Previously:
1. last_error wouldn't be updated with errors from is_dir;
2. We'd always issue a stat(), even for binaries without execute;
3. We used stat() instead of access(), which is cheaper.
This change avoids all of those, by only checking inside X_OK-positive
case whether access() works on the path with an extra slash appended.
Thanks to Lennart for the suggestion.
This allows us to later port nss-resolve to use Varlink rather than
D-Bus for resolution. This has the benefit that nss-resolve based
resoluton works even without D-Bus being up. And it's faster too.
It's strictly bus-specific, hence let's move this to resolved-bus.c like
the rest of the bus specific logic.
This is also in preparation for adding an alternative varlink transport,
which needs similar functionality, but varlink instead of bus-specific.
Let's prepare for adding a new varlink interface, and thus rename the
"request" field to "bus_request", so that we can later add a
varlink_request field too.
It's pretty useful to be able to access the bytes generically, without
acknowledging a specific family, hence let's a third way to access an
in_addr_union.
Imagine $PATH /a:/b. There is an echo command at /b/echo. Under this
configuration, this works fine:
% systemd-run --user --scope echo .
Running scope as unit: run-rfe98e0574b424d63a641644af511ff30.scope
.
However, if I do `mkdir /a/echo`, this happens:
% systemd-run --user --scope echo .
Running scope as unit: run-rcbe9369537ed47f282ee12ce9f692046.scope
Failed to execute: Permission denied
We check whether the resulting file is executable for the performing
user, but of course, most directories are anyway, since that's needed to
list within it. As such, another is_dir() check is needed prior to
considering the search result final.
Another approach might be to check S_ISREG, but there may be more gnarly
edge cases there than just eliminating this obviously pathological
example, so let's just do this for now.
