In virPolkitAgentCreate neglected to initialize agent to NULL. If
there was an error in the pipe, then we jump to error and would have
an issue. Found by coverity.

Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

Introduce virPolkitAgentCreate and virPolkitAgentDestroy

virPolkitAgentCreate will run the polkit pkttyagent image as an asynchronous
command in order to handle the local agent authentication via stdin/stdout.
The code makes use of the pkttyagent --notify-fd mechanism to let it know
when the agent is successfully registered.

virPolkitAgentDestroy will close the command effectively reaping our
child process

When there isn't a ssh -X type session running and a user has not
been added to the libvirt group, attempts to run 'virsh -c qemu:///system'
commands from an otherwise unprivileged user will fail with rather
generic or opaque error message:

    "error: authentication failed: no agent is available to authenticate"

This patch will adjust the error code and message to help reflect the
situation that the problem is the requested mechanism is UNAVAILABLE and
a slightly more descriptive error. The result on a failure then becomes:

    "error: authentication unavailable: no polkit agent available to
            authenticate action 'org.libvirt.unix.manage'"

A bit more history on this - at one time a failure generated the
following type message when running the 'pkcheck' as a subprocess:

"error: authentication failed: polkit\56retains_authorization_after_challenge=1
Authorization requires authentication but no agent is available."

but, a patch was generated to adjust the error message to help provide
more details about what failed. This was pushed as commit id '96a108c9'.
That patch prepended a "polkit: " to the output. It really didn't solve
the problem, but gave a hint.

After some time it was deemed using DBus API calls directly was a
better way to go (since pkcheck calls them anyway). So, commit id
'1b854c76' (more or less) copied the code from remoteDispatchAuthPolkit
and adjusted it. Then commit id 'c7542573' adjusted the remote.c
code to call the new API (virPolkitCheckAuth). Finally, commit id
'308c0c5a' altered the code to call DBus APIs directly. In doing
so, it reverted the failing error message to the generic message
that would have been received from DBus anyway.

It does exactly the same thing as virStringListLength() and it's used in
one place only.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

Use of an 'int' to represent a 'bool' value is confusing.  Just
because dbus made the mistake of cementing their 4-byte wire
format of dbus_bool_t into their API doesn't mean we have to
repeat the mistake.  With a little bit of finesse, we can
guarantee that we provide a large-enough value to the DBus
code, while still copying only the relevant one-byte bool
to the client code, and isolate the rest of our code base from
the DBus stupidity.

* src/util/virdbus.c (GET_NEXT_VAL): Add parameter.
(virDBusMessageIterDecode): Adjust all clients.
* src/util/virpolkit.c (virPolkitCheckAuth): Use nicer type.
* tests/virdbustest.c (testMessageSimple, testMessageStruct):
Test new behavior.
Signed-off-by: NEric Blake <eblake@redhat.com>

The commit 1b854c76 introduced a new function 'virPolkitCheckAuth' and
in the #else section when you don't have polkit all attributes should be
follwed by ATTRIBUTE_UNUSED.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Spawning the pkcheck program every time a permission check is
required is hugely expensive on CPU. The pkcheck program is just
a dumb wrapper for the DBus API, so rewrite the code to use the
DBus API directly. This also simplifies error handling a bit.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

There are now two places in libvirt which use polkit. Currently
they use pkexec, which is set to be replaced by direct DBus API
calls. Add a common API which they will both be able to use for
this purpose.

No tests are added at this time, since the impl will be gutted
in favour of a DBus API call shortly.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>