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[Bug translator/3887] New: Need a syntax to specify probes alternatives

This enhancement idea is a close cousin to bug #2645.

I would like to have a way to list several alternatives for a probe point, and
have the translator pick exactly one for implementation.  With this, one could
specify a preferred probe point, with an alternate probe point if the first is
not available.

My syntax suggestion is to use the boolean-OR ('||') operator when defining a
probepoint, and the translator will resolve in the typical left-to-right,
short-circuit fashion.  For example, given 'probe kernel.function("foo") ||
kernel.function("bar") {}', the translator will resolve as follows:

 probeable: | probe
  foo | bar | used
   Y  |  Y  | foo
   Y  |  N  | foo
   N  |  Y  | bar
   N  |  N  | *error*

This sort of feature will be useful for defining portable tapsets and scripts. 
Some example usage ideas:

  probe scheduler.context_switch = kernel.mark("context_switch")
        || kernel.function("__switch_to")
        || kernel.inline("context_switch") {}
If markers are available, use that.  Otherwise, try __switch_to, which has good
parameter information.  Finally, try context_switch, but parameters might not
work in inlines.  Note that on some kernels all three might be ok, but only
*one* is used.

  probe scsi.ioentry = kernel.function("scsi_prep_fn")
        || module("scsi_mod").function("scsi_prep_fn") {}
This explicitly declares how to search for the function "scsi_prep_fn".  The
same can almost be achieved using the '?' operator on both alternatives, except
that this new methods *requires* that one resolves correctly.  (Consider the use
of "scsi.*" -- if using '?' the ioentry might silently disappear on failure.)

  probe event.foobar = kernel.function("foobar") || foo_and_bar {}
  probe foo_and_bar = kernel.function("foo"), kernel.function("bar") {}
This demonstrates how to achieve grouping.  The translator will generate a
single probe on "foobar" if it exists, or else it will use two probes on "foo"
and "bar".

We'll have to consider operator precedence and semantics.  For example, what
should happen in the following cases?
  1. probe foobar = foo ? || bar {}
  2. probe foobar = foo || bar ? {}

           Summary: Need a syntax to specify probes alternatives
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: translator
        AssignedTo: systemtap at sources dot redhat dot com
        ReportedBy: joshua dot i dot stone at intel dot com

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