Synchronise probe start times

[ajor]

Problem

Probes are currently attached one-at-a-time, roughly in the order that they're declared in a script (with some extra ordering logic that I'm not sure is 100% correct).

That means that this script may print many lines of "a" before a single "b" appears:

tracepoint:syscalls:sys_enter_read { print("a") }
tracepoint:syscalls:sys_exit_read { print("b"); exit() }

Attaching 2 probes...
a
a
a
a
b

Reversing the order of the probes, the script would always print "b" then exit without printing a single "a":

tracepoint:syscalls:sys_exit_read { print("b"); exit() }
tracepoint:syscalls:sys_enter_read { print("a") }

These staggered start times can be confusing for users.

Solution

It would be nice to provide the option to synchronise probe start times.

We don't have any way to attach them all at the same time, so one alternative solution is to add an implicit predicate to every probe which references a global variable:

/ is_started == true /

Then just toggle the variable to true from userspace once all probes are attached and ready to go.

There is some small overhead to adding a predicate to every probe, so we could make this a configurable setting.