Improve Next implementation

Fix bug involving detecting whether or not we have stepped into another
function when we plan on return from the function we are currently in.

_fixtures/testnextprog.go

@@ -37,7 +37,7 @@ func testnext() {
 func main() {
 	runtime.LockOSThread()
 	for {
-		helloworld()
 		testnext()
+		fmt.Println("foo")
 	}
 }

dwarf/frame/table_test.go

@@ -56,7 +56,7 @@ func TestFindReturnAddress(t *testing.T) {
 		syscall.PtracePeekText(p.Pid, uintptr(addr), data)
 		addr = binary.LittleEndian.Uint64(data)
 
-		expected := uint64(0x400ed4)
+		expected := uint64(0x400ed3)
 		if addr != expected {
 			t.Fatalf("return address not found correctly, expected %#v got %#v", expected, addr)
 		}

proctl/proctl_linux_amd64.go

@@ -254,27 +254,28 @@ func (dbp *DebuggedProcess) Next() error {
 
 	loc := dbp.DebugLine.NextLocation(pc, f, l)
 	if !fde.Cover(loc.Address) {
-		// Step once to ensure we're not going to step
-		// into another function before returning.
-		pc, err = step()
-		if err != nil {
-			return err
+		ret := dbp.ReturnAddressFromOffset(fde.ReturnAddressOffset(pc))
+
+		// Attempt to step out of function.
+		for fde.Cover(pc) {
+			pc, err = step()
+			if err != nil {
+				return err
+			}
 		}
 
-		if fde.Cover(pc) {
-			// Unconditionally step out of current function
-			// Don't bother looking up ret addr, next line is
-			// outside of current fn, should only be a few
-			// instructions left to RET
-			for fde.Cover(pc) {
-				pc, err = step()
-				if err != nil {
-					return err
-				}
-			}
+		if pc == ret {
 			return nil
 		}
 
+		// We have stepped into another function, return from it
+		// and continue single stepping through until we
+		// reach our real destination.
+		err = dbp.continueToReturnAddress(pc, fde)
+		if err != nil {
+			return err
+		}
+
 	}
 
 	for {

proctl/proctl_test.go

@@ -177,9 +177,9 @@ func TestNext(t *testing.T) {
 		{26, 27},
 		{27, 34},
 		{34, 35},
-		{35, 40},
-		{40, 41},
+		{35, 41},
 		{41, 40},
+		{40, 41},
 	}
 
 	fp, err := filepath.Abs("../_fixtures/testnextprog.go")