proc,prettyprint: guard against autodereferenced escaped pointers (#1077)

Much like the bug in issue #1031 and commit
f6f6f0bf13e4c708cb501202b83a6327a0f00e31 pointers can also escape to
the heap and then have a zero address (and no children) when we
autodereference.

1. Mark autodereferenced escaped variables with a 0 address as
   unreadable.
2. Add guards to the pretty printers for unsafe.Pointer and pointers.

Fixes #1075

_fixtures/clientdo.go

@@ -0,0 +1,10 @@
+package main
+
+import (
+	"net/http"
+	"net/url"
+)
+
+func main() {
+	http.DefaultClient.Do(&http.Request{URL: &url.URL{}})
+}

pkg/proc/variables.go

@@ -1938,6 +1938,9 @@ func (scope *EvalScope) variablesByTag(tag dwarf.Tag, cfg *LoadConfig) ([]*Varia
 	for i, v := range vars {
 		if name := v.Name; len(name) > 1 && name[0] == '&' {
 			v = v.maybeDereference()
+			if v.Addr == 0 {
+				v.Unreadable = fmt.Errorf("no address for escaped variable")
+			}
 			v.Name = name[1:]
 			v.Flags |= VariableEscaped
 			vars[i] = v

service/api/prettyprint.go

@@ -49,7 +49,7 @@ func (v *Variable) writeTo(buf io.Writer, top, newlines, includeType bool, inden
 	case reflect.Array:
 		v.writeArrayTo(buf, newlines, includeType, indent)
 	case reflect.Ptr:
-		if v.Type == "" {
+		if v.Type == "" || len(v.Children) == 0 {
 			fmt.Fprint(buf, "nil")
 		} else if v.Children[0].OnlyAddr && v.Children[0].Addr != 0 {
 			fmt.Fprintf(buf, "(%s)(0x%x)", v.Type, v.Children[0].Addr)
@@ -58,7 +58,11 @@ func (v *Variable) writeTo(buf io.Writer, top, newlines, includeType bool, inden
 			v.Children[0].writeTo(buf, false, newlines, includeType, indent)
 		}
 	case reflect.UnsafePointer:
+		if len(v.Children) == 0 {
+			fmt.Fprintf(buf, "unsafe.Pointer(nil)")
+		} else {
 			fmt.Fprintf(buf, "unsafe.Pointer(0x%x)", v.Children[0].Addr)
+		}
 	case reflect.String:
 		v.writeStringTo(buf)
 	case reflect.Chan:

service/test/variables_test.go

@@ -976,3 +976,28 @@ func TestConstants(t *testing.T) {
 		}
 	})
 }
+
+func setFunctionBreakpoint(p proc.Process, fname string) (*proc.Breakpoint, error) {
+	addr, err := proc.FindFunctionLocation(p, fname, true, 0)
+	if err != nil {
+		return nil, err
+	}
+	return p.SetBreakpoint(addr, proc.UserBreakpoint, nil)
+}
+
+func TestIssue1075(t *testing.T) {
+	withTestProcess("clientdo", t, func(p proc.Process, fixture protest.Fixture) {
+		_, err := setFunctionBreakpoint(p, "net/http.(*Client).Do")
+		assertNoError(err, t, "setFunctionBreakpoint")
+		assertNoError(proc.Continue(p), t, "Continue()")
+		for i := 0; i < 10; i++ {
+			scope, err := proc.GoroutineScope(p.CurrentThread())
+			assertNoError(err, t, fmt.Sprintf("GoroutineScope (%d)", i))
+			vars, err := scope.LocalVariables(pnormalLoadConfig)
+			assertNoError(err, t, fmt.Sprintf("LocalVariables (%d)", i))
+			for _, v := range vars {
+				api.ConvertVar(v).SinglelineString()
+			}
+		}
+	})
+}