Makes BrokerChannel abstract over a rendezvousMethod. BrokerChannel
itself is responsible for keepLocalAddresses and the NAT type state, as
well as encoding and decoding client poll messages. rendezvousMethod is
only responsible for delivery of encoded messages.

Formerly, BrokerChannel represented the broker URL and possible domain
fronting as
	bc.url  *url.URL
        bc.Host string
That is, bc.url is the URL of the server which we contact directly, and
bc.Host is the Host header to use in the request. With no domain
fronting, bc.url points directly at the broker itself, and bc.Host is
blank. With domain fronting, we do the following reshuffling:
	if front != "" {
		bc.Host = bc.url.Host
		bc.url.Host = front
	}
That is, we alter bc.url to reflect that the server to which we send
requests directly is the CDN, not the broker, and store the broker's own
URL in the HTTP Host header.

The above representation was always confusing to me, because in my
mental model, we are always conceptually communicating with the broker;
but we may optionally be using a CDN proxy in the middle. The new
representation is
	bc.url   *url.URL
        bc.front string
bc.url is the URL of the broker itself, and never changes. bc.front is
the optional CDN front domain, and likewise never changes after
initialization. When domain fronting is in use, we do the swap in the
http.Request struct, not in BrokerChannel itself:
	if bc.front != "" {
		request.Host = request.URL.Host
		request.URL.Host = bc.front
	}

Compare to the representation in meek-client:

https://gitweb.torproject.org/pluggable-transports/meek.git/tree/meek-client/meek-client.go?h=v0.35.0#n94
	var options struct {
		URL       string
		Front     string
	}
https://gitweb.torproject.org/pluggable-transports/meek.git/tree/meek-client/meek-client.go?h=v0.35.0#n308
	if ok { // if front is set
		info.Host = info.URL.Host
		info.URL.Host = front
	}

The tests were using a broker URL of "test.broker" (i.e., a schema-less,
host-less, relative path), and running assertions on the value of
b.url.Path. This is strange, especially in tests regarding domain
fronting, where we care about b.url.Host, not b.url.Path. This commit
changes the broker URL to "http://test.broker" and changes tests to
check b.url.Host. I also added an additional assertion for an empty
b.Host in the non-domain-fronted case.

Closes: #40055

Simplify the code to use a ticker. Using a pattern to allow a first run
of the loop before hitting the ticker:
https://github.com/golang/go/issues/17601#issuecomment-311955879

Instead of copying the value.

Unused since 1364d7d4.

The legacy code does case matching on these exact strings so it's better
to ensure they're constant.

Move the logic for the legacy version into the http handlers and use a
shim when doing ipc.

Introduces an IPC struct and moves the logic out of the http handlers
and into methods on that.

So the assignment of proxies is based on the load. The number of clients
is ronded down to 8. Existing proxies that doesn't report the number
of clients will be distributed equaly to new proxies until they get 8
clients, that is okish as the existing proxies do have a maximum
capacity of 10.

Fixes #40048

Remove the -max 3 option because we only use one snowflake. Add
SocksPort auto because many testers have a tor process already bound to
port 9050.

Refactor out a separate handleStream function and ensure that all
connections are closed and the references are out of scope.

We used a WaitGroup to prevent a call to Peers.End from melting
snowflakes while a new one is being collected. However, calls to
WaitGroup.Add are in a race with WaitGroup.Wait. To fix this, we use a
Mutex instead.

Using the boolean value was unnecessary since we already have a channel
we can check for closure.

The race condition occurs because concurrent goroutines are intermixing
reads and writes of `WebRTCPeer.closed`.

Spotted when integrating Snowflake inside OONI in
https://github.com/ooni/probe-cli/pull/373.

sbs authored 3 years ago and Cecylia Bocovich committed 3 years ago

The race condition occurs because concurrent goroutines are
intermixing reads and writes of `WebRTCPeer.lastReceive`.

Spotted when integrating Snowflake inside OONI in
https://github.com/ooni/probe-cli/pull/373.

This fixes a stats collection bug where we were converting client
addresses between a string and net.Addr using the clientAddr function
multiple times, resulting in an empty string for all addresses.

In VSCode, the staticcheck tool emits this warning:

> should call wg.Add(1) before starting the goroutine to
> avoid a race (SA2000)go-staticcheck

To avoid this warning, just move wg.Add outside.

To be used by the debian package (#19409)

Send the client poll request and response in a json-encoded format in
the HTTP request body rather than sending the data in HTTP headers. This
will pave the way for using domain-fronting alternatives for the
Snowflake rendezvous.

Make a stack of cleanup functions to run (as with defer), but clear the
stack before returning if no error occurs.

Uselessly pushing the stream.Close() cleanup just before clearing the
stack is an intentional safeguard, for in case additional operations are
added before the return in the future.

Fixes #40042.

The code checked for and displayed an error, but would then go on to
call copyLoop on the nil Conn returned from transport.Dial. Add a return
in that case, and put the cleanup operations in defer. Also remove an
obsolete comment about an empty address. Obsolete because:
!31 (comment 2733279)

David Fifield authored 3 years ago and meskio committed 3 years ago

If we don't stop them explicitly, the timers will not get garbage collected
until they timeout:
https://medium.com/@oboturov/golang-time-after-is-not-garbage-collected-4cbc94740082

Related to #40039

Follow up to 160ae2dd

Analysis by @dcf,

> I don't think the sync.Once around logMetrics is necessary anymore.
Its original purpose was to inhibit logging on later file handles of
metrics.log, if there were more than one opened. See 171c55a9 and #29734
(comment 2593039) "Making a singleton *Metrics variable causes problems
with how Convey does tests. It shouldn't be called more than once, but
for now I'm using sync.Once on the logging at least so it's explicit."
Commit ba4fe1a7 changed it so that metrics.log is opened in main, used
to create a *log.Logger, and that same instance of *log.Logger is passed
to both NewMetrics and NewBrokerContext. It's safe to share the same
*log.Logger across multiple BrokerContext.

Doesn't seem like it needs to exist outside of the metrics struct.

Also, the call to logMetrics is moved to the constructor.  A metrics
instance is only created when a BrokerContext is created, which only
happens at startup.  The sync of only doing that once is left for
documentation purposes, since it doesn't hurt, but also seems redundant.

Introduce a waitgroup and done channel to ensure that both the read and
write gorouting for turbotunnel connections terminate when the
connection is closed.