runtime: fix, simplify, and improve scan state in goroutine header

Currently goroutineheader goes through some convolutions to *almost*
print the scan state of a G. However, the code path that would print
the scan state of the G refers to gStatusStrings where it almost
certainly meant to refer to gScanStatusStrings (which is unused), so
it winds up printing the regular status string without the scan state
either way. Furthermore, if the G is in _Gwaiting, we override the
status string and lose where this would indicate the scan state if it
worked.

This commit fixes this so the runtime prints the scan state. However,
rather than using a parallel list of status strings, this simply adds
a conditional print if the scan bit is set. This lets us remove the
string list, prints the scan state even in _Gwaiting, and lets us
strip off the scan bit at the beginning of the function, which
simplifies the rest of it.

Change-Id: Ic0adbe5c05abf4adda93da59f93b578172b28e3d
Reviewed-on: https://go-review.googlesource.com/18092Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/traceback.go

@@ -645,41 +645,34 @@ var gStatusStrings = [...]string{
 	_Gcopystack: "copystack",
 }
 
-var gScanStatusStrings = [...]string{
-	0:          "scan",
-	_Grunnable: "scanrunnable",
-	_Grunning:  "scanrunning",
-	_Gsyscall:  "scansyscall",
-	_Gwaiting:  "scanwaiting",
-	_Gdead:     "scandead",
-	_Genqueue:  "scanenqueue",
-}
-
 func goroutineheader(gp *g) {
 	gpstatus := readgstatus(gp)
 
+	isScan := gpstatus&_Gscan != 0
+	gpstatus &^= _Gscan // drop the scan bit
+
 	// Basic string status
 	var status string
 	if 0 <= gpstatus && gpstatus < uint32(len(gStatusStrings)) {
 		status = gStatusStrings[gpstatus]
-	} else if gpstatus&_Gscan != 0 && 0 <= gpstatus&^_Gscan && gpstatus&^_Gscan < uint32(len(gStatusStrings)) {
-		status = gStatusStrings[gpstatus&^_Gscan]
 	} else {
 		status = "???"
 	}
 
 	// Override.
-	if (gpstatus == _Gwaiting || gpstatus == _Gscanwaiting) && gp.waitreason != "" {
+	if gpstatus == _Gwaiting && gp.waitreason != "" {
 		status = gp.waitreason
 	}
 
 	// approx time the G is blocked, in minutes
 	var waitfor int64
-	gpstatus &^= _Gscan // drop the scan bit
 	if (gpstatus == _Gwaiting || gpstatus == _Gsyscall) && gp.waitsince != 0 {
 		waitfor = (nanotime() - gp.waitsince) / 60e9
 	}
 	print("goroutine ", gp.goid, " [", status)
+	if isScan {
+		print(" (scan)")
+	}
 	if waitfor >= 1 {
 		print(", ", waitfor, " minutes")
 	}