When gsutil cp fails, we can resume the session by running the exact same command on the same machine (session is tracked via ~/.gsutil/tracker-files). However, when we switch machines, this directory no longer exists, so when running the same command, the session is not resumed. See Resumable Uploads (CLI) and Background On Resumable Transfers.
Alternatively, I see that with Resumable Uploads (REST APIs), we can generate a session URI and pass this URI to different commands. However, I don't see the option to pass in a session URI in the gsutil cp options listed here.
With this information, is the only way to resume the session via gsutil CLI by sharing the ~/.gsutil/tracker-files volume among different machines?
Both gsutil and gcloud alpha storage use local tracker files to handle resuming uploads, so yes, you'd need to copy those files to a new machine if you wanted to resume the operation elsewhere.
As you noted, the API itself provides a "session URI" that you can use to query the upload progress and resume the upload from anywhere, but I don't believe specifying it explicitly is an option in either command line utility.
Our client libraries do support it, though. If recovering uploads from different machines is a regular part of your workflow, you could perhaps write a small, custom uploader program. Here's an example of how to use resumable uploads with the C++ client. The key section is:
gcs::ObjectWriteStream stream =
client.WriteObject(bucket_name, object_name,
gcs::RestoreResumableUploadSession(session_id));
std::cout << "I should start writing from byte " << stream.next_expected_byte();