[WIP] backupccl: add feature flag support for backup command

Adds a cluster setting to toggle a feature flag for the
BACKUP command off and on. It is being introduced to address a
Cockroach Cloud SRE use case of wanting to disable certain
categories of features in case of cluster failure.

Release note category, description TBD

Release note: None

docs/generated/settings/settings.html

@@ -13,6 +13,7 @@
 <tr><td><code>enterprise.license</code></td><td>string</td><td><code></code></td><td>the encoded cluster license</td></tr>
 <tr><td><code>external.graphite.endpoint</code></td><td>string</td><td><code></code></td><td>if nonempty, push server metrics to the Graphite or Carbon server at the specified host:port</td></tr>
 <tr><td><code>external.graphite.interval</code></td><td>duration</td><td><code>10s</code></td><td>the interval at which metrics are pushed to Graphite (if enabled)</td></tr>
+<tr><td><code>feature.backup.enabled</code></td><td>boolean</td><td><code>true</code></td><td>set to true to enable backups, false to disable; default is true</td></tr>
 <tr><td><code>jobs.retention_time</code></td><td>duration</td><td><code>336h0m0s</code></td><td>the amount of time to retain records for completed jobs before</td></tr>
 <tr><td><code>kv.allocator.load_based_lease_rebalancing.enabled</code></td><td>boolean</td><td><code>true</code></td><td>set to enable rebalancing of range leases based on load and latency</td></tr>
 <tr><td><code>kv.allocator.load_based_rebalancing</code></td><td>enumeration</td><td><code>leases and replicas</code></td><td>whether to rebalance based on the distribution of QPS across stores [off = 0, leases = 1, leases and replicas = 2]</td></tr>

pkg/ccl/backupccl/backup_planning.go

@@ -27,6 +27,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/server/telemetry"
+	"github.com/cockroachdb/cockroach/pkg/settings"
 	"github.com/cockroachdb/cockroach/pkg/settings/cluster"
 	"github.com/cockroachdb/cockroach/pkg/sql"
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog"
@@ -79,6 +80,12 @@ type backupKMSEnv struct {
 
 var _ cloud.KMSEnv = &backupKMSEnv{}
 
+// featureBackupEnabled is used to enable and disable the BACKUP feature.
+var featureBackupEnabled = settings.RegisterPublicBoolSetting(
+	"feature.backup.enabled",
+	"set to true to enable backups, false to disable; default is true",
+	true /* enabled by default */)
+
 func (p *backupKMSEnv) ClusterSettings() *cluster.Settings {
 	return p.settings
 }
@@ -501,6 +508,12 @@ func backupPlanHook(
 		return nil, nil, nil, false, nil
 	}
 
+	// Check whether feature backup is enabled or not.
+	if !featureBackupEnabled.Get(&p.ExecCfg().Settings.SV) {
+		return nil, nil, nil, false,
+			pgerror.Newf(pgcode.OperatorIntervention, "BACKUP feature was disabled by the database administrator.")
+	}
+
 	var err error
 	subdirFn := func() (string, error) { return "", nil }
 	if backupStmt.Subdir != nil {

pkg/ccl/backupccl/testdata/backup-restore/feature-flags

@@ -0,0 +1,22 @@
+new-server name=s1
+----
+
+# Test running backup when feature flag is disabled.
+exec-sql
+CREATE DATABASE d;
+CREATE TABLE d.t (x INT);
+INSERT INTO d.t VALUES (1), (2), (3);
+----
+
+# Cluster backup should fail.
+exec-sql
+SET CLUSTER SETTING feature.backup.enabled = FALSE;
+BACKUP TO 'nodelocal://0/test-root/';
+----
+pq: BACKUP feature was disabled by the database administrator.
+
+# Test running backup when feature flag is enabled.
+exec-sql
+SET CLUSTER SETTING feature.backup.enabled = TRUE;
+BACKUP TO 'nodelocal://0/test-root/';
+----