On many systems, the sockaddr_un.sun_path field is quite
small. Even on Linux, it is only 108 characters. A user of
the credential-cache daemon can easily surpass this,
especially if their home directory is in a deep directory
tree (since the default location expands ~/.git-credentials).
We can hack around this in the unix-socket.[ch] code by
doing a chdir() to the enclosing directory, feeding the
relative basename to the socket functions, and then
restoring the working directory.
This introduces several new possible error cases for
creating a socket, including an irrecoverable one in the
case that we can't restore the working directory. In the
case of the credential-cache code, we could perhaps get away
with simply chdir()-ing to the socket directory and never
coming back. However, I'd rather do it at the lower level
for a few reasons:
1. It keeps the hackery behind an opaque interface instead
of polluting the main program logic.
2. A hack in credential-cache won't help any unix-socket
users who come along later.
3. The chdir trickery isn't that likely to fail (basically
it's only a problem if your cwd is missing or goes away
while you're running). And because we only enable the
hack when we get a too-long name, it can only fail in
cases that would have failed under the previous code
anyway.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>