core: remove support for raft protocol version 2

This PR checks server config for raft_protocol, which must now
be set to 3 or unset (0). When unset, version 3 is used as the
default.

.changelog/13467.txt

@@ -0,0 +1,3 @@
+```release-note:breaking-change
+core: remove support for raft protocol version 2
+```

command/agent/agent.go

@@ -201,6 +201,9 @@ func convertServerConfig(agentConfig *Config) (*nomad.Config, error) {
 	if agentConfig.Server.RaftProtocol != 0 {
 		conf.RaftConfig.ProtocolVersion = raft.ProtocolVersion(agentConfig.Server.RaftProtocol)
 	}
+	if v := conf.RaftConfig.ProtocolVersion; v != 3 {
+		return nil, fmt.Errorf("raft_protocol must be 3 in Nomad v1.4 and later, got %d", v)
+	}
 	raftMultiplier := int(DefaultRaftMultiplier)
 	if agentConfig.Server.RaftMultiplier != nil && *agentConfig.Server.RaftMultiplier != 0 {
 		raftMultiplier = *agentConfig.Server.RaftMultiplier

command/agent/agent_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/hashicorp/nomad/helper/testlog"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/nomad/structs/config"
+	"github.com/shoenig/test/must"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -472,6 +473,31 @@ func TestAgent_ServerConfig_RaftMultiplier_Bad(t *testing.T) {
 	}
 }
 
+func TestAgent_ServerConfig_RaftProtocol_3(t *testing.T) {
+	ci.Parallel(t)
+
+	cases := []int{
+		0, 1, 2, 3, 4,
+	}
+
+	for _, tc := range cases {
+		t.Run(fmt.Sprintf("protocol_version %d", tc), func(t *testing.T) {
+			conf := DevConfig(nil)
+			conf.Server.RaftProtocol = tc
+			must.NoError(t, conf.normalizeAddrs())
+			_, err := convertServerConfig(conf)
+
+			switch tc {
+			case 0, 3: // 0 defers to default
+				must.NoError(t, err)
+			default:
+				exp := fmt.Sprintf("raft_protocol must be 3 in Nomad v1.4 and later, got %d", tc)
+				must.EqError(t, err, exp)
+			}
+		})
+	}
+}
+
 func TestAgent_ClientConfig(t *testing.T) {
 	ci.Parallel(t)
 	conf := DefaultConfig()

website/content/docs/configuration/server.mdx

@@ -166,7 +166,8 @@ server {
 - `raft_protocol` `(int: 3)` - Specifies the Raft protocol version to use when
   communicating with other Nomad servers. This affects available Autopilot
   features and is typically not required as the agent internally knows the
-  latest version, but may be useful in some upgrade scenarios.
+  latest version, but may be useful in some upgrade scenarios. Must be `3` in
+  Nomad v1.4 or later.
 
 - `raft_multiplier` `(int: 1)` - An integer multiplier used by Nomad servers to
   scale key Raft timing parameters. Omitting this value or setting it to 0 uses

website/content/docs/upgrade/upgrade-specific.mdx

@@ -22,6 +22,16 @@ upgrade. However, specific versions of Nomad may have more details provided for
 their upgrades as a result of new features or changed behavior. This page is
 used to document those details separately from the standard upgrade flow.
 
+## Nomad 1.4.0
+
+#### Raft Protocol Version 2 Unsupported
+
+Raft protocol version 2 was deprecated in Nomad v1.3.0, and is being removed
+in Nomad v1.4.0. In Nomad 1.3.0, the default raft protocol version was updated
+to version 3, and in Nomad 1.4.0 Nomad requires the use of raft protocol version
+3. If [`raft_protocol`] version is explicitly set, it must now be set to `3`.
+For more information see the [Upgrading to Raft Protocol 3] guide.
+
 ## Nomad 1.3.1, 1.2.8, 1.1.14
 
 #### Default `artifact` limits
@@ -42,7 +52,7 @@ Raft protocol version 2 will be removed from Nomad in the next major
 release of Nomad, 1.4.0.
 
 In Nomad 1.3.0, the default raft protocol version has been updated to
-3. If the [`raft_protocol_version`] is not explicitly set, upgrading a
+3. If the [`raft_protocol`] version is not explicitly set, upgrading a
 server will automatically upgrade that server's raft protocol. See the
 [Upgrading to Raft Protocol 3] guide.
 
@@ -1407,7 +1417,7 @@ deleted and then Nomad 0.3.0 can be launched.
 [preemption]: /docs/internals/scheduling/preemption
 [proxy_concurrency]: /docs/job-specification/sidecar_task#proxy_concurrency
 [`sidecar_task.config`]: /docs/job-specification/sidecar_task#config
-[`raft_protocol_version`]: /docs/configuration/server#raft_protocol
+[`raft_protocol`]: /docs/configuration/server#raft_protocol
 [`raft protocol`]: /docs/configuration/server#raft_protocol
 [reserved]: /docs/configuration/client#reserved-parameters
 [task-config]: /docs/job-specification/task#config