debug/dwarf: add test for split DWARF

This adds a test that debug/dwarf can read the skeleton DWARF data
from a split DWARF image (though it doesn't currently support piecing
the external DWARF data back together). This should work because
there's nothing particularly different about skeleton DWARF data, but
previously failed because of poor handling of unrecognized attributes.

Updates #12592.

Change-Id: I2fc5f4679883b05ebd7ec9f0b5c398a758181a32
Reviewed-on: https://go-review.googlesource.com/14542Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: jcd . <jcd@golang.org>

src/debug/dwarf/entry_test.go

+// Copyright 2009 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.
+
+package dwarf_test
+
+import (
+	. "debug/dwarf"
+	"testing"
+)
+
+func TestSplit(t *testing.T) {
+	// debug/dwarf doesn't (currently) support split DWARF, but
+	// the attributes that pointed to the split DWARF used to
+	// cause loading the DWARF data to fail entirely (issue
+	// #12592). Test that we can at least read the DWARF data.
+	d := elfData(t, "testdata/split.elf")
+	r := d.Reader()
+	e, err := r.Next()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if e.Tag != TagCompileUnit {
+		t.Fatalf("bad tag: have %s, want %s", e.Tag, TagCompileUnit)
+	}
+	// Check that we were able to parse the unknown section offset
+	// field, even if we can't figure out its DWARF class.
+	const AttrGNUAddrBase Attr = 0x2133
+	f := e.AttrField(AttrGNUAddrBase)
+	if _, ok := f.Val.(int64); !ok {
+		t.Fatalf("bad attribute value type: have %T, want int64", f.Val)
+	}
+	if f.Class != ClassUnknown {
+		t.Fatalf("bad class: have %s, want %s", f.Class, ClassUnknown)
+	}
+}

src/debug/dwarf/testdata/split.c

+// gcc -gsplit-dwarf split.c -o split.elf
+
+int main() 
+{
+}