deal with removal of forEachG linkname availability

This is arguably a breaking change, but not removing this prevents the
package from building at all on go1.23, so given the universe we are in
this is being treated as a bugfix.

.github/workflows/go.yml

@@ -12,14 +12,13 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest]
         go-version:
-          - 1.14.x
-          - 1.15.x
           - 1.16.x
           - 1.17.x
           - 1.18.x
           - 1.19.x
           - 1.20.x
           - 1.21.x
+          - 1.22.x
     runs-on: ${{ matrix.os }}
 
     steps:
@@ -39,8 +38,6 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest]
         go-version:
-          - 1.14.x
-          - 1.15.x
           - 1.16.x
           - 1.17.x
           - 1.18.x

gid_family_g121.go

@@ -0,0 +1,32 @@
+// Copyright ©2023 Dan Kortschak. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.21 && !go1.23
+// +build go1.21,!go1.23
+
+package goroutine
+
+import "unsafe"
+
+// Link is a goroutine parent-child relationship.
+//
+// Only available for go1.21 and go1.22.
+type Link struct {
+	Parent, Child int64
+}
+
+// All returns all the known goroutine parent-child relationships.
+//
+// Only available for go1.21 and go1.22.
+func All() []Link {
+	var s []Link
+	forEachG(func(g unsafe.Pointer) {
+		s = append(s, Link{Parent: idOf(g, parentGoidoff), Child: idOf(g, goidoff)})
+	})
+	return s
+}
+
+//go:linkname forEachG runtime.forEachG
+//go:nosplit
+func forEachG(fn func(gp unsafe.Pointer))

gid_g121.go

@@ -7,30 +7,10 @@
 
 package goroutine
 
-import "unsafe"
-
 // ParentID returns the runtime ID of goroutine that created the calling
 // goroutine.
 func ParentID() int64 {
 	return idOf(getg(), parentGoidoff)
 }
 
 var parentGoidoff = offset("*runtime.g", "parentGoid")
-
-// Link is a goroutine parent-child relationship.
-type Link struct {
-	Parent, Child int64
-}
-
-// All returns all the known goroutine parent-child relationships.
-func All() []Link {
-	var s []Link
-	forEachG(func(g unsafe.Pointer) {
-		s = append(s, Link{Parent: idOf(g, parentGoidoff), Child: idOf(g, goidoff)})
-	})
-	return s
-}
-
-//go:linkname forEachG runtime.forEachG
-//go:nosplit
-func forEachG(fn func(gp unsafe.Pointer))