Deprecate V2 API: --enable-v2 and v2v3

Flags `--experimental-enable-v2v3` and '-enable-v2' will raise a warning in 3.5,
in 3.6 they are schedule for decomissioning, such that v2store can stop be written in 3.7.

Deprecation plan in: etcd-io#12913

CHANGELOG-3.5.md

@@ -71,6 +71,8 @@ See [code changes](https://github.com/etcd-io/etcd/compare/v3.4.0...v3.5.0) and
 - [Embed Etcd.Close()](https://github.com/etcd-io/etcd/pull/12828) needs to called exactly once and closes Etcd.Err() stream.
 - [Embed Etcd does not override global/grpc logger](https://github.com/etcd-io/etcd/pull/12861) be default any longer. If desired, please call `embed.Config::SetupGlobalLoggers()` explicitly. 
 - Errors: `context cancelled` or `context deadline exceeded` are exposed as codes.Canceled, codes.DeadlineExceeded instead of 'codes.Unknown'.
+- Version 3.5 is the last version that supports V2 API. Flags `--enable-v2` and `--experimental-enable-v2v3` [are now deprecated](https://github.com/etcd-io/etcd/pull/)
+  and will be removed in etcd v3.6 release.
 ###
 
 - Make sure [save snapshot downloads checksum for integrity checks](https://github.com/etcd-io/etcd/pull/11896).

server/embed/config.go

@@ -222,7 +222,11 @@ type Config struct {
 	InitialCluster        string `json:"initial-cluster"`
 	InitialClusterToken   string `json:"initial-cluster-token"`
 	StrictReconfigCheck   bool   `json:"strict-reconfig-check"`
-	EnableV2              bool   `json:"enable-v2"`
+
+	// EnableV2 exposes the deprecated V2 API surface.
+	// TODO: Delete in 3.6 (https://github.com/etcd-io/etcd/issues/12913)
+	// Deprecated in 3.5.
+	EnableV2 bool `json:"enable-v2"`
 
 	// AutoCompactionMode is either 'periodic' or 'revision'.
 	AutoCompactionMode string `json:"auto-compaction-mode"`
@@ -306,7 +310,10 @@ type Config struct {
 
 	ExperimentalInitialCorruptCheck bool          `json:"experimental-initial-corrupt-check"`
 	ExperimentalCorruptCheckTime    time.Duration `json:"experimental-corrupt-check-time"`
-	ExperimentalEnableV2V3          string        `json:"experimental-enable-v2v3"`
+	// ExperimentalEnableV2V3 configures URLs that expose deprecated V2 API working on V3 store.
+	// Deprecated in v3.5.
+	// TODO: Delete in v3.6 (https://github.com/etcd-io/etcd/issues/12913)
+	ExperimentalEnableV2V3 string `json:"experimental-enable-v2v3"`
 	// ExperimentalEnableLeaseCheckpoint enables primary lessor to persist lease remainingTTL to prevent indefinite auto-renewal of long lived leases.
 	ExperimentalEnableLeaseCheckpoint       bool          `json:"experimental-enable-lease-checkpoint"`
 	ExperimentalCompactionBatchLimit        int           `json:"experimental-compaction-batch-limit"`

server/embed/etcd.go

@@ -695,7 +695,9 @@ func (e *Etcd) serveClients() (err error) {
 	// Start a client server goroutine for each listen address
 	var h http.Handler
 	if e.Config().EnableV2 {
+		e.cfg.logger.Warn("Flag `enable-v2` is deprecated and will get removed in etcd 3.6.")
 		if len(e.Config().ExperimentalEnableV2V3) > 0 {
+			e.cfg.logger.Warn("Flag `experimental-enable-v2v3` is deprecated and will get removed in etcd 3.6.")
 			srv := v2v3.NewServer(e.cfg.logger, v3client.New(e.Server), e.cfg.ExperimentalEnableV2V3)
 			h = v2http.NewClientHandler(e.GetLogger(), srv, e.Server.Cfg.ReqTimeout())
 		} else {

server/etcdmain/config.go

@@ -190,7 +190,7 @@ func newConfig() *config {
 	fs.Var(cfg.cf.clusterState, "initial-cluster-state", "Initial cluster state ('new' or 'existing').")
 
 	fs.BoolVar(&cfg.ec.StrictReconfigCheck, "strict-reconfig-check", cfg.ec.StrictReconfigCheck, "Reject reconfiguration requests that would cause quorum loss.")
-	fs.BoolVar(&cfg.ec.EnableV2, "enable-v2", cfg.ec.EnableV2, "Accept etcd V2 client requests.")
+	fs.BoolVar(&cfg.ec.EnableV2, "enable-v2", cfg.ec.EnableV2, "Accept etcd V2 client requests. Deprecated in v3.5. Will be decommission in v3.6.")
 	fs.BoolVar(&cfg.ec.PreVote, "pre-vote", cfg.ec.PreVote, "Enable to run an additional Raft election phase.")
 
 	// proxy
@@ -268,7 +268,7 @@ func newConfig() *config {
 	// experimental
 	fs.BoolVar(&cfg.ec.ExperimentalInitialCorruptCheck, "experimental-initial-corrupt-check", cfg.ec.ExperimentalInitialCorruptCheck, "Enable to check data corruption before serving any client/peer traffic.")
 	fs.DurationVar(&cfg.ec.ExperimentalCorruptCheckTime, "experimental-corrupt-check-time", cfg.ec.ExperimentalCorruptCheckTime, "Duration of time between cluster corruption check passes.")
-	fs.StringVar(&cfg.ec.ExperimentalEnableV2V3, "experimental-enable-v2v3", cfg.ec.ExperimentalEnableV2V3, "v3 prefix for serving emulated v2 state.")
+	fs.StringVar(&cfg.ec.ExperimentalEnableV2V3, "experimental-enable-v2v3", cfg.ec.ExperimentalEnableV2V3, "v3 prefix for serving emulated v2 state. Deprecated in 3.5. Will be decomissioned in 3.6.")
 	fs.BoolVar(&cfg.ec.ExperimentalEnableLeaseCheckpoint, "experimental-enable-lease-checkpoint", false, "Enable to persist lease remaining TTL to prevent indefinite auto-renewal of long lived leases.")
 	fs.IntVar(&cfg.ec.ExperimentalCompactionBatchLimit, "experimental-compaction-batch-limit", cfg.ec.ExperimentalCompactionBatchLimit, "Sets the maximum revisions deleted in each compaction batch.")
 	fs.DurationVar(&cfg.ec.ExperimentalWatchProgressNotifyInterval, "experimental-watch-progress-notify-interval", cfg.ec.ExperimentalWatchProgressNotifyInterval, "Duration of periodic watch progress notifications.")

server/etcdmain/help.go

@@ -123,7 +123,7 @@ Clustering:
   --auto-compaction-mode 'periodic'
     Interpret 'auto-compaction-retention' one of: periodic|revision. 'periodic' for duration based retention, defaulting to hours if no time unit is provided (e.g. '5m'). 'revision' for revision number based retention.
   --enable-v2 '` + strconv.FormatBool(embed.DefaultEnableV2) + `'
-    Accept etcd V2 client requests.
+    Accept etcd V2 client requests. Deprecated and to be decommissioned in v3.6.
 
 Security:
   --cert-file ''
@@ -203,7 +203,7 @@ Experimental distributed tracing:
   --experimental-distributed-tracing-instance-id ''
     Distributed tracing instance ID, must be unique per each etcd instance.
 
-v2 Proxy (to be deprecated in v4):
+v2 Proxy (to be deprecated in v3.6):
   --proxy 'off'
     Proxy mode setting ('off', 'readonly' or 'on').
   --proxy-failure-wait 5000
@@ -223,7 +223,7 @@ Experimental feature:
   --experimental-corrupt-check-time '0s'
     Duration of time between cluster corruption check passes.
   --experimental-enable-v2v3 ''
-    Serve v2 requests through the v3 backend under a given prefix.
+    Serve v2 requests through the v3 backend under a given prefix. Deprecated and to be decommissioned in v3.6.
   --experimental-enable-lease-checkpoint 'false'
     ExperimentalEnableLeaseCheckpoint enables primary lessor to persist lease remainingTTL to prevent indefinite auto-renewal of long lived leases.
   --experimental-compaction-batch-limit 1000