When a provider source produces an HTTP URL location we'll expect it to
resolve to a zip file, which we'll first download to a temporary
directory and then treat it like a local archive.
When a provider source produces a local archive path we'll expect it to
be a zip file and extract it into the target directory.
This does not yet include an implementation of installing from an
already-unpacked local directory. That will follow in a subsequent commit,
likely following a similar principle as in Dir.LinkFromOtherCache.
This will eventually be responsible for actually retrieving a package from
a source and then installing it into the cache directory, but for the
moment it's just a stub to complete the proposed API, which I intend to
test in a subsequent commit by writing the full "Installer" API that will
encapsulate the full installation logic.
When a system-wide shared plugin cache is configured, we'll want to make
use of entries already in the shared cache when populating a local
(configuration-specific) cache.
This new method LinkFromOtherCache encapsulates the work of placing a link
from one cache to another. If possible it will create a symlink, therefore
retaining a key advantage of configuring a shared plugin cache, but
otherwise we'll do a deep copy of the package directory from one cache
to the other.
Our old provider installer would always skip trying to create symlinks on
Windows because Go standard library support for os.Symlink on Windows
was inconsistent in older versions. However, os.Symlink can now create
symlinks using a new API introduced in a Windows 10 update and cleanly
fail if symlink creation is impossible, so it's safe for us to just
try to create the symlink and react if that produces an error, just as we
used to do on non-Windows systems when possibly creating symlinks on
filesystems that cannot support them.
Martin Atkins