Built-in SSH transport

[Sebastian Thiel (Byron)]

Summary 💡

A built-in transport that allows to manually configure an SSH connection without the need for the ssh program.

Candidates for the ssh part are:

• russh - Rust
  • even with limited support for ssh-agent, which makes it something I'd seriously look at
• ssh2 - Rust bindings to libssh2
• libssh2 - C library
  • used by libgit2
  • desirable license, but different feature set, maybe less mature than libssh
• libssh - C library
  • Maybe more mature than libssh2, but has undesirable license

Motivation 🔦

Cargo should be standalone, and right now it needs the ssh-program to clone ssh:// URLs. Even though this is beneficial on Linux, it's usually not very portable and often won't work at all on Windows. git2 has ssh support built-in using libssh which works, but has a less desirable license than libssh2.

References

• needs a notion of a SSH-Key in this struct by making it an enum
  • Maybe more is needed to support all git2 ssh configuration options, which is what cargo caters to and is implemented with.
• Implement a sync (or async if needed) version of the Transport trait
• The entrypoint to connect using the ssh program, along with the the git-upload-pack invocation

Suggestions

• Try to add an actual test using the existing ssh transport to create a connection to a local (and controlled) ssh daemon. This could be very useful when testing a new implementation.

Requirement

• Needs a feature toggle, and added dependencies are optional (but pulled in by the feature)

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Original issue: #1246