Context

I ran into this while working on adding native GCS multipart upload support to s3proxy (see gaul/s3proxy#76). We use setHost() to configure the GCS endpoint, and the MultipartUploadClient operations fail with DNS errors.

What happens

With setHost("https://storage.googleapis.com") (no trailing slash):

Expected: https://storage.googleapis.com/my-bucket?uploads
Actual:   https://storage.googleapis.commy-bucket?uploads
                                       ^^^ missing slash!

This causes a DNS lookup failure since storage.googleapis.commy-bucket isn't a valid hostname.

Reproduction

HttpStorageOptions options = StorageOptions.http()
    .setHost("https://storage.googleapis.com")  // no trailing slash
    .build();

MultipartUploadClient client = MultipartUploadClient.create(
    MultipartUploadSettings.of(options));

// Fails with DNS error:
client.createMultipartUpload(...);  // StorageException: storage.googleapis.commy-bucket

Workaround

Add a trailing slash to your host:

.setHost("https://storage.googleapis.com/")  // note the trailing slash

Or just don't call setHost() at all if you're using the default GCS endpoint.

Root cause

In MultipartUploadHttpRequestManager.java, the URI templates don't start with /:

UriTemplate.expand(uri.toString(), "{bucket}/{key}?uploads", params, false);

Looking at the UriTemplate.expand implementation:

When the template doesn't start with /, it simply concatenates baseUrl + uriTemplate:

Why tests didn't catch this

The test harness (FakeHttpServer) always creates endpoints with a trailing slash:

URI endpoint = URI.create("http://localhost:" + port + "/");

So the bug only shows up when explicitly using setHost()