Always add core scheduler

CHANGELOG.md

@@ -46,7 +46,9 @@ BUG FIXES:
  * client: Improve auto-detection of network interface when interface name has a
    space in it on Windows [[GH-3855](https://github.com/hashicorp/nomad/issues/3855)]
  * client/vault: Recognize renewing non-renewable Vault lease as fatal [[GH-3727](https://github.com/hashicorp/nomad/issues/3727)]
- * config: Revert minimum CPU limit back to 20 from 100.
+ * config: Revert minimum CPU limit back to 20 from 100 [[GH-3706](https://github.com/hashicorp/nomad/issues/3706)]
+ * config: Always add core scheduler to enabled schedulers and add invalid
+   EnabledScheduler detection [[GH-3978](https://github.com/hashicorp/nomad/issues/3978)]
  * driver/exec: Properly disable swapping [[GH-3958](https://github.com/hashicorp/nomad/issues/3958)]
  * driver/lxc: Cleanup LXC containers after errors on container startup. [[GH-3773](https://github.com/hashicorp/nomad/issues/3773)]
  * ui: Fix ui on non-leaders when ACLs are enabled [[GH-3722](https://github.com/hashicorp/nomad/issues/3722)]

command/agent/agent.go

@@ -149,7 +149,20 @@ func convertServerConfig(agentConfig *Config, logOutput io.Writer) (*nomad.Confi
 		conf.NumSchedulers = agentConfig.Server.NumSchedulers
 	}
 	if len(agentConfig.Server.EnabledSchedulers) != 0 {
-		conf.EnabledSchedulers = agentConfig.Server.EnabledSchedulers
+		// Convert to a set and require the core scheduler
+		set := make(map[string]struct{}, 4)
+		set[structs.JobTypeCore] = struct{}{}
+		for _, sched := range agentConfig.Server.EnabledSchedulers {
+			set[sched] = struct{}{}
+		}
+
+		schedulers := make([]string, 0, len(set))
+		for k := range set {
+			schedulers = append(schedulers, k)
+		}
+
+		conf.EnabledSchedulers = schedulers
+
 	}
 	if agentConfig.ACL.Enabled {
 		conf.ACLEnabled = true

nomad/server.go

@@ -30,6 +30,7 @@ import (
 	"github.com/hashicorp/nomad/nomad/state"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/nomad/structs/config"
+	"github.com/hashicorp/nomad/scheduler"
 	"github.com/hashicorp/raft"
 	raftboltdb "github.com/hashicorp/raft-boltdb"
 	"github.com/hashicorp/serf/serf"
@@ -1193,6 +1194,22 @@ func (s *Server) setupWorkers() error {
 		return nil
 	}
 
+	// Check if the core scheduler is not enabled
+	foundCore := false
+	for _, sched := range s.config.EnabledSchedulers {
+		if sched == structs.JobTypeCore {
+			foundCore = true
+			continue
+		}
+
+		if _, ok := scheduler.BuiltinSchedulers[sched]; !ok {
+			return fmt.Errorf("invalid configuration: unknown scheduler %q in enabled schedulers", sched)
+		}
+	}
+	if !foundCore {
+		return fmt.Errorf("invalid configuration: %q scheduler not enabled", structs.JobTypeCore)
+	}
+
 	// Start the workers
 	for i := 0; i < s.config.NumSchedulers; i++ {
 		if w, err := NewWorker(s); err != nil {

nomad/server_test.go

@@ -3,19 +3,23 @@ package nomad
 import (
 	"fmt"
 	"io/ioutil"
+	"log"
 	"os"
 	"path"
 	"strings"
 	"testing"
 	"time"
 
 	msgpackrpc "github.com/hashicorp/net-rpc-msgpackrpc"
+	"github.com/hashicorp/nomad/command/agent/consul"
+	"github.com/hashicorp/nomad/helper/testlog"
 	"github.com/hashicorp/nomad/helper/uuid"
 	"github.com/hashicorp/nomad/nomad/mock"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/nomad/structs/config"
 	"github.com/hashicorp/nomad/testutil"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func tmpDir(t *testing.T) string {
@@ -412,3 +416,26 @@ func TestServer_Reload_TLSConnections_Raft(t *testing.T) {
 
 	testutil.WaitForLeader(t, s2.RPC)
 }
+
+func TestServer_InvalidSchedulers(t *testing.T) {
+	t.Parallel()
+	require := require.New(t)
+
+	config := DefaultConfig()
+	config.DevMode = true
+	config.LogOutput = testlog.NewWriter(t)
+	logger := log.New(config.LogOutput, "", log.LstdFlags)
+	catalog := consul.NewMockCatalog(logger)
+
+	// Set the config to not have the core scheduler
+	config.EnabledSchedulers = []string{"batch"}
+	_, err := NewServer(config, catalog, logger)
+	require.NotNil(err)
+	require.Contains(err.Error(), "scheduler not enabled")
+
+	// Set the config to have an unknown scheduler
+	config.EnabledSchedulers = []string{"batch", structs.JobTypeCore, "foo"}
+	_, err = NewServer(config, catalog, logger)
+	require.NotNil(err)
+	require.Contains(err.Error(), "foo")
+}