Fix: Improve handling of soft signals on darwin

Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

_fixtures/sigchldprog.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"os/exec"
+)
+
+func main() {
+	cmd := exec.Command("date")
+	reader, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Fatalln(err)
+	}
+	scanner := bufio.NewScanner(reader)
+	go func() {
+		for scanner.Scan() {
+			fmt.Println(scanner.Text())
+		}
+		reader.Close()
+	}()
+	cmd.Start()
+	cmd.Wait()
+}

proc/proc_darwin.c

@@ -113,6 +113,8 @@ mach_port_t
 mach_port_wait(mach_port_t port_set) {
 	kern_return_t kret;
 	thread_act_t thread;
+	NDR_record_t *ndr;
+	integer_t *data;
 	union
 	{
 		mach_msg_header_t hdr;
@@ -128,28 +130,21 @@ mach_port_wait(mach_port_t port_set) {
 	mach_msg_body_t *bod = (mach_msg_body_t*)(&msg.hdr + 1);
 	mach_msg_port_descriptor_t *desc = (mach_msg_port_descriptor_t *)(bod + 1);
 	thread = desc[0].name;
+	ndr = (NDR_record_t *)(desc + 2);
+	data = (integer_t *)(ndr + 1);
 
 	switch (msg.hdr.msgh_id) {
 		case 2401: // Exception
-			kret = thread_suspend(thread);
+			if (thread_suspend(thread) != KERN_SUCCESS) return 0;
-			if (kret != KERN_SUCCESS) return 0;
-
 			// Send our reply back so the kernel knows this exception has been handled.
-			mig_reply_error_t reply;
+			kret = mach_send_reply(msg.hdr);
-			mach_msg_header_t *rh = &reply.Head;
-			rh->msgh_bits = MACH_MSGH_BITS(MACH_MSGH_BITS_REMOTE(msg.hdr.msgh_bits), 0);
-			rh->msgh_remote_port = msg.hdr.msgh_remote_port;
-			rh->msgh_size = (mach_msg_size_t) sizeof(mig_reply_error_t);
-			rh->msgh_local_port = MACH_PORT_NULL;
-			rh->msgh_id = msg.hdr.msgh_id + 100;
-
-			reply.NDR = NDR_record;
-			reply.RetCode = KERN_SUCCESS;
-
-			kret = mach_msg(&reply.Head, MACH_SEND_MSG|MACH_SEND_INTERRUPT, rh->msgh_size, 0,
-					MACH_PORT_NULL, MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);
-
 			if (kret != MACH_MSG_SUCCESS) return 0;
+			if (data[2] == EXC_SOFT_SIGNAL) {
+				if (data[3] != SIGTRAP) {
+					if (thread_resume(thread) != KERN_SUCCESS) return 0;
+					return mach_port_wait(port_set);
+				}
+			}
 			break;
 
 		case 72: // Death
@@ -159,6 +154,23 @@ mach_port_wait(mach_port_t port_set) {
 	return thread;
 }
 
+kern_return_t
+mach_send_reply(mach_msg_header_t hdr) {
+	mig_reply_error_t reply;
+	mach_msg_header_t *rh = &reply.Head;
+	rh->msgh_bits = MACH_MSGH_BITS(MACH_MSGH_BITS_REMOTE(hdr.msgh_bits), 0);
+	rh->msgh_remote_port = hdr.msgh_remote_port;
+	rh->msgh_size = (mach_msg_size_t) sizeof(mig_reply_error_t);
+	rh->msgh_local_port = MACH_PORT_NULL;
+	rh->msgh_id = hdr.msgh_id + 100;
+
+	reply.NDR = NDR_record;
+	reply.RetCode = KERN_SUCCESS;
+
+	return mach_msg(&reply.Head, MACH_SEND_MSG|MACH_SEND_INTERRUPT, rh->msgh_size, 0,
+			MACH_PORT_NULL, MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);
+}
+
 kern_return_t
 raise_exception(mach_port_t task, mach_port_t thread, mach_port_t exception_port, exception_type_t exception) {
 	return exception_raise(exception_port, thread, task, exception, 0, 0);

proc/proc_darwin.go

@@ -301,26 +301,25 @@ func (dbp *Process) trapWait(pid int) (*Thread, error) {
 		dbp.updateThreadList()
 		th, err = dbp.handleBreakpointOnThread(int(port))
 		if err != nil {
-			if _, ok := err.(NoBreakpointError); ok {
+			if _, ok := err.(NoBreakpointError); !ok {
+				return nil, err
+			}
+			thread := dbp.Threads[int(port)]
 			if dbp.halt {
 				dbp.halt = false
-					return dbp.Threads[int(port)], nil
+				return thread, nil
 			}
-				th := dbp.Threads[int(port)]
+			if dbp.firstStart || thread.singleStepping {
-				if dbp.firstStart || th.singleStepping {
 				dbp.firstStart = false
-					return dbp.Threads[int(port)], nil
+				return thread, nil
 			}
-				if err := th.Continue(); err != nil {
+			if err := thread.Continue(); err != nil {
 				return nil, err
 			}
 			continue
 		}
-			return nil, err
-		}
 		return th, nil
 	}
-	return th, nil
 }
 
 func wait(pid, tgid, options int) (int, *sys.WaitStatus, error) {

proc/proc_darwin.h

@@ -38,6 +38,9 @@ thread_count(task_t task);
 mach_port_t
 mach_port_wait(mach_port_t);
 
+kern_return_t
+mach_send_reply(mach_msg_header_t);
+
 kern_return_t
 raise_exception(mach_port_t, mach_port_t, mach_port_t, exception_type_t);
 

proc/proc_test.go

@@ -694,3 +694,13 @@ func TestBreakpointOnFunctionEntry(t *testing.T) {
 		}
 	})
 }
+
+func TestProcessReceivesSIGCHLD(t *testing.T) {
+	withTestProcess("sigchldprog", t, func(p *Process, fixture protest.Fixture) {
+		err := p.Continue()
+		_, ok := err.(ProcessExitedError)
+		if !ok {
+			t.Fatalf("Continue() returned unexpected error type %s", err)
+		}
+	})
+}