proc: do not panic if we can't satisfy a composite location for a slice var

The fix in 7f53117e for Issue #1416 had a bug, fix it and add a test.

Fixes #1419

pkg/dwarf/dwarfbuilder/info.go

@@ -5,6 +5,7 @@ import (
 	"debug/dwarf"
 	"encoding/binary"
 
+	"github.com/derekparker/delve/pkg/dwarf/godwarf"
 	"github.com/derekparker/delve/pkg/dwarf/util"
 )
 
@@ -282,3 +283,12 @@ func (b *Builder) AddMember(fieldname string, typ dwarf.Offset, memberLoc []byte
 	b.TagClose()
 	return r
 }
+
+// AddPointerType adds a new pointer type to debug_info.
+func (b *Builder) AddPointerType(typename string, typ dwarf.Offset) dwarf.Offset {
+	r := b.TagOpen(dwarf.TagPointerType, typename)
+	b.Attr(dwarf.AttrType, typ)
+	b.Attr(godwarf.AttrGoKind, uint8(22))
+	b.TagClose()
+	return r
+}

pkg/proc/dwarf_expr_test.go

@@ -142,10 +142,7 @@ func TestDwarfExprComposite(t *testing.T) {
 
 	byteoff := dwb.AddBaseType("uint8", dwarfbuilder.DW_ATE_unsigned, 1)
 
-	byteptroff := dwb.TagOpen(dwarf.TagPointerType, "*uint8")
-	dwb.Attr(godwarf.AttrGoKind, uint8(22))
-	dwb.Attr(dwarf.AttrType, byteoff)
-	dwb.TagClose()
+	byteptroff := dwb.AddPointerType("*uint8", byteoff)
 
 	pairoff := dwb.AddStructType("main.pair", 4)
 	dwb.Attr(godwarf.AttrGoKind, uint8(25))
@@ -222,3 +219,44 @@ func TestDwarfExprLoclist(t *testing.T) {
 	scope.PC = 0x40800
 	uintExprCheck(t, scope, "a", after)
 }
+
+func TestIssue1419(t *testing.T) {
+	// trying to read a slice variable with a location list that tries to read
+	// from registers we don't have should not cause a panic.
+
+	dwb := dwarfbuilder.New()
+
+	uint64off := dwb.AddBaseType("uint64", dwarfbuilder.DW_ATE_unsigned, 8)
+	intoff := dwb.AddBaseType("int", dwarfbuilder.DW_ATE_signed, 8)
+	intptroff := dwb.AddPointerType("*int", intoff)
+
+	sliceoff := dwb.AddStructType("[]int", 24)
+	dwb.Attr(godwarf.AttrGoKind, uint8(23))
+	dwb.AddMember("array", intptroff, dwarfbuilder.LocationBlock(op.DW_OP_plus_uconst, uint(0)))
+	dwb.AddMember("len", uint64off, dwarfbuilder.LocationBlock(op.DW_OP_plus_uconst, uint(8)))
+	dwb.AddMember("cap", uint64off, dwarfbuilder.LocationBlock(op.DW_OP_plus_uconst, uint(16)))
+	dwb.TagClose()
+
+	dwb.AddSubprogram("main.main", 0x40100, 0x41000)
+	dwb.AddVariable("a", sliceoff, dwarfbuilder.LocationBlock(op.DW_OP_reg2, op.DW_OP_piece, uint(8), op.DW_OP_reg2, op.DW_OP_piece, uint(8), op.DW_OP_reg2, op.DW_OP_piece, uint(8)))
+	dwb.TagClose()
+
+	bi := fakeBinaryInfo(t, dwb)
+
+	mainfn := bi.LookupFunc["main.main"]
+
+	mem := newFakeMemory(defaultCFA)
+
+	scope := &proc.EvalScope{Location: proc.Location{PC: 0x40100, Fn: mainfn}, Regs: op.DwarfRegisters{}, Mem: mem, Gvar: nil, BinInfo: bi}
+
+	va, err := scope.EvalExpression("a", normalLoadConfig)
+	assertNoError(err, t, "EvalExpression(a)")
+	t.Logf("%#x\n", va.Addr)
+	t.Logf("%v", va)
+	if va.Unreadable == nil {
+		t.Fatalf("expected 'a' to be unreadable but it wasn't")
+	}
+	if va.Unreadable.Error() != "could not read 8 bytes from register 2 (size: 0)" {
+		t.Fatalf("wrong unreadable reason for variable 'a': %v", va.Unreadable)
+	}
+}

pkg/proc/variables.go

@@ -908,9 +908,10 @@ func (scope *EvalScope) extractVarInfoFromEntry(varEntry *dwarf.Entry) (*Variabl
 	mem := scope.Mem
 	if pieces != nil {
 		addr = fakeAddress
-		mem, err = newCompositeMemory(scope.Mem, scope.Regs, pieces)
-		if mem == nil {
-			mem = scope.Mem
+		var cmem *compositeMemory
+		cmem, err = newCompositeMemory(scope.Mem, scope.Regs, pieces)
+		if cmem != nil {
+			mem = cmem
 		}
 	}