2 files changed, 208 insertions, 0 deletions
diff --git a/internal/queue/inmemqueue/queue.go b/internal/queue/inmemqueue/queue.go
new file mode 100644
index 00000000..f930f5d5
--- /dev/null
+++ b/internal/queue/inmemqueue/queue.go
@@ -0,0 +1,90 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package inmemqueue provides an in-memory queue implementation that can be
+// used for scheduling of fetch actions.
+package inmemqueue
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"golang.org/x/pkgsite/internal"
+	"golang.org/x/pkgsite/internal/experiment"
+	"golang.org/x/pkgsite/internal/log"
+	"golang.org/x/pkgsite/internal/queue"
+)
+
+// InMemory is a Queue implementation that schedules in-process fetch
+// operations. Unlike the GCP task queue, it will not automatically retry tasks
+// on failure.
+//
+// This should only be used for local development.
+type InMemory struct {
+	queue       chan internal.Modver
+	done        chan struct{}
+	experiments []string
+}
+
+type InMemoryProcessFunc func(context.Context, string, string) (int, error)
+
+// New creates a new InMemory that asynchronously fetches from proxyClient and
+// stores in db. It uses workerCount parallelism to execute these fetches.
+func New(ctx context.Context, workerCount int, experiments []string, processFunc InMemoryProcessFunc) *InMemory {
+	q := &InMemory{
+		queue:       make(chan internal.Modver, 1000),
+		experiments: experiments,
+		done:        make(chan struct{}),
+	}
+	sem := make(chan struct{}, workerCount)
+	go func() {
+		for v := range q.queue {
+			select {
+			case <-ctx.Done():
+				return
+			case sem <- struct{}{}:
+			}
+
+			// If a worker is available, make a request to the fetch service inside a
+			// goroutine and wait for it to finish.
+			go func(v internal.Modver) {
+				defer func() { <-sem }()
+
+				log.Infof(ctx, "Fetch requested: %s (workerCount = %d)", v, cap(sem))
+
+				fetchCtx, cancel := context.WithTimeout(ctx, 5*time.Minute)
+				fetchCtx = experiment.NewContext(fetchCtx, experiments...)
+				defer cancel()
+
+				if _, err := processFunc(fetchCtx, v.Path, v.Version); err != nil {
+					log.Error(fetchCtx, err)
+				}
+			}(v)
+		}
+		for i := 0; i < cap(sem); i++ {
+			select {
+			case <-ctx.Done():
+				panic(fmt.Sprintf("InMemory queue context done: %v", ctx.Err()))
+			case sem <- struct{}{}:
+			}
+		}
+		close(q.done)
+	}()
+	return q
+}
+
+// ScheduleFetch pushes a fetch task into the local queue to be processed
+// asynchronously.
+func (q *InMemory) ScheduleFetch(ctx context.Context, modulePath, version string, _ *queue.Options) (bool, error) {
+	q.queue <- internal.Modver{Path: modulePath, Version: version}
+	return true, nil
+}
+
+// WaitForTesting waits for all queued requests to finish. It should only be
+// used by test code.
+func (q *InMemory) WaitForTesting(ctx context.Context) {
+	close(q.queue)
+	<-q.done
+}
diff --git a/internal/queue/inmemqueue/queue_test.go b/internal/queue/inmemqueue/queue_test.go
new file mode 100644
index 00000000..79847140
--- /dev/null
+++ b/internal/queue/inmemqueue/queue_test.go
@@ -0,0 +1,118 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package inmemqueue
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"testing"
+)
+
+func TestInMemory_ScheduleAndProcess(t *testing.T) {
+	ctx := context.Background()
+
+	var processed int
+	var mu sync.Mutex
+
+	processFunc := func(ctx context.Context, path, version string) (int, error) {
+		mu.Lock()
+		processed++
+		mu.Unlock()
+		return 200, nil
+	}
+
+	q := New(ctx, 2, nil, processFunc)
+
+	tests := []struct {
+		path    string
+		version string
+	}{
+		{"example.com/mod1", "v1.0.0"},
+		{"example.com/mod2", "v1.1.0"},
+		{"example.com/mod3", "v2.0.0"},
+	}
+
+	for _, tc := range tests {
+		ok, err := q.ScheduleFetch(ctx, tc.path, tc.version, nil)
+		if err != nil {
+			t.Fatalf("ScheduleFetch(%q, %q) returned error: %v", tc.path, tc.version, err)
+		}
+		if !ok {
+			t.Errorf("ScheduleFetch(%q, %q) = false, want true", tc.path, tc.version)
+		}
+	}
+
+	q.WaitForTesting(ctx)
+
+	if got, want := processed, len(tests); got != want {
+		t.Errorf("processed tasks = %d, want %d", got, want)
+	}
+}
+
+func TestInMemory_ProcessFuncError(t *testing.T) {
+	ctx := context.Background()
+
+	processFunc := func(ctx context.Context, path, version string) (int, error) {
+		return 500, errors.New("simulated fetch error")
+	}
+
+	q := New(ctx, 1, nil, processFunc)
+
+	_, err := q.ScheduleFetch(ctx, "example.com/error-mod", "v1.0.0", nil)
+	if err != nil {
+		t.Fatalf("ScheduleFetch returned error: %v", err)
+	}
+
+	// This should complete without panicking or deadlocking.
+	q.WaitForTesting(ctx)
+}
+
+func TestInMemory_WorkerConcurrency(t *testing.T) {
+	ctx := context.Background()
+	workerCount := 2
+
+	var active, max int
+	var mu sync.Mutex
+	var wg sync.WaitGroup
+
+	blockCh := make(chan struct{})
+
+	processFunc := func(ctx context.Context, path, version string) (int, error) {
+		mu.Lock()
+		active++
+		if active > max {
+			max = active
+		}
+		mu.Unlock()
+
+		<-blockCh
+
+		mu.Lock()
+		active--
+		mu.Unlock()
+		wg.Done()
+		return 200, nil
+	}
+
+	q := New(ctx, workerCount, nil, processFunc)
+
+	taskCount := 5
+	wg.Add(taskCount)
+	for range taskCount {
+		_, err := q.ScheduleFetch(ctx, "example.com/concurrent", "v1.0.0", nil)
+		if err != nil {
+			t.Fatalf("ScheduleFetch returned error: %v", err)
+		}
+	}
+
+	close(blockCh)
+	wg.Wait()
+	q.WaitForTesting(ctx)
+
+	if max > workerCount {
+		t.Errorf("max concurrent workers = %d, want <= %d", max, workerCount)
+	}
+}