from mutex to semaphore chan

src/db/ctx.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"path"
 	"strings"
-	"sync"
 	"testing"
 	"time"
 
@@ -70,13 +69,13 @@ func Inject(ctx context.Context, conn string) (context.Context, error) {
 		return ctx, err
 	}
 
-	return context.WithValue(context.WithValue(ctx, ctxKey+"_lock", &sync.Mutex{}), ctxKey, db), ctx.Err()
+	return context.WithValue(context.WithValue(ctx, ctxKey+"_lock", newSemaphore()), ctxKey, db), ctx.Err()
 }
 
-func extract(ctx context.Context) (*sql.DB, *sync.Mutex, error) {
+func extract(ctx context.Context) (*sql.DB, semaphore, error) {
 	db := ctx.Value(ctxKey)
 	if db == nil {
 		return nil, nil, fmt.Errorf("db not injected")
 	}
-	return db.(*sql.DB), ctx.Value(ctxKey + "_lock").(*sync.Mutex), nil
+	return db.(*sql.DB), ctx.Value(ctxKey + "_lock").(semaphore), nil
 }

src/db/db.go

@@ -129,11 +129,11 @@ func Exec(ctx context.Context, q string, args ...any) error {
 }
 
 func with(ctx context.Context, foo func(*sql.DB) error) error {
-	db, lock, err := extract(ctx)
+	db, sem, err := extract(ctx)
 	if err != nil {
 		return err
 	}
-	lock.Lock()
-	defer lock.Unlock()
-	return foo(db)
+	return sem.With(ctx, func() error {
+		return foo(db)
+	})
 }

src/db/sem.go

@@ -0,0 +1,21 @@
+package db
+
+import "context"
+
+type semaphore chan struct{}
+
+func newSemaphore() semaphore {
+	return make(semaphore, 1)
+}
+
+func (semaphore semaphore) With(ctx context.Context, cb func() error) error {
+	select {
+	case semaphore <- struct{}{}:
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+	defer func() {
+		<-semaphore
+	}()
+	return cb()
+}