This replaces the NIH epee http server which does not work all that well
with an external C++ library called uWebSockets. Fundamentally this
gives the following advantages:
- Much less code to maintain
- Just one thread for handling HTTP connections versus epee's pool of
threads
- Uses existing LokiMQ job server and existing thread pool for handling
the actual tasks; they are processed/scheduled in the same "rpc" or
"admin" queues as lokimq rpc calls. One notable benefit is that "admin"
rpc commands get their own queue (and thus cannot be delayed by long rpc
commands). Currently the lokimq threads and the http rpc thread pool
and the p2p thread pool and the job queue thread pool and the dns lookup
thread pool and... are *all* different thread pools; this is a step
towards consolidating them.
- Very little mutex contention (which has been a major problem with epee
RPC in the past): there is one mutex (inside uWebSockets) for putting
responses back into the thread managing the connection; everything
internally gets handled through (lock-free) lokimq inproc sockets.
- Faster RPC performance on average, and much better worst case
performance. Epee's http interface seems to have some race condition
that ocassionally stalls a request (even a very simple one) for a dozen
or more seconds for no good reason.
- Long polling gets redone here to no longer need threads; instead we
just store the request and respond when the thread pool, or else in a
timer (that runs once/second) for timing out long polls.
---
The basic idea of how this works from a high level:
We launch a single thread to handle HTTP RPC requests and response data.
This uWebSockets thread is essentially running an event loop: it never
actually handles any logic; it only serves to shuttle data that arrives
in a request to some other thread, and then, at some later point, to
send some reply back to that waiting connection. Everything is
asynchronous and non-blocking here: the basic uWebSockets event loop
just operates as things arrive, passes it off immediately, and goes back
to waiting for the next thing to arrive.
The basic flow is like this:
0. uWS thread -- listens on localhost:22023
1. uWS thread -- incoming request on localhost:22023
2. uWS thread -- fires callback, which injects the task into the LokiMQ job queue
3. LMQ main loop -- schedules it as an RPC job
4. LMQ rpc thread -- Some LokiMQ thread runs it, gets the result
5. LMQ rpc thread -- Result gets queued up for the uWS thread
6. uWS thread -- takes the request and starts sending it
(asynchronously) back to the requestor.
In more detail:
uWebSockets has registered has registered handlers for non-jsonrpc
requests (legacy JSON or binary). If the port is restricted then admin
commands get mapped to a "Access denied" response handler, otherwise
public commands (and admin commands on an unrestricted port) go to the
rpc command handler.
POST requests to /json_rpc have their own handler; this is a little
different than the above because it has to parse the request before it
can determine whether it is allowed or not, but once this is done it
continues roughly the same as legacy/binary requests.
uWebSockets then listens on the given IP/port for new incoming requests,
and starts listening for requests in a thread (we own this thread).
When a request arrives, it fires the event handler for that request.
(This may happen multiple times, if the client is sending a bunch of
data in a POST request). Once we have the full request, we then queue
the job in LokiMQ, putting it in the "rpc" or "admin" command
categories. (The one practical different here is that "admin" is
configured to be allowed to start up its own thread if all other threads
are busy, while "rpc" commands are prioritized along with everything
else.) LokiMQ then schedules this, along with native LokiMQ "rpc." or
"admin." requests.
When a LMQ worker thread becomes available, the RPC command gets called
in it and runs. Whatever output it produces (or error message, if it
throws) then gets wrapped up in jsonrpc boilerplate (if necessary), and
delivered to the uWebSockets thread to be sent in reply to that request.
uWebSockets picks up the data and sends whatever it can without
blocking, then buffers whatever it couldn't send to be sent again in a
later event loop iteration once the requestor can accept more data.
(This part is outside lokid; we only have to give uWS the data and let
it worry about delivery).
---
PR specifics:
Things removed from this PR:
1. ssl settings; with this PR the HTTP RPC interface is plain-text. The
previous default generated a self-signed certificate for the server on
startup and then the client accepted any certificate. This is actually
*worse* than unencrypted because it is entirely MITM-readable and yet
might make people think that their RPC communication is encrypted, and
setting up actual certificates is difficult enough that I think most
people don't bother.
uWebSockets *does* support HTTPS, and we could glue the existing options
into it, but I'm not convinced it's worthwhile: it works much better to
put HTTPS in a front-end proxy holding the certificate that proxies
requests to the backend (which can then listen in restricted mode on
some localhost port). One reason this is better is that it is much
easier to reload and/or restart such a front-end server, while
certificate updates with lokid require a full restart. Another reason
is that you get an error page instead of a timeout if something is wrong
with the backend. Finally we also save having to generate a temporary
certificate on *every* lokid invocation.
2. HTTP Digest authentication. Digest authentication is obsolete (and
was already obsolete when it got added to Monero). HTTP-Digest was
originally an attempt to provide a password authentication mechanism
that does not leak the password in transit, but still required that the
server know the password. It only has marginal value against replay
attacks, and is made entirely obsolete by sending traffic over HTTPS
instead. No client out there supports Digest but *not* Basic auth, and
so given the limited usefulness it seems pointless to support more than
Basic auth for HTTP RPC login.
What's worse is that epee's HTTP Digest authentication is a terrible
implementation: it uses boost::spirit -- a recursive descent parser
meant for building complex language grammars -- just to parse a single
HTTP header for Digest auth. This is a big load of crap that should
never have been accepted upstream, and that we should get rid of (even
if we wanted to support Digest auth it takes less than 100 lines of code
to do it when *not* using a recursive descent parser).
- Adds a tools::trim that operates on a string_view to replace boost
string trim usage.
- Replaces some boost::split's with tools::split's
- updates vercmp to operate on string_views instead of c strings
common/util.h has become something of a dumping ground of random
functions. This splits them up a little by moving the filesystem bits
to common/file.h, the sha256sum functions to common/sha256sum.h, and the
(singleton) signal handler to common/signal_handler.h.
Add number parsing and basic string splitting to common/string_util.h
and use it (replacing some regexes and boost string utilities).
Includes unit tests.
