migration: fix crash in when incoming client channel setup fails

migration: fix crash in when incoming client channel setup fails

The way we determine if we can start the incoming migration was
changed to use migration_has_all_channels() in:

  commit 428d89084c709e568f9cd301c2f6416a54c53d6d
  Author: Juan Quintela <quintela@redhat.com>
  Date:   Mon Jul 24 13:06:25 2017 +0200

    migration: Create migration_has_all_channels

This method in turn calls multifd_recv_all_channels_created()
which is hardcoded to always return 'true' when multifd is
not in use. This is a latent bug...

...activated in a following commit where that return result
ends up acting as the flag to indicate whether it is possible
to start processing the migration:

  commit 36c2f8be2c4eb0003ac77a14910842b7ddd7337e
  Author: Juan Quintela <quintela@redhat.com>
  Date:   Wed Mar 7 08:40:52 2018 +0100

    migration: Delay start of migration main routines

This means that if channel initialization fails with normal
migration, it'll never notice and attempt to start the
incoming migration regardless and crash on a NULL pointer.

This can be seen, for example, if a client connects to a server
requiring TLS, but has an invalid x509 certificate:

qemu-system-x86_64: The certificate hasn't got a known issuer
qemu-system-x86_64: migration/migration.c:386: process_incoming_migration_co: Assertion `mis->from_src_file' failed.

 #0  0x00007fffebd24f2b in raise () at /lib64/libc.so.6
 #1  0x00007fffebd0f561 in abort () at /lib64/libc.so.6
 #2  0x00007fffebd0f431 in _nl_load_domain.cold.0 () at /lib64/libc.so.6
 #3  0x00007fffebd1d692 in  () at /lib64/libc.so.6
 #4  0x0000555555ad027e in process_incoming_migration_co (opaque=<optimized out>) at migration/migration.c:386
 #5  0x0000555555c45e8b in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at util/coroutine-ucontext.c:116
 #6  0x00007fffebd3a6a0 in __start_context () at /lib64/libc.so.6
 #7  0x0000000000000000 in  ()

To handle the non-multifd case, we check whether mis->from_src_file
is non-NULL. With this in place, the migration server drops the
rejected client and stays around waiting for another, hopefully
valid, client to arrive.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180619163552.18206-1-berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>