Reindexer: Mock reindex function to reduce parallel contention

This one's aimed at fixing an intermittently failing test case:

https://github.com/riverqueue/river/actions/runs/24322049420/job/71009998068?pr=1208

    --- FAIL: TestReindexer (0.00s)
        --- FAIL: TestReindexer/ReindexesConfiguredIndexes (10.07s)
            reindexer_test.go:219: Reusing idle postgres schema "maintenance_2026_04_13t01_54_14_schema_04" [user facing: "maintenance_2026_04_13t01_54_14_schema_04"] after cleaning in 26.436456ms [4 generated] [7 reused]
            test_signal.go:95: timed out waiting on test signal after 10s
            logger.go:256: time=2026-04-13T01:54:27.581Z level=INFO msg="maintenance.Reindexer: Signaled to stop during index build; attempting to clean up concurrent artifacts"
            riverdbtest.go:293: Checked in postgres schema "maintenance_2026_04_13t01_54_14_schema_04"; 1 idle schema(s) [4 generated] [10 reused]
        --- FAIL: TestReindexer/ReindexesMinimalSubsetofIndexes (10.14s)
            reindexer_test.go:183: Reusing idle postgres schema "maintenance_2026_04_13t01_54_14_schema_01" [user facing: "maintenance_2026_04_13t01_54_14_schema_01"] after cleaning in 28.042877ms [4 generated] [10 reused]
            test_signal.go:95: timed out waiting on test signal after 10s
            reindexer_test.go:211:
                    Error Trace:	/home/runner/work/river/river/internal/maintenance/reindexer_test.go:211
                    Error:      	Should be false
                    Test:       	TestReindexer/ReindexesMinimalSubsetofIndexes
            logger.go:256: time=2026-04-13T01:54:28.444Z level=INFO msg="maintenance.Reindexer: Signaled to stop during index build; attempting to clean up concurrent artifacts"
            riverdbtest.go:293: Checked in postgres schema "maintenance_2026_04_13t01_54_14_schema_01"; 1 idle schema(s) [5 generated] [24 reused]
    FAIL
    FAIL	github.com/riverqueue/river/internal/maintenance	18.764s

I'm diagnosing with Claude's help here, but what appears to be happening
is that although a reindex operation in Postgres is often fast, it is
still a heavy operation, and can slow down even further when there's a
lot of concurrent activity hammering a database.

Many reindexer test cases run in parallel, and it appears that was
happening here is that we got a reindex that exceeded our maximum
timeout of 10x. We have some evidence this during the test run from the
runtime of 10.07s and the line:

    Signaled to stop during index build; attempting to clean up concurrent artifacts

Here, I'm putting forward a solution proposed by Claude, which is to
mock out the reindex operation, especially where we have a number of
reindexes going in parallel. The tests `ReindexOneSuccess` and
`ReindexSkippedWithReindexArtifact` still put load on the real reindex
operation, so we're not going full mock here, and should see our
intermittency considerably reduced while still being confident that
everything still works.

Author: brandur

internal/maintenance/reindexer_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -24,13 +25,16 @@ type reindexerExecutorMock struct {
 	indexesExistCalls  atomic.Int32
 	indexesExistFunc   func(ctx context.Context, params *riverdriver.IndexesExistParams) (map[string]bool, error)
 	indexesExistSignal chan struct{}
+
+	indexReindexFunc func(ctx context.Context, params *riverdriver.IndexReindexParams) error
 }
 
 func newReindexerExecutorMock(exec riverdriver.Executor) *reindexerExecutorMock {
 	return &reindexerExecutorMock{
 		Executor:           exec,
 		indexesExistFunc:   exec.IndexesExist,
 		indexesExistSignal: make(chan struct{}, 10),
+		indexReindexFunc:   exec.IndexReindex,
 	}
 }
 
@@ -45,6 +49,10 @@ func (m *reindexerExecutorMock) IndexesExist(ctx context.Context, params *riverd
 	return m.indexesExistFunc(ctx, params)
 }
 
+func (m *reindexerExecutorMock) IndexReindex(ctx context.Context, params *riverdriver.IndexReindexParams) error {
+	return m.indexReindexFunc(ctx, params)
+}
+
 func TestReindexer(t *testing.T) {
 	t.Parallel()
 
@@ -182,6 +190,20 @@ func TestReindexer(t *testing.T) {
 
 		svc, bundle := setup(t)
 
+		// Mock IndexReindex so the test doesn't depend on the speed of real
+		// REINDEX CONCURRENTLY operations on the shared CI database. Track
+		// which indexes got reindexed so we can verify the expected set.
+		mockExec := newReindexerExecutorMock(bundle.exec)
+		var reindexedNames []string
+		var reindexedMu sync.Mutex
+		mockExec.indexReindexFunc = func(ctx context.Context, params *riverdriver.IndexReindexParams) error {
+			reindexedMu.Lock()
+			defer reindexedMu.Unlock()
+			reindexedNames = append(reindexedNames, params.Index)
+			return nil
+		}
+		svc.exec = mockExec
+
 		svc.Config.IndexNames = []string{
 			"river_job_kind",
 			"river_job_prioritized_fetching_index",
@@ -198,25 +220,23 @@ func TestReindexer(t *testing.T) {
 		case <-time.After(100 * time.Millisecond):
 		}
 
-		// Make sure that no `CONCURRENTLY` artifacts exist after reindexing is
-		// supposed to be done. Postgres creates a new index suffixed with
-		// `_ccnew` before swapping it in as the new index. The existing index
-		// is renamed `_ccold` before being dropped concurrently.
-		//
-		// https://www.postgresql.org/docs/current/sql-reindex.html#SQL-REINDEX-CONCURRENTLY
-		for _, indexName := range svc.Config.IndexNames {
-			for _, reindexArtifactName := range []string{indexName + "_ccnew", indexName + "_ccold"} {
-				indexExists, err := bundle.exec.IndexExists(ctx, &riverdriver.IndexExistsParams{Index: reindexArtifactName, Schema: bundle.schema})
-				require.NoError(t, err)
-				require.False(t, indexExists)
-			}
-		}
+		reindexedMu.Lock()
+		require.ElementsMatch(t, svc.Config.IndexNames, reindexedNames)
+		reindexedMu.Unlock()
 	})
 
 	t.Run("ReindexesConfiguredIndexes", func(t *testing.T) {
 		t.Parallel()
 
-		svc, _ := setup(t)
+		svc, bundle := setup(t)
+
+		// Mock IndexReindex so the test doesn't depend on the speed of real
+		// REINDEX CONCURRENTLY operations on the shared CI database.
+		mockExec := newReindexerExecutorMock(bundle.exec)
+		mockExec.indexReindexFunc = func(ctx context.Context, params *riverdriver.IndexReindexParams) error {
+			return nil
+		}
+		svc.exec = mockExec
 
 		svc.Config.ScheduleFunc = runImmediatelyThenOnceAnHour()