Cgo respects build constraints. Simplify implementation.

Author: petermattis

internal/goid13/goid13.go goid_go1.3.c

@@ -13,8 +13,11 @@
 // permissions and limitations under the License. See the AUTHORS file
 // for names of contributors.
 
-// Package goid provides a function to get an identifier for the current goroutine.
-// This package is not compatible with go 1.4 and higher.
-package goid13
+// +build !go1.4
 
-func GetGoID() int64
+#include <runtime.h>
+
+void ·Get(int64 ret) {
+  ret = g->goid;
+  USED(&ret);
+}

goid_go1.3.go

@@ -13,13 +13,8 @@
 // permissions and limitations under the License. See the AUTHORS file
 // for names of contributors.
 
-// On go 1.3 (and older), we can import the goid13 package.
 // +build !go1.4
 
 package goid
 
-import "github.com/petermattis/goid/internal/goid13"
-
-func GoID() int64 {
-	return goid13.GetGoID()
-}
+func Get() int64

goid_go1.4.go

@@ -13,13 +13,21 @@
 // permissions and limitations under the License. See the AUTHORS file
 // for names of contributors.
 
-// On go 1.3 (and newer), we can import the goid14 package.
 // +build go1.4
 
 package goid
 
-import "github.com/petermattis/goid/internal/goid14"
+import "unsafe"
 
-func GoID() int64 {
-	return goid14.GetGoID()
+var pointerSize = unsafe.Sizeof(uintptr(0))
+
+// Backdoor access to runtime·getg().
+func getg() uintptr // in goid_go1.4.s
+
+func Get() int64 {
+	// The goid is the 16th field in the G struct where each field is a
+	// pointer, uintptr or padded to that size. See runtime.h from the
+	// Go sources. I'm not aware of a cleaner way to determine the
+	// offset.
+	return *(*int64)(unsafe.Pointer(getg() + 16*pointerSize))
 }

internal/goid14/goid14.s goid_go1.4.s

@@ -26,7 +26,7 @@ import (
 // Parse the goid from runtime.Stack() output. Slow, but it works.
 var goroutineRE = regexp.MustCompile(`^goroutine\s+(\d+)\s+.*`)
 
-func getGoIDSlow() int64 {
+func getSlow() int64 {
 	var buf [1024]byte
 	s := buf[0:runtime.Stack(buf[:], false)]
 	m := goroutineRE.FindSubmatch(s)
@@ -37,12 +37,12 @@ func getGoIDSlow() int64 {
 	return v
 }
 
-func TestGetGoID(t *testing.T) {
+func TestGet(t *testing.T) {
 	ch := make(chan *string, 100)
 	for i := 0; i < cap(ch); i++ {
 		go func(i int) {
-			goid := GoID()
-			expected := getGoIDSlow()
+			goid := Get()
+			expected := getSlow()
 			if goid == expected {
 				ch <- nil
 				return