kvserver: allowlist spanconfig updates to bypass range-size backpressure

This commit modifies `backpressurableSpans` to exclude the
`system.span_configurations` table by splitting the original span into
two ranges that create a gap around the `system.span_configurations`
table. This allows spanconfig updates to bypass backpressure entirely,
preventing the catch-22 deadlock.

We only allowlist the `system.span_configurations` table as the table
is self-referential: updates to it trigger spanconfig reconciliation
that reads from the same table. When backpressure blocks these updates,
protected timestamp and GC TTL updates cannot proceed, creating a
deadlock where the system cannot update its own configuration.
Other system tables lack this circular dependency, and are not exluded.

Fixes: #146982
Release note: None

Author: dodeca12

pkg/keys/constants.go

@@ -374,6 +374,10 @@ var (
 	NamespaceTableMin = SystemSQLCodec.TablePrefix(NamespaceTableID)
 	// NamespaceTableMax is the end key of system.namespace.
 	NamespaceTableMax = SystemSQLCodec.TablePrefix(NamespaceTableID + 1)
+	// SpanConfigTableDataMin is the start key of system.span_configurations.
+	SpanConfigTableDataMin = SystemSQLCodec.TablePrefix(SpanConfigurationsTableID)
+	// SpanConfigTableDataMax is the end key of system.span_configurations.
+	SpanConfigTableDataMax = SystemSQLCodec.TablePrefix(SpanConfigurationsTableID + 1)
 
 	// 4. Non-system tenant SQL keys
 	//

pkg/kv/kvserver/client_spanconfig_backpressure_test.go

@@ -73,7 +73,8 @@ func TestSpanConfigUpdatesBlockedByRangeSizeBackpressureOnDefaultRanges(t *testi
 				return err
 			}
 			if conf.RangeMaxBytes != expRangeMaxBytes {
-				return fmt.Errorf("expected RangeMaxBytes %d, got %d", expRangeMaxBytes, conf.RangeMaxBytes)
+				return fmt.Errorf("expected RangeMaxBytes %d, got %d",
+					expRangeMaxBytes, conf.RangeMaxBytes)
 			}
 			return nil
 		})
@@ -215,8 +216,8 @@ func TestSpanConfigUpdatesBlockedByRangeSizeBackpressureOnDefaultRanges(t *testi
 	}
 
 	// Assert that the operation failed due to backpressure.
-	require.Error(t, err, `Expected span config writes to fail due to 
-	backpressure, but they succeeded`)
+	require.NoError(t, err, `Expected span config writes to succeed due to 
+		allowlist; they should bypass backpressure`)
 
 	t.Logf("Completed %d direct KV writes\n", i)
 
@@ -242,9 +243,9 @@ func TestSpanConfigUpdatesBlockedByRangeSizeBackpressureOnDefaultRanges(t *testi
 		[]spanconfig.Target{testTarget}, []spanconfig.Record{smallSpancofnRecord},
 		hlc.MinTimestamp, hlc.MaxTimestamp)
 
-	require.Error(t, smallSpancofnRecordWriteErr, `Expected smallSpancofnRecord 
-		write to fail`)
-	t.Logf(`SUCCESS: smallSpancofnRecord write failed as expected; 
-		still getting backpressure\n`)
+	require.NoError(t, smallSpancofnRecordWriteErr, `Expected smallSpancofnRecord 
+		write to succeed`)
+	t.Logf(`SUCCESS: smallSpancofnRecord write succeeded; 
+		spanconfigs should bypass backpressure\n`)
 
 }

pkg/kv/kvserver/replica_backpressure.go

@@ -88,9 +88,11 @@ var backpressureByteTolerance = settings.RegisterByteSizeSetting(
 // to be backpressured.
 var backpressurableSpans = []roachpb.Span{
 	{Key: keys.TimeseriesPrefix, EndKey: keys.TimeseriesKeyMax},
-	// Backpressure from the end of the system config forward instead of
-	// over all table data to avoid backpressuring unsplittable ranges.
-	{Key: keys.SystemConfigTableDataMax, EndKey: keys.TableDataMax},
+	// Split the span to exclude the span_configurations table to avoid
+	// catch-22 situations where protected timestamp updates or garbage
+	// collection TTL updates are blocked by backpressure.
+	{Key: keys.SystemConfigTableDataMax, EndKey: keys.SpanConfigTableDataMin},
+	{Key: keys.SpanConfigTableDataMax, EndKey: keys.TableDataMax},
 }
 
 // canBackpressureBatch returns whether the provided BatchRequest is eligible