runtime: fix GDB goroutine N command when N is running

The current implementation of "goroutine N cmd" assumes it can get
goroutine N's state from the goroutine's sched buffer. But this only
works if the goroutine is blocked. Extend find_goroutine so that, if
there is no saved scheduler state for a goorutine, it tries to find
the thread the goroutine is running on and use the thread's current
register state. We also extend find_goroutine to understand saved
syscall register state.

Fixes #13887.

Change-Id: I739008a8987471deaa4a9da918655e4042cf969b
Reviewed-on: https://go-review.googlesource.com/45031
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime-gdb.py

@@ -416,8 +416,37 @@ def find_goroutine(goid):
 		if ptr['atomicstatus'] == 6:  # 'gdead'
 			continue
 		if ptr['goid'] == goid:
-			return (ptr['sched'][x].cast(vp) for x in ('pc', 'sp'))
-	return None, None
+			break
+	else:
+		return None, None
+	# Get the goroutine's saved state.
+	pc, sp = ptr['sched']['pc'], ptr['sched']['sp']
+	# If the goroutine is stopped, sched.sp will be non-0.
+	if sp != 0:
+		return pc.cast(vp), sp.cast(vp)
+	# If the goroutine is in a syscall, use syscallpc/sp.
+	pc, sp = ptr['syscallpc'], ptr['syscallsp']
+	if sp != 0:
+		return pc.cast(vp), sp.cast(vp)
+	# Otherwise, the goroutine is running, so it doesn't have
+	# saved scheduler state. Find G's OS thread.
+	m = ptr['m']
+	if m == 0:
+		return None, None
+	for thr in gdb.selected_inferior().threads():
+		if thr.ptid[1] == m['procid']:
+			break
+        else:
+		return None, None
+	# Get scheduler state from the G's OS thread state.
+	curthr = gdb.selected_thread()
+	try:
+		thr.switch()
+		pc = gdb.parse_and_eval('$pc')
+		sp = gdb.parse_and_eval('$sp')
+	finally:
+		curthr.switch()
+	return pc.cast(vp), sp.cast(vp)
 
 
 class GoroutineCmd(gdb.Command):

src/runtime/runtime-gdb_test.go

@@ -157,6 +157,9 @@ func testGdbPython(t *testing.T, cgo bool) {
 		"-ex", "info locals",
 		"-ex", "echo END\n",
 		"-ex", "down", // back to fmt.Println (goroutine 2 below only works at bottom of stack.  TODO: fix that)
+		"-ex", "echo BEGIN goroutine 1 bt\n",
+		"-ex", "goroutine 1 bt",
+		"-ex", "echo END\n",
 		"-ex", "echo BEGIN goroutine 2 bt\n",
 		"-ex", "goroutine 2 bt",
 		"-ex", "echo END\n",
@@ -213,8 +216,13 @@ func testGdbPython(t *testing.T, cgo bool) {
 		t.Fatalf("info locals failed: %s", bl)
 	}
 
-	btGoroutineRe := regexp.MustCompile(`^#0\s+(0x[0-9a-f]+\s+in\s+)?runtime.+at`)
-	if bl := blocks["goroutine 2 bt"]; !btGoroutineRe.MatchString(bl) {
+	btGoroutine1Re := regexp.MustCompile(`(?m)^#0\s+(0x[0-9a-f]+\s+in\s+)?fmt\.Println.+at`)
+	if bl := blocks["goroutine 1 bt"]; !btGoroutine1Re.MatchString(bl) {
+		t.Fatalf("goroutine 1 bt failed: %s", bl)
+	}
+
+	btGoroutine2Re := regexp.MustCompile(`(?m)^#0\s+(0x[0-9a-f]+\s+in\s+)?runtime.+at`)
+	if bl := blocks["goroutine 2 bt"]; !btGoroutine2Re.MatchString(bl) {
 		t.Fatalf("goroutine 2 bt failed: %s", bl)
 	}
 }