debug/dwarf: more graceful handling of unsupported types

Enhance the type decoder to do a better job handling unknown type
tags. DWARF has a number of type DIEs that this package doesn't handle
(things like "pointer to member" types in C++); avoid crashing for
such types, but instead return a placeholder "UnsupportedType" object
(this idea suggested by Austin). This provides a compromise between
implementing the entire kitchen sink and simply returning an error
outright on any unknown type DIE.

Fixes #29601.

Change-Id: I2eeffa094c86ef3a2c358ee42e8e629d74cec2ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/158797
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 34 insertions, 0 deletions

diff --git a/src/debug/dwarf/testdata/cppunsuptypes.cc b/src/debug/dwarf/testdata/cppunsuptypes.cc
new file mode 100644
index 0000000000..e9281c7dec
--- /dev/null
+++ b/src/debug/dwarf/testdata/cppunsuptypes.cc
@@ -0,0 +1,34 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// cppunsuptypes.elf built with g++ 7.3
+//    g++ -g -c -o cppunsuptypes.elf cppunsuptypes.cc
+
+int i = 3;
+double d = 3;
+
+// anonymous reference type
+int &culprit = i;
+
+// named reference type
+typedef double &dref;
+dref dr = d;
+
+// incorporated into another type
+typedef struct {
+  dref q;
+  int &r;
+} hasrefs;
+
+hasrefs hr = { d, i };
+
+// This code is intended to trigger a DWARF "pointer to member" type DIE
+struct CS { int dm; };
+
+int foo()
+{
+  int CS::* pdm = &CS::dm;
+  CS cs = {42};
+  return cs.*pdm;
+}
diff --git a/src/debug/dwarf/testdata/cppunsuptypes.elf b/src/debug/dwarf/testdata/cppunsuptypes.elf
new file mode 100644
index 0000000000..e955512ecd
--- /dev/null
+++ b/src/debug/dwarf/testdata/cppunsuptypes.elf