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Must wait for process P0 to enter fun F2
before starting purge, to make sure it's not suspended.
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Ensure that we cannot get any dangling pointers into code that
has been purged. This is done by a two phase purge. At first
phase all fun entries pointing into the code to purge are marked
for purge. All processes trying to call these funs will be suspended
and by this we avoid getting new direct references into the code.
When all processes has been checked, these processes are resumed.
The new purge strategy now also completely ignore the existence of
indirect references to the code (funs). If such exist, they will
cause bad fun exceptions to the caller, but will not prevent a
soft purge or cause a kill of a process having such live references
during a hard purge. This since it is impossible to give any
guarantees that no processes in the system have such indirect
references. Even when the system is completely clean from such
references, new ones can appear via distribution and/or disk.
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Repeatedly reload a literals module while sending
the references literals around in a process ring.
This will smoke test the non-copying literals message sending does
not corrupt code unloading.
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We did not have test case that ensures that the loader refuses to
load a module if there already exists old code for the module.
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To simplify the implementation of literal pools (constant pools)
for the R12 release, a shortcut was taken regarding binaries --
all binaries would be stored as heap binaries regardless of size.
To allow a module containing literals to be unloaded, literal
terms are copied when sent to another process. That means that
huge literal binaries will also be copied if they are sent to
another process, which could be surprising.
Another problem is that the arity field in the header for the heap
object may not be wide enough to handle big binaries.
Therefore, bite the bullet and allow refc binaries to be stored
in literal pools. In short, the following need to be changed:
* Each loaded module needs a MSO list, linking all refc binaries
in the literal pool.
* When check_process_code/2 copies literals to a process heap,
it must link each referenced binary into the MSO list for the
process and increment the reference counter for the binary.
* purge_module/1 must decrement the reference counter for each
refc binary in the literal pool.
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Funs are identified by a triple, <Module,Uniq,Index>, where Module is
the module name, Uniq is a 27 bit hash value of some intermediate
representation of the code for the fun, and index is a small integer.
When a fun is loaded, the triple for the fun will be compared to
previously loaded funs. If all elements in the triple in the newly
loaded fun are the same, the newly loaded fun will replace the previous
fun. The idea is that if Uniq are the same, the code for the fun is also
the same.
The problem is that Uniq is only based on the intermediate representation
of the fun itself. If the fun calls local functions in the same module,
Uniq may remain the same even if the behavior of the fun has been changed.
See
http://erlang.org/pipermail/erlang-bugs/2007-June/000368.htlm
for an example.
As a long-term plan to fix this problem, the NewIndex and NewUniq
fields was added to each fun in the R8 release (where NewUniq is the
MD5 of the BEAM code for the module). Unfortunately, it turns
out that the compiler does not assign unique value to NewIndex (if it
isn't tested, it doesn't work), so we cannot use the
<Module,NewUniq,NewIndex> triple as identification.
It would be possible to use <Module,NewUniq,Index>, but that seems
ugly. Therefore, fix the problem by making Uniq more unique by
taking 27 bits from the MD5 for the BEAM code. That only requires
a change to the compiler.
Also update a test case for cover, which now fails because of the
stronger Uniq calculation. (The comment in test case about why the
Pid2 process survived is not correct.)
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