internal/database/webhook_db_manager.go

@@ -41,6 +41,11 @@ type WebhookDBManager struct {
 	dataDir string
 	dbs     sync.Map // map[webhookID]*gorm.DB
 	log     *slog.Logger
+
+	// mu is held while a database is opened, deleted, or closed, so
+	// each file has at most one open handle. Reading an already cached
+	// handle does not take it.
+	mu sync.Mutex
 }
 
 // NewWebhookDBManager creates a new WebhookDBManager and
@@ -86,35 +91,34 @@ func (m *WebhookDBManager) GetDB(
 ) (*gorm.DB, error) {
 	// Fast path: already open
 	if val, ok := m.dbs.Load(webhookID); ok {
-		cachedDB, castOK := val.(*gorm.DB)
-		if !castOK {
-			return nil, fmt.Errorf(
-				"%w for webhook %s",
-				errInvalidCachedDBType,
-				webhookID,
-			)
+		return asGormDB(val, webhookID)
 	}
 
-		return cachedDB, nil
+	// Slow path: open the database under the lock, looking in the
+	// cache again first. A caller that raced another one here then
+	// waits for its handle instead of opening a second one.
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if val, ok := m.dbs.Load(webhookID); ok {
+		return asGormDB(val, webhookID)
 	}
 
-	// Slow path: open/create the database
 	db, err := m.openDB(webhookID)
 	if err != nil {
 		return nil, err
 	}
 
-	// Store it; if another goroutine beat us, close ours
-	actual, loaded := m.dbs.LoadOrStore(webhookID, db)
-	if loaded {
-		// Another goroutine created it first; close our duplicate
-		sqlDB, closeErr := db.DB()
-		if closeErr == nil {
-			_ = sqlDB.Close()
+	m.dbs.Store(webhookID, db)
+
+	return db, nil
 }
 
-		existingDB, castOK := actual.(*gorm.DB)
-		if !castOK {
+// asGormDB returns a value read from the cache as the database
+// handle it is.
+func asGormDB(val any, webhookID string) (*gorm.DB, error) {
+	db, ok := val.(*gorm.DB)
+	if !ok {
 		return nil, fmt.Errorf(
 			"%w for webhook %s",
 			errInvalidCachedDBType,
@@ -122,9 +126,6 @@ func (m *WebhookDBManager) GetDB(
 		)
 	}
 
-		return existingDB, nil
-	}
-
 	return db, nil
 }
 
@@ -153,6 +154,11 @@ func (m *WebhookDBManager) DBExists(
 func (m *WebhookDBManager) DeleteDB(
 	webhookID string,
 ) error {
+	// Held until the files are gone, so GetDB cannot open the file
+	// again between the close and the removal.
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
 	// Close and remove from cache
 	if val, ok := m.dbs.LoadAndDelete(webhookID); ok {
 		if gormDB, castOK := val.(*gorm.DB); castOK {
@@ -186,6 +192,11 @@ func (m *WebhookDBManager) DeleteDB(
 // CloseAll closes all open per-webhook database connections.
 // Called during application shutdown.
 func (m *WebhookDBManager) CloseAll() error {
+	// An open already under way finishes and is cached first, so it
+	// is closed here rather than cached after this loop has passed.
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
 	var lastErr error
 
 	m.dbs.Range(func(key, value any) bool {

internal/database/webhook_db_manager_test.go

@@ -1,10 +1,14 @@
 package database_test
 
 import (
+	"bytes"
 	"context"
+	"log/slog"
 	"net/http"
 	"os"
 	"path/filepath"
+	"strings"
+	"sync"
 	"testing"
 
 	"github.com/google/uuid"
@@ -104,6 +108,54 @@ func TestWebhookDBManager_CreateAndGetDB(t *testing.T) {
 	assert.Equal(t, `{"test": true}`, readEvent.Body)
 }
 
+// Many callers ask for one webhook's database at the same moment,
+// before it is cached. Only one of them may open the file; the others
+// must wait for its handle. openDB logs one "opened per-webhook
+// database" line per open, and those lines are what is counted.
+func TestWebhookDBManager_ConcurrentFirstTouchOpensOnce(t *testing.T) {
+	t.Parallel()
+
+	var logs bytes.Buffer
+
+	mgr := database.NewTestWebhookDBManagerWithLogger(
+		t.TempDir(),
+		slog.New(slog.NewTextHandler(&logs, nil)),
+	)
+
+	t.Cleanup(func() { assert.NoError(t, mgr.CloseAll()) })
+
+	webhookID := uuid.New().String()
+
+	const callers = 16
+
+	start := make(chan struct{})
+	handles := make([]*gorm.DB, callers)
+	errs := make([]error, callers)
+
+	var wg sync.WaitGroup
+
+	for i := range callers {
+		wg.Go(func() {
+			<-start
+
+			handles[i], errs[i] = mgr.GetDB(webhookID)
+		})
+	}
+
+	close(start)
+	wg.Wait()
+
+	for i := range callers {
+		require.NoError(t, errs[i])
+		assert.Same(t, handles[0], handles[i])
+	}
+
+	assert.Equal(
+		t, 1,
+		strings.Count(logs.String(), "opened per-webhook database"),
+	)
+}
+
 func TestWebhookDBManager_DeleteDB(t *testing.T) {
 	t.Parallel()