join path properly

[midnight-wonderer]

This thing messed up my setup.
I believe this is the proper way to join paths.

Warning: untested code; I have no professional Golang experience.

[ss098]

Hello,

Thanks for the PR.

Is it because the Prefix is not configured that the module cannot work?

[midnight-wonderer]

Hey, it kinda complicated

The S3-compatible storage I am using, allows setting key prefixes access control. For example, I can grant access to some/prefix/path to access key A; and some/other/path to access key B.

S3 is just a big KV store, it doesn't actually have a concept of directories.

That why:
To mimic the directory access grant, the only logical way to do it is to add a trailing / to the rule. Key A, which should be able to access some/prefix/path, should have the prefix rule of some/prefix/path/ (with trailing slash), because otherwise, it will be able to access some/prefix/path-and/some/other/adjacent/path too.

But once I add a trailing slash to the prefix rule:

• I can configure prefix to certmagic-s3 with a trailing slash.
• Or config prefix to certmagic-s3 without trailing slash.

Both have an issue:

• with trailing slash, I'll get objects with double slashes as shown by the patch.
• without trailing slash, certmagic-s3 somehow checks for permission at the root and determines that it doesn't have write permission and fails. To illustrate this, the prefix rule is some/path/ but certmagic checks for permission at some/path, which is correctly denied.

There are two possible solutions on the top of my head, but this PR is the better one. Simply join paths like what average people expected. I want double slashes in the middle of file paths; said no one ever.