From bc70e73f9c4910ddbe80161a1aa181dc92d456a1 Mon Sep 17 00:00:00 2001
From: Adam Harrison <harrisonadamw@gmail.com>
Date: Wed, 24 Feb 2016 14:43:06 +0000
Subject: [PATCH] Implement `weave consense`

---
 ipam/allocator.go               | 12 ++++++++++++
 ipam/consense.go                | 24 ++++++++++++++++++++++++
 ipam/http.go                    |  4 ++++
 site/ipam/overview-init-ipam.md | 16 ++++++++++++++--
 weave                           |  5 +++++
 5 files changed, 59 insertions(+), 2 deletions(-)
 create mode 100644 ipam/consense.go

diff --git a/ipam/allocator.go b/ipam/allocator.go
index 4ea0eb8167..256822c2a0 100644
--- a/ipam/allocator.go
+++ b/ipam/allocator.go
@@ -54,6 +54,7 @@ type Allocator struct {
 	nicknames        map[mesh.PeerName]string     // so we can map nicknames for rmpeer
 	pendingAllocates []operation                  // held until we get some free space
 	pendingClaims    []operation                  // held until we know who owns the space
+	pendingConsenses []operation                  // held until consensus achieved
 	dead             map[string]time.Time         // containers we heard were dead, and when
 	db               db.DB                        // persistence
 	gossip           mesh.Gossip                  // our link to the outside world for sending messages
@@ -165,6 +166,8 @@ func (alloc *Allocator) tryOps(ops *[]operation) {
 
 // Try all pending operations
 func (alloc *Allocator) tryPendingOps() {
+	// Unblock pending consenses first
+	alloc.tryOps(&alloc.pendingConsenses)
 	// Process existing claims before servicing new allocations
 	alloc.tryOps(&alloc.pendingClaims)
 	alloc.tryOps(&alloc.pendingAllocates)
@@ -193,6 +196,14 @@ func (e *errorCancelled) Error() string {
 
 // Actor client API
 
+// Consense (Sync) - wait for consensus
+func (alloc *Allocator) Consense() {
+	resultChan := make(chan struct{})
+	op := &consense{resultChan: resultChan}
+	alloc.doOperation(op, &alloc.pendingConsenses)
+	<-resultChan
+}
+
 // Allocate (Sync) - get new IP address for container with given name in range
 // if there isn't any space in that range we block indefinitely
 func (alloc *Allocator) Allocate(ident string, r address.Range, hasBeenCancelled func() bool) (address.Address, error) {
@@ -348,6 +359,7 @@ func (alloc *Allocator) Shutdown() {
 		alloc.shuttingDown = true
 		alloc.cancelOps(&alloc.pendingClaims)
 		alloc.cancelOps(&alloc.pendingAllocates)
+		alloc.cancelOps(&alloc.pendingConsenses)
 		if heir := alloc.pickPeerForTransfer(); heir != mesh.UnknownPeerName {
 			alloc.ring.Transfer(alloc.ourName, heir)
 			alloc.space.Clear()
diff --git a/ipam/consense.go b/ipam/consense.go
new file mode 100644
index 0000000000..a4e36c15a6
--- /dev/null
+++ b/ipam/consense.go
@@ -0,0 +1,24 @@
+package ipam
+
+type consense struct {
+	resultChan chan<- struct{}
+}
+
+func (c *consense) Try(alloc *Allocator) bool {
+	if !alloc.ring.Empty() {
+		close(c.resultChan)
+		return true
+	}
+
+	alloc.establishRing()
+
+	return false
+}
+
+func (c *consense) Cancel() {
+	close(c.resultChan)
+}
+
+func (c *consense) ForContainer(ident string) bool {
+	return false
+}
diff --git a/ipam/http.go b/ipam/http.go
index eb51b87a10..7ca1312e1d 100644
--- a/ipam/http.go
+++ b/ipam/http.go
@@ -77,6 +77,10 @@ func (alloc *Allocator) HandleHTTP(router *mux.Router, defaultSubnet address.CID
 		w.WriteHeader(204)
 	})
 
+	router.Methods("GET").Path("/consensus").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		alloc.Consense()
+	})
+
 	router.Methods("GET").Path("/ip/{id}/{ip}/{prefixlen}").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		vars := mux.Vars(r)
 		if subnet, ok := parseCIDR(w, vars["ip"]+"/"+vars["prefixlen"]); ok {
diff --git a/site/ipam/overview-init-ipam.md b/site/ipam/overview-init-ipam.md
index 727216c9e8..afd651a52e 100644
--- a/site/ipam/overview-init-ipam.md
+++ b/site/ipam/overview-init-ipam.md
@@ -15,6 +15,7 @@ The following automatic IP address managment topics are discussed:
 
  * [Initializing Peers on a Weave Network](#initialization)
  * [`--init-peer-count` and How Quorum is Achieved](#quorum)
+ * [Forcing Consensus](#forcing-consensus)
  * [Choosing an Allocation Range](#range)
 
  
@@ -100,6 +101,19 @@ is safe with respect to Weave's startup quorum:
     ...host1 is rebooted...
     host1$ weave launch $HOST2 $HOST3
 
+### <a name="forcing-consensus"></a>Forcing Consensus
+
+Under certain circumstances (for example when adding new nodes to an
+existing cluster) it is desirable to ensure that a node has
+successfully joined and received a copy of the IPAM data structure
+shared amongst the peers. An administrative command is provided for
+this purpose:
+
+    host1$ weave consense
+
+This operation will block until the node on which it is run has joined
+successfully.
+
 ### <a name="range"></a>Choosing an Allocation Range
 
 By default, Weave allocates IP addresses in the 10.32.0.0/12
@@ -126,5 +140,3 @@ to the rest of the network without any conflicts arising.
 
  * [Automatic Allocation Across Multiple Subnets](/site/ipam/allocation-multi-ipam.md)
  * [Plugin Command-line Arguments](/site/plugin/plug-in-command-line.md)
- 
-    
\ No newline at end of file
diff --git a/weave b/weave
index 6364bcfd3a..69bc9e07a9 100755
--- a/weave
+++ b/weave
@@ -51,6 +51,8 @@ weave launch        <same arguments as 'weave launch-router'>
                       [--rewrite-inspect]
       launch-plugin [--no-multicast-route]
 
+weave consense
+
 weave env           [--restore]
       config
       dns-args
@@ -2186,6 +2188,9 @@ EOF
         echo "The 'stop-dns command has been removed; DNS is stopped as part of 'stop' and 'stop-router'." >&2
         exit 0
         ;;
+    consense)
+        call_weave GET /consensus
+        ;;
     *)
         echo "Unknown weave command '$COMMAND'" >&2
         usage