cmd/coordinator, internal/coordinator/remote: move post handlers

This change moves the SSH handlers into the internal packages. It also
adds the handler which will use the session pool instead of the remote
buildlets.

Updates golang/go#52594
For golang/go#47521

Change-Id: I7e99fdbb16e0f80a871696cec79a9b638354e662
Reviewed-on: https://go-review.googlesource.com/c/build/+/405257
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Carlos Amedee <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Carlos Amedee <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: cagedmantis

cmd/coordinator/coordinator.go

@@ -406,16 +406,7 @@ func main() {
 		if err != nil {
 			return nil, fmt.Errorf("unable to retrieve keys for SSH Server: %v", err)
 		}
-		privateHostKeyFile, err := remote.WriteSSHPrivateKeyToTempFile(privateKey)
-		log.Printf("unable to write private host key file: %s", err)
-		if err != nil {
-			return nil, fmt.Errorf("error writing ssh private key to temp file: %v; not running SSH server", err)
-		}
-		sshHandlers := &sshHandlers{
-			gomotePublicKey:   string(publicKey),
-			sshPrivateKeyFile: privateHostKeyFile,
-		}
-		return remote.NewSSHServer(*sshAddr, privateKey, sshCA, sshHandlers.handleIncomingSSHPostAuth, sp)
+		return remote.NewSSHServer(*sshAddr, privateKey, publicKey, sshCA, sp, remoteBuildlets)
 	}
 	sshServ, err := configureSSHServer()
 	if err != nil {