gdbserial: propagate signals to target while stepping (go-delve#1624)

Propagate signals when stepping because debugserver will report them,
from the issue:

2019-07-11T16:31:25+02:00 debug layer=gdbconn <- $z0,105525d,1#c9
2019-07-11T16:31:25+02:00 debug layer=gdbconn -> $OK#00
2019-07-11T16:31:25+02:00 debug layer=gdbconn <- $vCont;s:c41c3#50
2019-07-11T16:31:25+02:00 debug layer=gdbconn -> $T1cthread:c41c3;threads:c41c3,c41d7,c41d8,c41d9,c41da;thread-pcs:105525d,7fffc464bf46,7fffc464bbf2,7fffc464bbf2,7fffc46...
2019-07-11T16:31:25+02:00 debug layer=gdbconn <- $Z0,105525d,1#a9
2019-07-11T16:31:25+02:00 debug layer=gdbconn -> $OK#00

in this case we request a single step on thread c41c3 but debugserver
reports instead a signal (in this case SIGWINCH).

Fixes go-delve#1610

pkg/proc/gdbserial/gdbserver.go

@@ -610,8 +610,16 @@ func (p *Process) SelectedGoroutine() *proc.G {
 const (
 	interruptSignal  = 0x2
 	breakpointSignal = 0x5
+	faultSignal      = 0xb
 	childSignal      = 0x11
 	stopSignal       = 0x13
+
+	debugServerTargetExcBadAccess      = 0x91
+	debugServerTargetExcBadInstruction = 0x92
+	debugServerTargetExcArithmetic     = 0x93
+	debugServerTargetExcEmulation      = 0x94
+	debugServerTargetExcSoftware       = 0x95
+	debugServerTargetExcBreakpoint     = 0x96
 )
 
 // ContinueOnce will continue execution of the process until
@@ -677,7 +685,8 @@ continueLoop:
 		// The following are fake BSD-style signals sent by debugserver
 		// Unfortunately debugserver can not convert them into signals for the
 		// process so we must stop here.
-		case 0x91, 0x92, 0x93, 0x94, 0x95, 0x96: /* TARGET_EXC_BAD_ACCESS */
+		case debugServerTargetExcBadAccess, debugServerTargetExcBadInstruction, debugServerTargetExcArithmetic, debugServerTargetExcEmulation, debugServerTargetExcSoftware, debugServerTargetExcBreakpoint:
+
 			break continueLoop
 
 		// Signal 0 is returned by rr when it reaches the start of the process
@@ -1274,8 +1283,7 @@ func (t *Thread) stepInstruction(tu *threadUpdater) error {
 		}
 		defer t.p.conn.setBreakpoint(pc)
 	}
-	_, _, err := t.p.conn.step(t.strID, tu)
-	return err
+	return t.p.conn.step(t.strID, tu, false)
 }
 
 // StepInstruction will step exactly 1 CPU instruction.
@@ -1485,7 +1493,7 @@ func (t *Thread) reloadGAtPC() error {
 		}
 	}()
 
-	_, _, err = t.p.conn.step(t.strID, nil)
+	err = t.p.conn.step(t.strID, nil, true)
 	if err != nil {
 		if err == threadBlockedError {
 			t.regs.tls = 0
@@ -1538,7 +1546,7 @@ func (t *Thread) reloadGAlloc() error {
 		}
 	}()
 
-	_, _, err = t.p.conn.step(t.strID, nil)
+	err = t.p.conn.step(t.strID, nil, true)
 	if err != nil {
 		if err == threadBlockedError {
 			t.regs.tls = 0

pkg/proc/gdbserial/gdbserver_conn.go

@@ -575,21 +575,51 @@ func (conn *gdbConn) resume(sig uint8, tu *threadUpdater) (string, uint8, error)
 }
 
 // step executes a 'vCont' command on the specified thread with 's' action.
-func (conn *gdbConn) step(threadID string, tu *threadUpdater) (string, uint8, error) {
-	if conn.direction == proc.Forward {
-		conn.outbuf.Reset()
-		fmt.Fprintf(&conn.outbuf, "$vCont;s:%s", threadID)
-	} else {
+func (conn *gdbConn) step(threadID string, tu *threadUpdater, ignoreFaultSignal bool) error {
+	if conn.direction != proc.Forward {
 		if err := conn.selectThread('c', threadID, "step"); err != nil {
-			return "", 0, err
+			return err
 		}
 		conn.outbuf.Reset()
 		fmt.Fprint(&conn.outbuf, "$bs")
+		if err := conn.send(conn.outbuf.Bytes()); err != nil {
+			return err
+		}
+		_, _, err := conn.waitForvContStop("singlestep", threadID, tu)
+		return err
 	}
-	if err := conn.send(conn.outbuf.Bytes()); err != nil {
-		return "", 0, err
+	var sig uint8 = 0
+	for {
+		conn.outbuf.Reset()
+		if sig == 0 {
+			fmt.Fprintf(&conn.outbuf, "$vCont;s:%s", threadID)
+		} else {
+			fmt.Fprintf(&conn.outbuf, "$vCont;S%02x:%s", sig, threadID)
+		}
+		if err := conn.send(conn.outbuf.Bytes()); err != nil {
+			return err
+		}
+		var err error
+		_, sig, err = conn.waitForvContStop("singlestep", threadID, tu)
+		if err != nil {
+			return err
+		}
+		switch sig {
+		case faultSignal:
+			if ignoreFaultSignal { // we attempting to read the TLS, a fault here should be ignored
+				return nil
+			}
+		case interruptSignal, breakpointSignal, stopSignal:
+			return nil
+		case childSignal: // stop on debugserver but SIGCHLD on lldb-server/linux
+			if conn.isDebugserver {
+				return nil
+			}
+		case debugServerTargetExcBadAccess, debugServerTargetExcBadInstruction, debugServerTargetExcArithmetic, debugServerTargetExcEmulation, debugServerTargetExcSoftware, debugServerTargetExcBreakpoint:
+			return nil
+		}
+		// any other signal is propagated to the inferior
 	}
-	return conn.waitForvContStop("singlestep", threadID, tu)
 }
 
 var threadBlockedError = errors.New("thread blocked")