webdav: prevent webdav client race condition (#626)

The race is described in
https://github.com/studio-b12/gowebdav/issues/36

We need this workaround until the fix is merged upstream,
to avoid maintaining a webdav client fork.

Fixes #624

repo/blob/rclone/rclone_storage_test.go

@@ -6,8 +6,13 @@
 	"os/exec"
 	"testing"
 
+	"github.com/google/uuid"
+	"github.com/pkg/errors"
+	"golang.org/x/sync/errgroup"
+
 	"github.com/kopia/kopia/internal/blobtesting"
 	"github.com/kopia/kopia/internal/testlogging"
+	"github.com/kopia/kopia/repo/blob"
 	"github.com/kopia/kopia/repo/blob/rclone"
 )
 
@@ -36,6 +41,24 @@ func TestRCloneStorage(t *testing.T) {
 
 	defer st.Close(ctx)
 
+	var eg errgroup.Group
+
+	// trigger multiple parallel reads to ensure we're properly preventing race
+	// described in https://github.com/kopia/kopia/issues/624
+	for i := 0; i < 100; i++ {
+		eg.Go(func() error {
+			if _, err := st.GetBlob(ctx, blob.ID(uuid.New().String()), 0, -1); !errors.Is(err, blob.ErrBlobNotFound) {
+				return errors.Errorf("unexpected error when downloading non-existent blob: %v", err)
+			}
+
+			return nil
+		})
+	}
+
+	if err := eg.Wait(); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
 	blobtesting.VerifyStorage(ctx, t, st)
 	blobtesting.AssertConnectionInfoRoundTrips(ctx, t, st)
 }

repo/blob/webdav/webdav_storage.go

@@ -12,6 +12,7 @@
 	"strings"
 	"time"
 
+	"github.com/google/uuid"
 	"github.com/pkg/errors"
 	"github.com/studio-b12/gowebdav"
 
@@ -201,7 +202,7 @@ func New(ctx context.Context, opts *Options) (blob.Storage, error) {
 		cli.SetTransport(tlsutil.TransportTrustingSingleCertificate(opts.TrustedServerCertificateFingerprint))
 	}
 
-	return &davStorage{
+	s := &davStorage{
 		sharded.Storage{
 			Impl: &davStorageImpl{
 				Options: *opts,
@@ -211,7 +212,16 @@ func New(ctx context.Context, opts *Options) (blob.Storage, error) {
 			Suffix:   fsStorageChunkSuffix,
 			Shards:   opts.shards(),
 		},
-	}, nil
+	}
+
+	// temporary workaround to a race condition problem in https://github.com/studio-b12/gowebdav/issues/36
+	// the race condition is only during first request to the server, so to fix it we force the first request
+	// to read a non-existent blob, which sets the authentication method.
+	if _, err := s.GetBlob(ctx, blob.ID(uuid.New().String()), 0, -1); !errors.Is(err, blob.ErrBlobNotFound) {
+		return nil, errors.Errorf("unexpected error when initializing webdav: %v", err)
+	}
+
+	return s, nil
 }
 
 func init() {