Implement push support.

Pushing is achieved by inverting the roles on the established
connection, i.e. the client tells the server what data it should pull
from the client (PullMeRequest).

Role inversion is achieved by moving the server loop to the serverLoop
function of ByteStreamRPC, which can be called from both the Listen()
function (server-side) and the PullMeRequest() client-side function.

A donwside of this PullMe approach is that the replication policies
become part of the rpc, because the puller must follow the policy.

cmd/handler.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"github.com/zrepl/zrepl/rpc"
 	"github.com/zrepl/zrepl/zfs"
 	"io"
@@ -9,18 +10,21 @@ import (
 type Handler struct {
 	Logger          Logger
 	PullACL         zfs.DatasetMapping
+	SinkMappingFunc func(clientIdentity string) (mapping zfs.DatasetMapping, err error)
 }
 
 func (h Handler) HandleFilesystemRequest(r rpc.FilesystemRequest) (roots []zfs.DatasetPath, err error) {
 
 	h.Logger.Printf("handling fsr: %#v", r)
 
+	h.Logger.Printf("using PullACL: %#v", h.PullACL)
+
 	if roots, err = zfs.ZFSListMapping(h.PullACL); err != nil {
 		h.Logger.Printf("handle fsr err: %v\n", err)
 		return
 	}
 
-	h.Logger.Printf("got filesystems: %#v", roots)
+	h.Logger.Printf("returning: %#v", roots)
 
 	return
 }
@@ -88,3 +92,36 @@ func (h Handler) HandleIncrementalTransferRequest(r rpc.IncrementalTransferReque
 	return
 
 }
+
+func (h Handler) HandlePullMeRequest(r rpc.PullMeRequest, clientIdentity string, client rpc.RPCRequester) (err error) {
+
+	// Check if we have a sink for this request
+	// Use that mapping to do what happens in doPull
+
+	h.Logger.Printf("handling PullMeRequest: %#v", r)
+
+	var sinkMapping zfs.DatasetMapping
+	sinkMapping, err = h.SinkMappingFunc(clientIdentity)
+	if err != nil {
+		h.Logger.Printf("no sink mapping for client identity '%s', denying PullMeRequest", clientIdentity)
+		err = fmt.Errorf("no sink for client identity '%s'", clientIdentity)
+		return
+	}
+
+	h.Logger.Printf("doing pull...")
+
+	err = doPull(PullContext{
+		Remote:            client,
+		Log:               h.Logger,
+		Mapping:           sinkMapping,
+		InitialReplPolicy: r.InitialReplPolicy,
+	})
+	if err != nil {
+		h.Logger.Printf("PullMeRequest failed with error: %s", err)
+		return
+	}
+
+	h.Logger.Printf("finished handling PullMeRequest: %#v", r)
+
+	return
+}