file.managed: list source eagerly calls cp.list_master before checking HTTP sources

Description

When file.managed uses a list of sources, file.source_list calls cp.list_master and cp.list_master_dirs before iterating through the list — even if the first source is HTTP and would resolve immediately.

https://github.com/saltstack/salt/blob/master/salt/modules/file.py#L4730-L4740

mfiles = [(f, saltenv) for f in __salt__["cp.list_master"](saltenv)]
mdirs = [(d, saltenv) for d in __salt__["cp.list_master_dirs"](saltenv)]

This means every state with a list source pays the cost of a full fileserver scan upfront, regardless of source order.

Reproduce

/tmp/test_file:
  file.managed:
    - source:
      - https://example.com/myfile
      - salt://myfile
    - skip_verify: True

time salt-call state.apply test

With a large fileserver (10k+ files), this is noticeably slow or times out — even though the HTTP source would succeed in milliseconds.

Expected

Salt should check sources in order and only call cp.list_master if it actually reaches a salt:// entry.

Suggested Fix

Lazy-evaluate — only fetch the file listing when a salt:// source is reached:

_cache = {}

def _get_mfiles(env):
    if env not in _cache:
        _cache[env] = set(__salt__["cp.list_master"](env))
    return _cache[env]

Replace (path, senv) in mfiles with path in _get_mfiles(senv).

Also changes lookup from O(n) list scan to O(1) set membership.

Versions

All versions affected. Confirmed on 3006.x, code unchanged in master.

Start in salt/modules/file.py around lines 4730-4740 and trace file.source_list while reproducing with the provided file.managed state and salt-call state.apply test command. Ensure HTTP sources are checked in order without an upfront fileserver listing, while salt:// sources still resolve correctly and repeated lookups avoid unnecessary scans.