os: report Windows exit status in hex

We print things like “exit status 3221225477”
but the standard Windows form is 0xc0000005.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: Iefe447d4d1781b53bef9619f68d386f2866b2934
Reviewed-on: https://go-review.googlesource.com/c/go/+/288792
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/exec_posix.go

@@ -102,7 +102,12 @@ func (p *ProcessState) String() string {
 	res := ""
 	switch {
 	case status.Exited():
-		res = "exit status " + itoa(status.ExitStatus())
+		code := status.ExitStatus()
+		if runtime.GOOS == "windows" && code >= 1<<16 { // windows uses large hex numbers
+			res = "exit status " + uitox(uint(code))
+		} else { // unix systems use small decimal integers
+			res = "exit status " + itoa(code) // unix
+		}
 	case status.Signaled():
 		res = "signal: " + status.Signal().String()
 	case status.Stopped():

src/os/str.go

@@ -6,7 +6,7 @@
 
 package os
 
-// Convert integer to decimal string
+// itoa converts val (an int) to a decimal string.
 func itoa(val int) string {
 	if val < 0 {
 		return "-" + uitoa(uint(-val))
@@ -14,7 +14,7 @@ func itoa(val int) string {
 	return uitoa(uint(val))
 }
 
-// Convert unsigned integer to decimal string
+// uitoa converts val (a uint) to a decimal string.
 func uitoa(val uint) string {
 	if val == 0 { // avoid string allocation
 		return "0"
@@ -31,3 +31,35 @@ func uitoa(val uint) string {
 	buf[i] = byte('0' + val)
 	return string(buf[i:])
 }
+
+// itox converts val (an int) to a hexdecimal string.
+func itox(val int) string {
+	if val < 0 {
+		return "-" + uitox(uint(-val))
+	}
+	return uitox(uint(val))
+}
+
+const hex = "0123456789abcdef"
+
+// uitox converts val (a uint) to a hexdecimal string.
+func uitox(val uint) string {
+	if val == 0 { // avoid string allocation
+		return "0x0"
+	}
+	var buf [20]byte // big enough for 64bit value base 16 + 0x
+	i := len(buf) - 1
+	for val >= 16 {
+		q := val / 16
+		buf[i] = hex[val%16]
+		i--
+		val = q
+	}
+	// val < 16
+	buf[i] = hex[val%16]
+	i--
+	buf[i] = 'x'
+	i--
+	buf[i] = '0'
+	return string(buf[i:])
+}