This is used to forward ssh-agent connection to Git Bash' ssh-agent.

I'm not sure this is the cleanest way to implement this, but I've not found existing library functions that could reduce the amount of code.

I've checked that the forwarding work using a "test" and a pipe server to be able to test this case:
ssh-add => \\.\pipe\mypipe => test => call connectLocal to forward to Git Bash's ssh-agent socket

A way to test with VS Code would be better than using a manual test, but I don't how I could integrate it in VS Code.

I've also tried to test the connection to gpg-agent (via the socket file named S.gpg-agent.ssh), but I couldn't make it works, despite expected packets are sent to gpg-agent according to wireshark. It seems that something broke on gpg-agent side according to:

• https://github.com/rupor-github/win-gpg-agent

  Assuan S.gpg-agent.ssh support in GnuPG code is presently broken under Windows (at least in GnuPG 2.2.25), so we have to resort to putty/pageant method instead

• https://dev.gnupg.org/T3883#113453

  It appears that the actual ssh-agent message coming across the bridge is never read or acted upon, but the socket connection isn't closed despite the log message stating that the handler is terminated.

• https://dev.gnupg.org/T3883#150791

  With Assuan S.gpg-agent.ssh support in GnuPG code was broken under Windows, [...]

• https://dev.gnupg.org/T3883#150854

  Also last time I looked for whatever reasons Assuan S.gpg-agent.ssh support in GnuPG code was broken under Windows (there was a comment in the code on that)

Here is the explanation of what is required to connect to a cygwin / git bash unix domain socket on Windows:

• Port parsing:
  • Git Bash' unix sockets requires connecting to the port whose number is in the socket file along with a cookie.
  • The socket file contains something like !<socket >63488 s 44693F4F-E2572CA5-537862AB-248DFDEF
  • The port here is 63488 and the cookie is 44693F4F-E2572CA5-537862AB-248DFDEF.
  • So I retrieve the port and cookie using a regex and convert it to a number.
    • If the file content does not match the regex, I assume this is a GPG socket and use the existing code to parse it.
  • When I have the port and the cookie, I connect to 127.0.0.1:<port>, then I do the following handshake.
• Cygwin / Git Bash socket Handshake:
  • The handshake consists in:
  • the client must send the cookie as 16 raw bytes
    - The cookie is formatted in the socket file as 4 32 bits hex integers. They must be send to the ssh-agent server in little endian as 16 raw bytes (this means according to the above example: 0x4F 0x3F 0x69 0x44 0xA5 0x72 ...).
  • the server send back the same 16 bytes if the cookie is valid, else closes the connection (so the client must skip these 16 bytes, as done in skipHeader)
  • the client must send pid and user id and user effective id information in a 12 bytes packet
    • I set the pid to a real value from process.pid, but ssh-agent ignores it
    • user id and user effective id are both set to 0
  • the server send back the same information, but about the server, I just ignore these 12 bytes too in skipHeader; this is a function that just skip the handshake data).

As the server send back data in the handshake phase (16 + 12 bytes), I need to skip them through the use of skipHeader.

Then actual data transfer can take place.

See also:
https://stackoverflow.com/questions/23086038/what-mechanism-is-used-by-msys-cygwin-to-emulate-unix-domain-sockets
https://github.com/abourget/secrets-bridge/blob/094959a1553943e0727f6524289e12e8aab697bf/pkg/agentfwd/agentconn_windows.go#L15

Fix: #62