See #3832.

since slave isn't replying to it's master, these errors go unnoticed.
since we don't expect the master to send garbadge to the slave, this should be safe.
(as long as we don't log OOM errors there)

When feeding the master with a high rate traffic the the slave's feed is much slower.
This causes the replication buffer to grow (indefinitely) which leads to slave disconnection.
The problem is that writeToClient() decides to stop writing after NET_MAX_WRITES_PER_EVENT
writes (In order to be fair to clients).
We should ignore this when the client is a slave.
It's better if clients wait longer, the alternative is that the slave has no chance to stay in
sync in this situation.

Related to #4568.

- protocol parsing (processMultibulkBuffer) was limitted to 32big positions in the buffer
  readQueryFromClient potential overflow
- rioWriteBulkCount used int, although rioWriteBulkString gave it size_t
- several places in sds.c that used int for string length or index.
- bugfix in RM_SaveAuxField (return was 1 or -1 and not length)
- RM_SaveStringBuffer was limitted to 32bit length

See issue #3844 for more information.

Redis clients need to have an instantaneous idea of the amount of memory
they are consuming (if the number is not exact should at least be
proportional to the actual memory usage). We do that adding and
subtracting the SDS length when pushing / popping from the client->reply
list. However it is quite simple to add bugs in such a setup, by not
taking the objects in the list and the count in sync. For such reason,
Redis has an assertion to track counts near 2^64: those are always the
result of the counter wrapping around because we subtract more than we
add. This commit adds the symmetrical assertion: when the list is empty
since we sent everything, the reply_bytes count should be zero. Thanks
to the new assertion it should be simple to also detect the other
problem, where the count slowly increases because of over-counting.
The assertion adds a conditional in the code that sends the buffer to
the socket but should not create any measurable performance slowdown,
listLength() just accesses a structure field, and this code path is
totally dominated by write(2).

Related to #4100.

More work to do with server.unixtime and similar. Need to write Helgrind
suppression file in order to suppress the valse positives.

This bug was discovered by @kevinmcgehee and constituted a major hidden
bug in the PSYNC2 implementation, caused by the propagation from the
master of incomplete commands to slaves.

The bug had several results:

1. Borrowing from Kevin text in the issue: "Given that slaves blindly
copy over their master's input into their own replication backlog over
successive read syscalls, it's possible that with large commands or
small TCP buffers, partial commands are present in this buffer. If the
master were to fail before successfully propagating the entire command
to a slave, the slaves will never execute the partial command (since the
client is invalidated) but will copy it to replication backlog which may
relay those invalid bytes to its slaves on PSYNC2, corrupting the
backlog and possibly other valid commands that follow the failover.
Simple command boundaries aren't sufficient to capture this, either,
because in the case of a MULTI/EXEC block, if the master successfully
propagates a subset of the commands but not the EXEC, then the
transaction in the backlog becomes corrupt and could corrupt other
slaves that consume this data."

2. As identified by @yangsiran later, there is another effect of the
bug. For the same mechanism of the first problem, a slave having another
slave, could receive a full resynchronization request with an already
half-applied command in the backlog. Once the RDB is ready, it will be
sent to the slave, and the replication will continue sending to the
sub-slave the other half of the command, which is not valid.

The fix, designed by @yangsiran and @antirez, and implemented by
@antirez, uses a secondary buffer in order to feed the sub-masters and
update the replication backlog and offsets, only when a given part of
the query buffer is actually *applied* to the state of the instance,
that is, when the command gets processed and the command is not pending
in the Redis transaction buffer because of CLIENT_MULTI state.

Given that now the backlog and offsets representation are in agreement
with the actual processed commands, both issue 1 and 2 should no longer
be possible.

Thanks to @kevinmcgehee, @yangsiran and @oranagra for their work in
identifying and designing a fix for this problem.

This actually includes two changes:

1) No newlines to take the master-slave link up when the upstream master
is down. Doing this is dangerous because the sub-slave often is received
replication protocol for an half-command, so can't receive newlines
without desyncing the replication link, even with the code in order to
cancel out the bytes that PSYNC2 was using. Moreover this is probably
also not needed/sane, because anyway the slave can keep serving
requests, and because if it's configured to don't serve stale data, it's
a good idea, actually, to break the link.

2) When a +CONTINUE with a different ID is received, we now break
connection with the sub-slaves: they need to be notified as well. This
was part of the original specification but for some reason it was not
implemented in the code, and was alter found as a PSYNC2 bug in the
integration testing.

The gist of the changes is that now, partial resynchronizations between
slaves and masters (without the need of a full resync with RDB transfer
and so forth), work in a number of cases when it was impossible
in the past. For instance:

1. When a slave is promoted to mastrer, the slaves of the old master can
partially resynchronize with the new master.

2. Chained slalves (slaves of slaves) can be moved to replicate to other
slaves or the master itsef, without requiring a full resync.

3. The master itself, after being turned into a slave, is able to
partially resynchronize with the new master, when it joins replication
again.

In order to obtain this, the following main changes were operated:

* Slaves also take a replication backlog, not just masters.

* Same stream replication for all the slaves and sub slaves. The
replication stream is identical from the top level master to its slaves
and is also the same from the slaves to their sub-slaves and so forth.
This means that if a slave is later promoted to master, it has the
same replication backlong, and can partially resynchronize with its
slaves (that were previously slaves of the old master).

* A given replication history is no longer identified by the `runid` of
a Redis node. There is instead a `replication ID` which changes every
time the instance has a new history no longer coherent with the past
one. So, for example, slaves publish the same replication history of
their master, however when they are turned into masters, they publish
a new replication ID, but still remember the old ID, so that they are
able to partially resynchronize with slaves of the old master (up to a
given offset).

* The replication protocol was slightly modified so that a new extended
+CONTINUE reply from the master is able to inform the slave of a
replication ID change.

* REPLCONF CAPA is used in order to notify masters that a slave is able
to understand the new +CONTINUE reply.

* The RDB file was extended with an auxiliary field that is able to
select a given DB after loading in the slave, so that the slave can
continue receiving the replication stream from the point it was
disconnected without requiring the master to insert "SELECT" statements.
This is useful in order to guarantee the "same stream" property, because
the slave must be able to accumulate an identical backlog.

* Slave pings to sub-slaves are now sent in a special form, when the
top-level master is disconnected, in order to don't interfer with the
replication stream. We just use out of band "\n" bytes as in other parts
of the Redis protocol.

An old design document is available here:

https://gist.github.com/antirez/ae068f95c0d084891305

However the implementation is not identical to the description because
during the work to implement it, different changes were needed in order
to make things working well.

This is an attempt at mitigating problems due to cross protocol
scripting, an attack targeting services using line oriented protocols
like Redis that can accept HTTP requests as valid protocol, by
discarding the invalid parts and accepting the payloads sent, for
example, via a POST request.

For this to be effective, when we detect POST and Host: and terminate
the connection asynchronously, the networking code was modified in order
to never process further input. It was later verified that in a
pipelined request containing a POST command, the successive commands are
not executed.

This feature is useful, especially in deployments using Sentinel in
order to setup Redis HA, where the slave is executed with NAT or port
forwarding, so that the auto-detected port/ip addresses, as listed in
the "INFO replication" output of the master, or as provided by the
"ROLE" command, don't match the real addresses at which the slave is
reachable for connections.

Send a long double or double as a bulk reply, in a human friendly
format.

An exposed Redis instance on the internet can be cause of serious
issues. Since Redis, by default, binds to all the interfaces, it is easy
to forget an instance without any protection layer, for error.

Protected mode try to address this feature in a soft way, providing a
layer of protection, but giving clues to Redis users about why the
server is not accepting connections.

When protected mode is enabeld (the default), and if there are no
minumum hints about the fact the server is properly configured (no
"bind" directive is used in order to restrict the server to certain
interfaces, nor a password is set), clients connecting from external
intefaces are refused with an error explaining what to do in order to
fix the issue.

Clients connecting from the IPv4 and IPv6 lookback interfaces are still
accepted normally, similarly Unix domain socket connections are not
restricted in any way.

We need to process replies after errors in order to delete keys
successfully transferred. Also argument rewriting was fixed since
it was broken in several ways. Now a fresh argument vector is created
and set if we are acknowledged of at least one key.

The old version only flushed data to slaves if there were strings
pending in the client->reply list. Now also static buffers are flushed.
Does not help to free memory (which is the only use we have right now in
the fuction), but is more correct conceptually, and may be used in
other contexts.

Sometimes it can be useful for clients to completely disable replies
from the Redis server. For example when the client sends fire and forget
commands or performs a mass loading of data, or in caching contexts
where new data is streamed constantly. In such contexts to use server
time and bandwidth in order to send back replies to clients, which are
going to be ignored, is a shame.

Multiple mechanisms are possible to implement such a feature. For
example it could be a feature of MULTI/EXEC, or a command prefix
such as "NOREPLY SADD myset foo", or a different mechanism that allows
to switch on/off requests using the CLIENT command.

The MULTI/EXEC approach has the problem that transactions are not
strictly part of the no-reply semantics, and if we want to insert a lot
of data in a bulk way, creating a huge MULTI/EXEC transaction in the
server memory is bad.

The prefix is the best in this specific use case since it does not allow
desynchronizations, and is pretty clear semantically. However Redis
internals and client libraries are not prepared to handle this
currently.

So the implementation uses the CLIENT command, providing a new REPLY
subcommand with three options:

    CLIENT REPLY OFF disables the replies, and does not reply itself.
    CLIENT REPLY ON re-enables the replies, replying +OK.
    CLIENT REPLY SKIP only discards the reply of the next command, and
                      like OFF does not reply anything itself.

The reason to add the SKIP command is that it allows to have an easy
way to send conceptually "single" commands that don't need a reply
as the sum of two pipelined commands:

    CLIENT REPLY SKIP
    SET key value

Note that CLIENT REPLY ON replies with +OK so it should be used when
sending multiple commands that don't need a reply. However since it
replies with +OK the client can check that the connection is still
active and all the previous commands were received.

This is currently just into Redis "unstable" so the proposal can be
modified or abandoned based on users inputs.

After the introduction of the list with clients with pending writes, to
process clients incrementally outside of the event loop we also need to
process the pending writes list.