selftests: net: lib: Introduce deferred commands

selftests: net: lib: Introduce deferred commands

In commit 8510801a9dbd ("selftests: drv-net: add ability to schedule
cleanup with defer()"), a defer helper was added to Python selftests.
The idea is to keep cleanup commands close to their dirtying counterparts,
thereby making it more transparent what is cleaning up what, making it
harder to miss a cleanup, and make the whole cleanup business exception
safe. All these benefits are applicable to bash as well, exception safety
can be interpreted in terms of safety vs. a SIGINT.

This patch therefore introduces a framework of several helpers that serve
to schedule cleanups in bash selftests:

- defer_scope_push(), defer_scope_pop(): Deferred statements can be batched
  together in scopes. When a scope is popped, the deferred commands
  scheduled in that scope are executed in the order opposite to order of
  their scheduling.

- defer(): Schedules a defer to the most recently pushed scope (or the
  default scope if none was pushed.)

- defer_prio(): Schedules a defer on the priority track. The priority defer
  queue is run before the default defer queue when scope is popped.

  The issue that this is addressing is specifically the one of restoring
  devlink shared buffer threshold type. When setting up static thresholds,
  one has to first change the threshold type to static, then override the
  individual thresholds. When cleaning up, it would be natural to reset the
  threshold values first, then change the threshold type. But the values
  that are valid for dynamic thresholds are generally invalid for static
  thresholds and vice versa. Attempts to restore the values first would be
  bounced. Thus one has to first reset the threshold type, then adjust the
  thresholds.

  (You could argue that the shared buffer threshold type API is broken and
  you would be right, but here we are.)

  This cannot be solved by pure defers easily. I considered making it
  possible to disable an existing defer, so that one could then schedule a
  new defer and disable the original. But this forward-shifting of the
  defer job would have to take place after every threshold-adjusting
  command, which would make it very awkward to schedule these jobs.

- defer_scopes_cleanup(): Pops any unpopped scopes, including the default
  one. The selftests that use defer should run this in their exit trap.
  This is important to get cleanups of interrupted scripts.

- in_defer_scope(): Sometimes a function would like to introduce a new
  defer scope, then run whatever it is that it wants to run, and then pop
  the scope to run the deferred cleanups. The helper in_defer_scope() can
  be used to run another command within such environment, such that any
  scheduled defers run after the command finishes.

The framework is added as a separate file lib/sh/defer.sh so that it can be
used by all bash selftests, including those that do not currently use
lib.sh. lib.sh however includes the file by default, because ideally all
tests would use these helpers instead of hand-rolling their cleanups.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>