I99c96aa67dd5525cd239ea96452e6e8fcb25ce02
Reviewed-on: https://go-review.googlesource.com/36891Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/pprof/pprof.go

@@ -267,13 +267,18 @@ func (p *Profile) Add(value interface{}, skip int) {
 
 	stk := make([]uintptr, 32)
 	n := runtime.Callers(skip+1, stk[:])
+	stk = stk[:n]
+	if len(stk) == 0 {
+		// The value for skip is too large, and there's no stack trace to record.
+		stk = []uintptr{funcPC(lostProfileEvent)}
+	}
 
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if p.m[value] != nil {
 		panic("pprof: Profile.Add of duplicate value")
 	}
-	p.m[value] = stk[:n]
+	p.m[value] = stk
 }
 
 // Remove removes the execution stack associated with value from the profile.

src/runtime/pprof/pprof_test.go

@@ -635,3 +635,22 @@ func containsCounts(prof *profile.Profile, counts []int64) bool {
 	}
 	return true
 }
+
+// Issue 18836.
+func TestEmptyCallStack(t *testing.T) {
+	t.Parallel()
+	var buf bytes.Buffer
+	p := NewProfile("test18836")
+	p.Add("foo", 47674)
+	p.WriteTo(&buf, 1)
+	p.Remove("foo")
+	got := buf.String()
+	prefix := "test18836 profile: total 1\n"
+	if !strings.HasPrefix(got, prefix) {
+		t.Fatalf("got:\n\t%q\nwant prefix:\n\t%q\n", got, prefix)
+	}
+	lostevent := "lostProfileEvent"
+	if !strings.Contains(got, lostevent) {
+		t.Fatalf("got:\n\t%q\ndoes not contain:\n\t%q\n", got, lostevent)
+	}
+}