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.

Fixes: #146982
Release note: None

Author: dodeca12

pkg/kv/kvserver/client_spanconfig_backpressure_test.go

@@ -218,8 +218,8 @@ func TestSpanConfigUpdatesBlockedByRangeSizeBackpressureOnDefaultRangesWithKVAcc
 	}
 
 	// Assert that the operation failed due to backpressure.
-	require.Error(t, err, "Expected span config writes to fail due to backpressure, but they succeeded")
-	log.Dev.Infof(ctx, "Verified that span config writes fail due to backpressure: %v\n", err)
+	require.NoError(t, err, "Expected span config writes to NOT be backpressured")
+	log.Dev.Infof(ctx, "Verified that span config writes NOT be backpressured")
 
 	log.Dev.Infof(ctx, "Completed %d direct KV writes\n", i)
 
@@ -244,7 +244,7 @@ func TestSpanConfigUpdatesBlockedByRangeSizeBackpressureOnDefaultRangesWithKVAcc
 	if smallSpancofnRecordWriteErr != nil {
 		log.Dev.Infof(ctx, "ERROR: smallSpancofnRecord write failed: %v\n", smallSpancofnRecordWriteErr)
 	}
-	require.Error(t, smallSpancofnRecordWriteErr, "Expected smallSpancofnRecord write to succeed")
-	log.Dev.Infof(ctx, "SUCCESS: smallSpancofnRecord write failed as expected; still getting backpressure\n")
+	require.NoError(t, smallSpancofnRecordWriteErr, "Expected smallSpancofnRecord write to succeed (spanconfigs should bypass backpressure)")
+	log.Dev.Infof(ctx, "SUCCESS: smallSpancofnRecord write succeeded as expected (spanconfigs bypassed backpressure)\n")
 
 }

pkg/kv/kvserver/replica_backpressure.go

@@ -90,7 +90,12 @@ 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},
+	// Note: The above comment is no longer true.
+	// 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.SystemSQLCodec.TablePrefix(keys.SpanConfigurationsTableID)},
+	{Key: keys.SystemSQLCodec.TablePrefix(keys.SpanConfigurationsTableID + 1), EndKey: keys.TableDataMax},
 }
 
 // canBackpressureBatch returns whether the provided BatchRequest is eligible