changefeedccl: fix unwatched column families memory monitoring bug

This patch fixes a bug where the memory monitor wouldn't reclaim the
memory allocated to events corresponding to unwatched column families
for a changefeed that targets only a subset of a table's families.

Release note (bug fix): A bug where a changefeed targeting only a subset
of a table's column families could become stuck has been fixed.

Author: andyyang890

pkg/ccl/changefeedccl/BUILD.bazel

@@ -202,6 +202,7 @@ go_test(
         "alter_changefeed_test.go",
         "avro_test.go",
         "changefeed_dist_test.go",
+        "changefeed_memory_test.go",
         "changefeed_processors_test.go",
         "changefeed_stmt_test.go",
         "changefeed_test.go",

pkg/ccl/changefeedccl/changefeed_memory_test.go

@@ -0,0 +1,86 @@
+// Copyright 2025 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package changefeedccl
+
+import (
+	"context"
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/ccl/changefeedccl/cdctest"
+	"github.com/cockroachdb/cockroach/pkg/ccl/changefeedccl/changefeedbase"
+	"github.com/cockroachdb/cockroach/pkg/testutils/sqlutils"
+	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/cockroach/pkg/util/randutil"
+)
+
+// TestChangefeedUnwatchedFamilyMemoryMonitoring verifies that changefeeds
+// correctly release memory allocations for events corresponding to unwatched
+// column families after discarding them. This is a regression test for #154776.
+func TestChangefeedUnwatchedFamilyMemoryMonitoring(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	ctx := context.Background()
+	rnd, _ := randutil.NewTestRand()
+
+	testFn := func(t *testing.T, s TestServer, f cdctest.TestFeedFactory) {
+		sqlDB := sqlutils.MakeSQLRunner(s.DB)
+
+		// Set a low memory limit.
+		changefeedbase.PerChangefeedMemLimit.Override(
+			ctx, &s.Server.ClusterSettings().SV, 1<<20 /* 1 MiB */)
+
+		// Create a table with two column families.
+		sqlDB.Exec(t, `CREATE TABLE foo (
+			id INT PRIMARY KEY,
+			a STRING,
+			b STRING,
+			FAMILY f1 (id, a),
+			FAMILY f2 (b)
+		)`)
+
+		// Insert initial data.
+		sqlDB.Exec(t, `INSERT INTO foo VALUES (1, 'a', 'b')`)
+
+		// Start changefeed watching only f1 with diff enabled.
+		// Events from f2 should trigger ErrUnwatchedFamily.
+		feed := feed(t, f,
+			`CREATE CHANGEFEED FOR foo FAMILY f1 WITH diff, initial_scan='no', resolved`)
+		defer closeFeed(t, feed)
+
+		// Update a watched column to generate an event.
+		sqlDB.Exec(t, `UPDATE foo SET a = 'a_1' WHERE id = 1`)
+		assertPayloads(t, feed, []string{
+			`foo.f1: [1]->{"after": {"a": "a_1", "id": 1}, "before": {"a": "a", "id": 1}}`,
+		})
+
+		// Generate a lot of events for the unwatched family. If the memory
+		// allocations are being leaked, this would cause the changefeed to
+		// exceed the previously configured 1 MiB limit and become stuck
+		// when it attempts to process more events after the limit is hit.
+		const charset = "abcdefghijklmnopqrstuvwxyz0123456789"
+		for range 1000 {
+			// Each update will create an event of size ~2 KiB
+			// (~1 KiB for each of before/after).
+			data := make([]byte, 1<<10 /* 1 KiB */)
+			for i := range data {
+				data[i] = charset[rnd.Intn(len(charset))]
+			}
+			sqlDB.Exec(t, `UPDATE foo SET b = $1 WHERE id = 1`, data)
+		}
+
+		// Update watched column again to verify the feed is still progressing.
+		// If the memory allocations leaked, this assertion would time out
+		// because the changefeed would be stuck.
+		sqlDB.Exec(t, `UPDATE foo SET a = 'a_2' WHERE id = 1`)
+		assertPayloads(t, feed, []string{
+			`foo.f1: [1]->{"after": {"a": "a_2", "id": 1}, "before": {"a": "a_1", "id": 1}}`,
+		})
+	}
+
+	cdcTest(t, testFn)
+}

pkg/ccl/changefeedccl/event_processing.go

@@ -337,6 +337,9 @@ func (c *kvEventToRowConsumer) ConsumeEvent(ctx context.Context, ev kvevent.Even
 		// Column families are stored contiguously, so we'll get
 		// events for each one even if we're not watching them all.
 		if errors.Is(err, cdcevent.ErrUnwatchedFamily) {
+			// Release the event's allocation since we're not processing it.
+			a := ev.DetachAlloc()
+			a.Release(ctx)
 			return nil
 		}
 		return err
@@ -353,6 +356,9 @@ func (c *kvEventToRowConsumer) ConsumeEvent(ctx context.Context, ev kvevent.Even
 		// Column families are stored contiguously, so we'll get
 		// events for each one even if we're not watching them all.
 		if errors.Is(err, cdcevent.ErrUnwatchedFamily) {
+			// Release the event's allocation since we're not processing it.
+			a := ev.DetachAlloc()
+			a.Release(ctx)
 			return nil
 		}
 		return err