We expect to read NUL terminated strings. Upon NUL, we should do
something. Treat it as a line break.

Fixes: 8ae9cf4698b4 ('Revert "libnm: buffer reads in nm_vpn_service_plugin_read_vpn_details()"')

This partially reverts commit 4a9fcb0fc32e, which replaced one-byte
reads with buffered ones in the VPN service plugin.

Unfortunately the buffering means that commands coming after the magic
"DONE" string were being pulled into the buffer. Secrets agents expect
a "QUIT" to come after the "DONE", and since with buffering "QUIT" was
in the buffer, this led to a twenty-second delay on every VPN
connection using a secrets manager.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1164

Fixes: 4a9fcb0fc32e ('libnm: buffer reads in nm_vpn_service_plugin_read_vpn_details()')

g_idle_add() uses G_PRIORITY_DEFAULT_IDLE priority. Most of the time we don't
care much about the priority.

But at the places that this patch changes, I think that using
G_PRIORITY_DEFAULT_IDLE (and following g_idle_add()) is more correct. The
reason for this is not very strong, except that it's probably the better
choice. And the old choice was made because I didn't realize that
g_idle_add() uses another default priority. Hence, the old choice was not
for good reasons either.

NMClient is strongly tied to the GMainContext with which it was created.
Several operations must only be called from within the context. There
was an assertion for that.

However, creating (and init_async()) should be allowed to call not
from within the GMainContext. So if the current context has no owner
(is not acquired), then it's also OK.

Fix the assertion for that.

Fixes: ce0e898fb476 ('libnm: refactor caching of D-Bus objects in NMClient')

Found with `git grep 'GError.*[^,)];'| grep ' *= *NULL;' -v`

Fixes: ce0e898fb476 ('libnm: refactor caching of D-Bus objects in NMClient')

When cloud-init job (metadata service crawler) starts, it sends the
SIGTERM signal to nm-cloud-setup and force the nm-cloud-setup to
restart, however, because the error is not initialized as NULL in
`_init_start_cancelled_cb()` before it is set, nm-cloud-setup will hit
a dumped core.

TO fix it, initialize the error as NULL in `_init_start_cancelled_cb()`.

https://bugzilla.redhat.com/show_bug.cgi?id=2027674

Fixes: ce0e898fb476 ('libnm: refactor caching of D-Bus objects in NMClient')

Backtrace:

 #0  g_logv (log_domain=0x7f833a872071 "GLib", log_level=G_LOG_LEVEL_WARNING, format=<optimized out>, args=<optimized out>) at ../glib/gmessages.c:1413
 #1  0x00007f833a81f043 in g_log (log_domain=<optimized out>, log_level=<optimized out>, format=<optimized out>) at ../glib/gmessages.c:1451
 #2  0x00007f833ab97230 in nm_utils_error_set_cancelled (is_disposing=<optimized out>, instance_name=<optimized out>, error=0x7ffff79cb980) at src/libnm-glib-aux/nm-shared-utils.c:2599
 #3  nm_utils_error_set_cancelled (is_disposing=0, instance_name=0x0, error=0x7ffff79cb980) at src/libnm-glib-aux/nm-shared-utils.c:2590
 #4  _init_start_cancelled_cb (cancellable=<optimized out>, user_data=0x5640ca292150) at src/libnm-client-impl/nm-client.c:7324
 #5  _init_start_cancelled_cb (cancellable=<optimized out>, user_data=0x5640ca292150) at src/libnm-client-impl/nm-client.c:7307
 #6  0x00007f833a93094a in _g_closure_invoke_va (param_types=0x0, n_params=<optimized out>, args=0x7ffff79cbb40, instance=0x5640ca267020, return_value=0x0, closure=0x5640ca29d430)
     at ../gobject/gclosure.c:873
 #7  g_signal_emit_valist (instance=0x5640ca267020, signal_id=<optimized out>, detail=0, var_args=var_args@entry=0x7ffff79cbb40) at ../gobject/gsignal.c:3406
 #8  0x00007f833a930a93 in g_signal_emit (instance=instance@entry=0x5640ca267020, signal_id=<optimized out>, detail=detail@entry=0) at ../gobject/gsignal.c:3553
 #9  0x00007f833a9a6475 in g_cancellable_cancel (cancellable=0x5640ca267020) at ../gio/gcancellable.c:513
 #10 g_cancellable_cancel (cancellable=0x5640ca267020) at ../gio/gcancellable.c:487
 #11 0x00005640ca1a8bd4 in sigterm_handler (user_data=0x5640ca267020) at src/nm-cloud-setup/main.c:599
 #12 0x00007f833a819d4f in g_main_dispatch (context=0x5640ca268ef0) at ../glib/gmain.c:3337
 #13 g_main_context_dispatch (context=0x5640ca268ef0) at ../glib/gmain.c:4055
 #14 0x00007f833a86e608 in g_main_context_iterate.constprop.0 (context=0x5640ca268ef0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:4131
 #15 0x00007f833a819463 in g_main_loop_run (loop=0x5640ca24fdb0) at ../glib/gmain.c:4329
 #16 0x00005640ca1a6d04 in nmc_client_new_waitsync (cancellable=0x5640ca267020, out_nmc=0x7ffff79cbfa0, error=0x7ffff79cbf98, first_property_name=0x5640ca1b11db "instance-flags",
     first_property_name=0x5640ca1b11db "instance-flags") at src/libnm-client-aux-extern/nm-libnm-aux.c:129
 #17 0x00005640ca1a3863 in main (argc=1, argv=<optimized out>) at src/nm-cloud-setup/main.c:639

The length argument is optional. That makes sense, because the returned
array is NUL-terminated.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1136

gcc-12.0.1-0.8.fc36 is annoying with false positives.
It's related to g_error() and its `for(;;) ;`.

For example:

    ../src/libnm-glib-aux/nm-shared-utils.c: In function 'nm_utils_parse_inaddr_bin_full':
    ../src/libnm-glib-aux/nm-shared-utils.c:1145:26: error: dangling pointer to 'error' may be used [-Werror=dangling-pointer=]
     1145 |                     error->message);
          |                          ^~
    /usr/include/glib-2.0/glib/gmessages.h:343:32: note: in definition of macro 'g_error'
      343 |                                __VA_ARGS__);         \
          |                                ^~~~~~~~~~~
    ../src/libnm-glib-aux/nm-shared-utils.c:1133:31: note: 'error' declared here
     1133 |         gs_free_error GError *error = NULL;
          |                               ^~~~~
    /usr/include/glib-2.0/glib/gmessages.h:341:25: error: dangling pointer to 'addrbin' may be used [-Werror=dangling-pointer=]
      341 |                         g_log (G_LOG_DOMAIN,         \
          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      342 |                                G_LOG_LEVEL_ERROR,    \
          |                                ~~~~~~~~~~~~~~~~~~~~~~~
      343 |                                __VA_ARGS__);         \
          |                                ~~~~~~~~~~~~
    ../src/libnm-glib-aux/nm-shared-utils.c:1141:13: note: in expansion of macro 'g_error'
     1141 |             g_error("unexpected assertion failure: could parse \"%s\" as %s, but not accepted by "
          |             ^~~~~~~
    ../src/libnm-glib-aux/nm-shared-utils.c:1112:14: note: 'addrbin' declared here
     1112 |     NMIPAddr addrbin;
          |              ^~~~~~~

I think the warning could potentially be useful and prevent real bugs.
So don't disable it altogether, but go through the effort to suppress it
at the places where it currently happens.

Note that NM_PRAGMA_WARNING_DISABLE_DANGLING_POINTER macro only expands
to suppressing the warning with __GNUC__ equal to 12. The purpose is to
only suppress the warning where we know we want to. Hopefully other gcc
versions don't have this problem.

I guess, we could also write a NM_COMPILER_WARNING() check in
"m4/compiler_options.m4", to disable the warning if we detect it. But
that seems too cumbersome.

Mark out_result as out argument. Fixes an issue when using libnm bindings.

Fixes: fbb038af5e5d ('all: return output dictionary from "AddAndActivate2"')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1092