runtime: add func TailscaleCurrentP

Updates #109

Signed-off-by: Brad Fitzpatrick <[email protected]>

api/go1.99999.txt

@@ -9,3 +9,4 @@ pkg net, type SockTrace struct, DidWrite func(int) #58
 pkg net, type SockTrace struct, WillCloseTCPConn func(syscall.RawConn) #58
 pkg net, type SockTrace struct, WillOverwrite func(*SockTrace) #58
 pkg net/http, func SetRoundTripEnforcer(func(*Request) error) #55
+pkg runtime, func TailscaleCurrentP() int #109

src/runtime/tailscale_runtime.go

@@ -0,0 +1,8 @@
+package runtime
+
+// TailscaleCurrentP returns the runtime's currently executing 'p' ID.
+//
+// See https://github.com/tailscale/go/issues/109.
+func TailscaleCurrentP() int {
+	return int(getg().m.p.ptr().id)
+}