GCC added an implicit-fallthrough warning a while back, where it
would complain if you had a nontrivial "case:" block that didn't end
with break, return, or something like that.  Clang recently added
the same thing.

GCC, however, would let you annotate a fall-through as intended by
any of various magic "/* fall through */" comments.  Clang, however,
only seems to like "__attribute__((fallthrough))".  Fortunately, GCC
accepts that too.

A previous commit in this branch defined a FALLTHROUGH macro to do
the right thing if GNUC is defined; here we replace all of our "fall
through" comments with uses of that macro.

This is an automated commit, made with the following perl one-liner:

  #!/usr/bin/perl -i -p
  s#/\* *falls? ?thr.*?\*/#FALLTHROUGH;#i;

The parts for handling cell formats should be in src/core/or.

The parts for handling onionskin queues should be in src/core/or.

Only the crypto wrapper belongs in src/core/crypto.

I am very glad to have written this script.

This commit won't build yet -- it just puts everything in a slightly
more logical place.

The reasoning here is that "src/core" will hold the stuff that every (or
nearly every) tor instance will need in order to do onion routing.
Other features (including some necessary ones) will live in
"src/feature".  The "src/app" directory will hold the stuff needed
to have Tor be an application you can actually run.

This commit DOES NOT refactor the former contents of src/or into a
logical set of acyclic libraries, or change any code at all.  That
will have to come in the future.

We will continue to move things around and split them in the future,
but I hope this lays a reasonable groundwork for doing so.

or.h should really include only the minimum of stuff from or/*,
common/*, and lib/*.

Since packed_cell and destroy_cell exist only to be queued, they go
in the same headers as the queues.

 * ADD new /src/common/crypto_rand.[ch] module.
 * ADD new /src/common/crypto_util.[ch] module (contains the memwipe()
   function, since all crypto_* modules need this).
 * FIXES part of #24658: https://bugs.torproject.org/24658

Instead, define a maximum size, and enforce it with an assertion.

Closes ticket 25150

Fixes bug 23106; bugfix on 0.2.4.8-alpha.

Fortunately, we only support big-endian and little-endian platforms,
and on both of those, hton*() and ntoh*() behave the same.  And if
we did start to support middle endian systems (haha, no), most of
_those_ have hton*(x) == ntoh*(x) too.

Fixes bug 20059; bugfix on 0.1.0.1-rc.

I need to be able to turn on Ed25519 support in client generation
of  extend cells so I can test it, but leave it off-by-default until
enough clients support it for us to turn it on for a bunch at once.

This is part of #15056 / prop#220.

(Not extended2/created2; that's too simple.)

Incidentally, add ed25519 identities to the mix when we have them.

This commit adds or improves the module-level documenation for:

  buffers.c circuitstats.c command.c connection_edge.c control.c
  cpuworker.c crypto_curve25519.c crypto_curve25519.h
  crypto_ed25519.c crypto_format.c dircollate.c dirserv.c dns.c
  dns_structs.h fp_pair.c geoip.c hibernate.c keypin.c ntmain.c
  onion.c onion_fast.c onion_ntor.c onion_tap.c periodic.c
  protover.c protover.h reasons.c rephist.c replaycache.c
  routerlist.c routerparse.c routerset.c statefile.c status.c
  tor_main.c workqueue.c

In particular, I've tried to explain (for each documented module)
what each module does, what's in it, what the big idea is, why it
belongs in Tor, and who calls it.  In a few cases, I've added TODO
notes about refactoring opportunities.

I've also renamed an argument, and fixed a few DOCDOC comments.

When a client connects to an intro point not in the client's consensus,
or a hidden service connects to a rend point not in the hidden service's
consensus, we are stuck with using TAP, because there is no ntor link
specifier.

This is a big-ish patch, but it's very straightforward.  Under this
clang warning, we're not actually allowed to have a global variable
without a previous extern declaration for it.  The cases where we
violated this rule fall into three roughly equal groups:
  * Stuff that should have been static.
  * Stuff that was global but where the extern was local to some
    other C file.
  * Stuff that was only global when built for the unit tests, that
    needed a conditional extern in the headers.

The first two were IMO genuine problems; the last is a wart of how
we build tests.

So, back long ago, XXX012 meant, "before Tor 0.1.2 is released, we
had better revisit this comment and fix it!"

But we have a huge pile of such comments accumulated for a large
number of released versions!  Not cool.

So, here's what I tried to do:

  * 0.2.9 and 0.2.8 are retained, since those are not yet released.

  * XXX+ or XXX++ or XXX++++ or whatever means, "This one looks
    quite important!"

  * The others, after one-by-one examination, are downgraded to
    plain old XXX.  Which doesn't mean they aren't a problem -- just
    that they cannot possibly be a release-blocking problem.

Because in 95 years, we or our successors will surely care about
enforcing the BSD license terms on this code.  Right?

By now, support in the network is widespread and it's time to require
more modern crypto on all Tor instances, whether they're clients or
servers. By doing this early in 0.2.6, we can be sure that at some point
all clients will have reasonable support.

Also, refactor the way we handle failed handshakes so that this
warning doesn't propagate itself to "onion_skin_client_handshake
failed" and "circuit_finish_handshake failed" and
"connection_edge_process_relay_cell (at origin) failed."

Resolves warning from 9635.

As it stands, it relies on the fact that onion_queue_entry_remove
will magically remove each onionskin from the right list.  This
patch changes the logic to be more resilient to possible bugs in
onion_queue_entry_remove, and less confusing to static analysis tools.

Fix for bug9635.

This is harmless in the Tor of today, but important for correctness.

Fixes bug 10536; bugfix on 0.2.4.8-alpha. Reported by "cypherpunks".

Now we explicitly check for overflow.

This approach seemed smarter than a cascade of "change int to unsigned
int and hope nothing breaks right before the release".

Nick, feel free to fix in a better way, maybe in master.