proxy: split the execProxy function into parts

Previously `execProxy` would
- create the proxy
- signal success/failure to the parent
- run forever (until signalled)

Since we want to add more proxy setup and teardown, this patch
removes the proxy creation from `execProxy` and renames it to
`proxyForever`. Later patches will be able to perform the necessary
side-effects before signalling success to the parent.

Signed-off-by: David Scott <dave.scott@docker.com>

alpine/packages/proxy/main.go

@@ -1,5 +1,11 @@
 package main
 
+import (
+	"pkg/proxy"
+)
+
 func main() {
-	execProxy()
+	host, container := parseHostContainerAddrs()
+
+	proxyForever(proxy.NewProxy(host, container))
 }

alpine/packages/proxy/proxy.go

@@ -12,14 +12,10 @@ import (
 	"pkg/proxy"
 )
 
-// From docker/libnetwork/portmapper/proxy.go
-
-// execProxy is the reexec function that is registered to start the userland proxies
-func execProxy() {
+// proxyForever signals the parent success/failure and runs the proxy forever
+func proxyForever(p proxy.Proxy, err error) {
 	f := os.NewFile(3, "signal-parent")
-	host, container := parseHostContainerAddrs()
 
-	p, err := proxy.NewProxy(host, container)
 	if err != nil {
 		fmt.Fprintf(f, "1\n%s", err)
 		f.Close()
@@ -33,6 +29,8 @@ func execProxy() {
 	p.Run()
 }
 
+// From docker/libnetwork/portmapper/proxy.go:
+
 // parseHostContainerAddrs parses the flags passed on reexec to create the TCP or UDP
 // net.Addrs to map the host and container ports
 func parseHostContainerAddrs() (host net.Addr, container net.Addr) {