fix: remove ListBuckets from s3client, add BucketExists check to NewS3Backend

internal/metadatadb/s3.go

@@ -32,7 +32,7 @@ type S3BackendConfig struct {
 	LockTTL time.Duration
 }
 
-func NewS3Backend(config S3BackendConfig) *S3Backend {
+func NewS3Backend(ctx context.Context, config S3BackendConfig) (*S3Backend, error) {
 	if config.Prefix == "" {
 		config.Prefix = "_meta"
 	}
@@ -43,13 +43,21 @@ func NewS3Backend(config S3BackendConfig) *S3Backend {
 	if logger == nil {
 		logger = slog.Default()
 	}
+	exists, err := config.Client.BucketExists(ctx, config.Bucket)
+	if err != nil {
+		return nil, errors.Errorf("failed to check if bucket exists: %w", err)
+	}
+	if !exists {
+		return nil, errors.Errorf("bucket %s does not exist", config.Bucket)
+	}
+
 	return &S3Backend{
 		client:  config.Client,
 		logger:  logger,
 		bucket:  config.Bucket,
 		prefix:  config.Prefix,
 		lockTTL: config.LockTTL,
-	}
+	}, nil
 }
 
 func (s *S3Backend) stateKey(namespace string) string { return s.prefix + "/" + namespace + ".json" }

internal/metadatadb/s3_test.go

@@ -4,6 +4,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/alecthomas/assert/v2"
+
 	"github.com/block/cachew/internal/metadatadb"
 	"github.com/block/cachew/internal/metadatadb/metadatadbtest"
 	"github.com/block/cachew/internal/s3client/s3clienttest"
@@ -14,24 +16,29 @@ func TestS3Backend(t *testing.T) {
 
 	metadatadbtest.Suite(t, func(t *testing.T) metadatadb.Backend {
 		t.Helper()
-		return metadatadb.NewS3Backend(metadatadb.S3BackendConfig{
+		b, err := metadatadb.NewS3Backend(t.Context(), metadatadb.S3BackendConfig{
 			Client:  s3clienttest.Client(t),
 			Bucket:  bucket,
 			Prefix:  "_meta-" + t.Name(),
 			LockTTL: 5 * time.Second,
 		})
+		assert.NoError(t, err)
+		return b
 	})
 }
 
 func TestS3BackendSoak(t *testing.T) {
 	bucket := s3clienttest.Start(t)
 
-	metadatadbtest.Soak(t, metadatadb.NewS3Backend(metadatadb.S3BackendConfig{
+	b, err := metadatadb.NewS3Backend(t.Context(), metadatadb.S3BackendConfig{
 		Client:  s3clienttest.Client(t),
 		Bucket:  bucket,
 		Prefix:  "_meta-soak",
 		LockTTL: 5 * time.Second,
-	}), metadatadbtest.SoakConfig{
+	})
+	assert.NoError(t, err)
+
+	metadatadbtest.Soak(t, b, metadatadbtest.SoakConfig{
 		Duration:    5 * time.Second,
 		Concurrency: 4,
 		NumKeys:     10,

internal/s3client/s3client.go

@@ -100,11 +100,5 @@ func NewClient(ctx context.Context, config Config) (*minio.Client, error) {
 		return nil, errors.Errorf("failed to create minio client: %w", err)
 	}
 
-	// Verify connectivity and credentials up front so misconfiguration is
-	// caught at startup rather than on the first cache request.
-	if _, err := mc.ListBuckets(ctx); err != nil {
-		return nil, errors.Errorf("failed to connect to S3 endpoint %s: %w", config.Endpoint, err)
-	}
-
 	return mc, nil
 }