[Bug dyninst/15566] New: Support multiple stap --dyninst sessions on the same processes

["jistone at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15566

            Bug ID: 15566
           Summary: Support multiple stap --dyninst sessions on the same
                    processes
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: dyninst
          Assignee: systemtap at sourceware dot org
          Reporter: jistone at redhat dot com

As it currently stands, stap --runtime=dyninst (via stapdyn) can either launch
new target processes or attach to existing processes.  However, this is an
exclusive action, so no other stapdyn scripts are able to attach to the same
process.  This is ultimately a limitation of ptrace.

So, it might be nice to have some sort of multiplexing layer to allow multiple
stap sessions on overlapping sets of processes.  This probably can't be all the
way down at the ptrace layer though, since Dyninst will expect to know about
all transformations in the process.  But it could be multiplexed at the Dyninst
layer, such that multiple stap modules are loaded at once in stapdyn, with
their respective instrumentation routed appropriately.

There is a slight notion of this in stapdyn's mutator.cxx, with the g_mutators
vector distributing BPatch callbacks.  But in reality this is always a
single-member vector right now.  All of the code will have to learn not to
expect a single mutator.  We'll also have to invent a mechanism for dynamically
adding and removing stap modules in the mix, from different stap --dyninst
invocations.

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