remote execution of funs in shell =/= in erts

[Matthias Lang]

Peter-Henry Mander writes:

 > So I guess that there's a difference between how the shell executes
 > remote funs and how the Erlang runtime executes remote funs. How do
 > nodes communicate what appears to be version information or beam digest
 > or checksums beween them? Is this documented (otherwise than by reading
 > code :-)? 

There are quite a few questions there. I don't know enough about the
ERTS internals to answer all of them confidently, but I'll have a go.

Yes, there's a difference between funs made in the shell and funs made
in code. An easy way to see that is to run term_to_binary() on the
'same' fun made two different ways:

8> io:fwrite("~w", [term_to_binary(e:a())]).              
<<131,112,0,0,0,66,1,119,221,89,18,158,97,22,203,233,28,220,41,82,49,106,16,0,0,0,0,0,0,0,0,100,0,1,101,97,0,98,5,195,163,107,103,100,0,13,110,111,110,111,100,101,64,110,111,104,111,115,116,0,0,0,46,0,0,0,0,0>>ok
9> io:fwrite("~w", [term_to_binary(fun(X) -> 2 * X end)]). 
<<131,112,0,0,0,227,1,242,35,237,99,76,83,218,107,189,28,220,154,229,136,183,25,0,0,0,1,0,0,0,3,100,0,8,101,114,108,95,101,118,97,108,97,5,98,3,208,36,215,103,100,0,13,110,111,110,111,100,101,64,110,111,104,111,115,116,0,0,0,46,0,0,0,0,0,104,3,100,0,4,101,118,97,108,104,2,100,0,5,115,104,101,108,108,100,0,10,108,111,99,97,108,95,102,117,110,99,108,0,0,0,1,103,100,0,13,110,111,110,111,100,101,64,110,111,104,111,115,116,0,0,0,22,0,0,0,0,0,106,106,108,0,0,0,1,104,5,100,0,6,99,108,97,117,115,101,97,1,108,0,0,0,1,104,3,100,0,3,118,97,114,97,1,100,0,1,88,106,106,108,0,0,0,1,104,5,100,0,2,111,112,97,1,100,0,1,42,104,3,100,0,7,105,110,116,101,103,101,114,97,1,97,2,104,3,100,0,3,118,97,114,97,1,100,0,1,88,106,106>>

(e:a() is the 'same' fun as the one I entered in the shell).

Taking a quick look at the source (erl_fun.h), it appears that a
'normal' fun includes an MD5sum of the module which created it, which
tallys with your observations. Taking an educated guess, a 'shell fun'
is actually a parse tree.

I would be surprised if it's possible to execute a compiled fun
without making the exact same code available at the other node, but
you don't necessarily have to do that via a filesystem.

 > Is it possible for the runtime to execute a remote fun without
 > the corresponding beam being available on the remote node, like the
 > shell appears to do?

I don't think there's a clean way of doing it. One dirty way is to use
erl_scan, erl_parse and erl_eval. I'd guess that's what the shell
does.

Matt