In Redis 4.0 replication, with the introduction of PSYNC2, masters and
slaves replicate commands to cascading slaves and to the replication
backlog itself in a different way compared to the past.

Masters actually replicate the effects of client commands.
Slaves just propagate what they receive from masters.

This mechanism can cause problems when the configuration of an instance
is changed from master to slave inside a transaction. For instance
we could send to a master instance the following sequence:

    MULTI
    SLAVEOF 127.0.0.1 0
    EXEC
    SLAVEOF NO ONE

Before the fixes in this commit, the MULTI command used to be propagated
into the replication backlog, however after the SLAVEOF command the
instance is a slave, so the EXEC implementation failed to also propagate
the EXEC command. When the slaves of the above instance reconnected,
they were incrementally synchronized just sending a "MULTI". This put
the master client (in the slaves) into MULTI state, breaking the
replication.

Notably even Redis Sentinel uses the above approach in order to guarantee
that configuration changes are always performed together with rewrites
of the configuration and with clients disconnection. Sentiel does:

    MULTI
    SLAVEOF ...
    CONFIG REWRITE
    CLIENT KILL TYPE normal
    EXEC

So this was a really problematic issue. However even with the fix in
this commit, that will add the final EXEC to the replication stream in
case the instance was switched from master to slave during the
transaction, the result would be to increment the slave replication
offset, so a successive reconnection with the new master, will not
permit a successful partial resynchronization: no way the new master can
provide us with the backlog needed, we incremented our offset to a value
that the new master cannot have.

However the EXEC implementation waits to emit the MULTI, so that if the
commands inside the transaction actually do not need to be replicated,
no commands propagation happens at all. From multi.c:

    if (!must_propagate && !(c->cmd->flags & (CMD_READONLY|CMD_ADMIN))) {
	execCommandPropagateMulti(c);
	must_propagate = 1;
    }

The above code is already modified by this commit you are reading.
Now also ADMIN commands do not trigger the emission of MULTI. It is actually
not clear why we do not just check for CMD_WRITE... Probably I wrote it this
way in order to make the code more reliable: better to over-emit MULTI
than not emitting it in time.

So this commit should indeed fix issue #3836 (verified), however it looks
like some reconsideration of this code path is needed in the long term.

BONUS POINT: The reverse bug.

Even in a read only slave "B", in a replication setup like:

	A -> B -> C

There are commands without the READONLY nor the ADMIN flag, that are also
not flagged as WRITE commands. An example is just the PING command.

So if we send B the following sequence:

    MULTI
    PING
    SLAVEOF NO ONE
    EXEC

The result will be the reverse bug, where only EXEC is emitted, but not the
previous MULTI. However this apparently does not create problems in practice
but it is yet another acknowledge of the fact some work is needed here
in order to make this code path less surprising.

Note that there are many different approaches we could follow. For instance
MULTI/EXEC blocks containing administrative commands may be allowed ONLY
if all the commands are administrative ones, otherwise they could be
denined. When allowed, the commands could simply never be replicated at all.

Issue #4084 shows how for a design error, GEORADIUS is a write command
because of the STORE option. Because of this it does not work
on readonly slaves, gets redirected to masters in Redis Cluster even
when the connection is in READONLY mode and so forth.

To break backward compatibility at this stage, with Redis 4.0 to be in
advanced RC state, is problematic for the user base. The API can be
fixed into the unstable branch soon if we'll decide to do so in order to
be more consistent, and reease Redis 5.0 with this incompatibility in
the future. This is still unclear.

However, the ability to scale GEO queries in slaves easily is too
important so this commit adds two read-only variants to the GEORADIUS
and GEORADIUSBYMEMBER command: GEORADIUS_RO and GEORADIUSBYMEMBER_RO.
The commands are exactly as the original commands, but they do not
accept the STORE and STOREDIST options.

The original RDB serialization format was not parsable without the
module loaded, becuase the structure was managed only by the module
itself. Moreover RDB is a streaming protocol in the sense that it is
both produce di an append-only fashion, and is also sometimes directly
sent to the socket (in the case of diskless replication).

The fact that modules values cannot be parsed without the relevant
module loaded is a problem in many ways: RDB checking tools must have
loaded modules even for doing things not involving the value at all,
like splitting an RDB into N RDBs by key or alike, or just checking the
RDB for sanity.

In theory module values could be just a blob of data with a prefixed
length in order for us to be able to skip it. However prefixing the values
with a length would mean one of the following:

1. To be able to write some data at a previous offset. This breaks
stremaing.
2. To bufferize values before outputting them. This breaks performances.
3. To have some chunked RDB output format. This breaks simplicity.

Moreover, the above solution, still makes module values a totally opaque
matter, with the fowllowing problems:

1. The RDB check tool can just skip the value without being able to at
least check the general structure. For datasets composed mostly of
modules values this means to just check the outer level of the RDB not
actually doing any checko on most of the data itself.
2. It is not possible to do any recovering or processing of data for which a
module no longer exists in the future, or is unknown.

So this commit implements a different solution. The modules RDB
serialization API is composed if well defined calls to store integers,
floats, doubles or strings. After this commit, the parts generated by
the module API have a one-byte prefix for each of the above emitted
parts, and there is a final EOF byte as well. So even if we don't know
exactly how to interpret a module value, we can always parse it at an
high level, check the overall structure, understand the types used to
store the information, and easily skip the whole value.

The change is backward compatible: older RDB files can be still loaded
since the new encoding has a new RDB type: MODULE_2 (of value 7).
The commit also implements the ability to check RDB files for sanity
taking advantage of the new feature.

commands.

This avoids Helgrind complaining, but we are actually not using
atomicGet() to get the unixtime value for now: too many places where it
is used and given tha time_t is word-sized it should be safe in all the
archs we support as it is.

On the other hand, Helgrind, when Redis is compiled with "make helgrind"
in order to force the __sync macros, will detect the write in
updateCachedTime() as a read (because atomic functions are used) and
will not complain about races.

This commit also includes minor refactoring of mutex initializations and
a "helgrind" target in the Makefile.

Only useful for when no atomic builtins are available.

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

Instead of giving the module background operations just a small time to
run in the beforeSleep() function, we can have the lock released for all
the time we are blocked in the multiplexing syscall.

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.

If a thread unblocks a client blocked in a module command, by using the
RedisMdoule_UnblockClient() API, the event loop may not be awaken until
the next timeout of the multiplexing API or the next unrelated I/O
operation on other clients. We actually want the client to be served
ASAP, so a mechanism is needed in order for the unblocking API to inform
Redis that there is a client to serve ASAP.

This commit fixes the issue using the old trick of the pipe: when a
client needs to be unblocked, a byte is written in a pipe. When we run
the list of clients blocked in modules, we consume all the bytes
written in the pipe. Writes and reads are performed inside the context
of the mutex, so no race is possible in which we consume the bytes that
are actually related to an awake request for a client that should still
be put into the list of clients to unblock.

It was verified that after the fix the server handles the blocked
clients with the expected short delay.

Thanks to @dvirsky for understanding there was such a problem and
reporting it.

This change attempts to switch to an hash function which mitigates
the effects of the HashDoS attack (denial of service attack trying
to force data structures to worst case behavior) while at the same time
providing Redis with an hash function that does not expect the input
data to be word aligned, a condition no longer true now that sds.c
strings have a varialbe length header.

Note that it is possible sometimes that even using an hash function
for which collisions cannot be generated without knowing the seed,
special implementation details or the exposure of the seed in an
indirect way (for example the ability to add elements to a Set and
check the return in which Redis returns them with SMEMBERS) may
make the attacker's life simpler in the process of trying to guess
the correct seed, however the next step would be to switch to a
log(N) data structure when too many items in a single bucket are
detected: this seems like an overkill in the case of Redis.

SPEED REGRESION TESTS:

In order to verify that switching from MurmurHash to SipHash had
no impact on speed, a set of benchmarks involving fast insertion
of 5 million of keys were performed.

The result shows Redis with SipHash in high pipelining conditions
to be about 4% slower compared to using the previous hash function.
However this could partially be related to the fact that the current
implementation does not attempt to hash whole words at a time but
reads single bytes, in order to have an output which is endian-netural
and at the same time working on systems where unaligned memory accesses
are a problem.

Further X86 specific optimizations should be tested, the function
may easily get at the same level of MurMurHash2 if a few optimizations
are performed.

This is of great interest because allows us to print debugging
informations that could be of useful when debugging, like in the
following example:

    serverPanic("Unexpected encoding for object %d, %d",
        obj->type, obj->encoding);

As suggested by @itamarhaber.

You can still force the logo in the normal logs.
For motivations, check issue #3112. For me the reason is that actually
the logo is nice to have in interactive sessions, but inside the logs
kinda loses its usefulness, but for the ability of users to recognize
restarts easily: for this reason the new startup sequence shows a one
liner ASCII "wave" so that there is still a bit of visual clue.

Startup logging was modified in order to log events in more obvious
ways, and to log more events. Also certain important informations are
now more easy to parse/grep since they are printed in field=value style.

The option --always-show-logo in redis.conf was added, defaulting to no.

The new algorithm provides the same speed with a smaller error for
cardinalities in the range 0-100k. Before switching, the new and old
algorithm behavior was studied in details in the context of
issue #3677. You can find a few graphs and motivations there.

Config option to use LogLog-Beta Algorithm for Cardinality

The commit improves ziplistRepr() and adds a new debugging subcommand so
that we can trigger the dump directly from the Redis API.
This command capability was used while investigating issue #3684.

We need to use a dictionary type that frees the key, since we copy the
keys in the dictionary we use to track expires created in the slave
side.

BACKGROUND AND USE CASEj

Redis slaves are normally write only, however the supprot a "writable"
mode which is very handy when scaling reads on slaves, that actually
need write operations in order to access data. For instance imagine
having slaves replicating certain Sets keys from the master. When
accessing the data on the slave, we want to peform intersections between
such Sets values. However we don't want to intersect each time: to cache
the intersection for some time often is a good idea.

To do so, it is possible to setup a slave as a writable slave, and
perform the intersection on the slave side, perhaps setting a TTL on the
resulting key so that it will expire after some time.

THE BUG

Problem: in order to have a consistent replication, expiring of keys in
Redis replication is up to the master, that synthesize DEL operations to
send in the replication stream. However slaves logically expire keys
by hiding them from read attempts from clients so that if the master did
not promptly sent a DEL, the client still see logically expired keys
as non existing.

Because slaves don't actively expire keys by actually evicting them but
just masking from the POV of read operations, if a key is created in a
writable slave, and an expire is set, the key will be leaked forever:

1. No DEL will be received from the master, which does not know about
such a key at all.

2. No eviction will be performed by the slave, since it needs to disable
eviction because it's up to masters, otherwise consistency of data is
lost.

THE FIX

In order to fix the problem, the slave should be able to tag keys that
were created in the slave side and have an expire set in some way.

My solution involved using an unique additional dictionary created by
the writable slave only if needed. The dictionary is obviously keyed by
the key name that we need to track: all the keys that are set with an
expire directly by a client writing to the slave are tracked.

The value in the dictionary is a bitmap of all the DBs where such a key
name need to be tracked, so that we can use a single dictionary to track
keys in all the DBs used by the slave (actually this limits the solution
to the first 64 DBs, but the default with Redis is to use 16 DBs).

This solution allows to pay both a small complexity and CPU penalty,
which is zero when the feature is not used, actually. The slave-side
eviction is encapsulated in code which is not coupled with the rest of
the Redis core, if not for the hook to track the keys.

TODO

I'm doing the first smoke tests to see if the feature works as expected:
so far so good. Unit tests should be added before merging into the
4.0 branch.

This means that stopping a slave and restarting it will still make it
able to PSYNC with the master. Moreover the master itself will retain
its ID/offset, in case it gets turned into a slave, or if a slave will
try to PSYNC with it with an exactly updated offset (otherwise there is
no backlog).

This change was possible thanks to PSYNC v2 that makes saving the current
replication state much simpler.

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 new command swaps two Redis databases, so that immediately all the
clients connected to a given DB will see the data of the other DB, and
the other way around. Example:

    SWAPDB 0 1

This will swap DB 0 with DB 1. All the clients connected with DB 0 will
immediately see the new data, exactly like all the clients connected
with DB 1 will see the data that was formerly of DB 0.

MOTIVATION AND HISTORY
---

The command was recently demanded by Pedro Melo, but was suggested in
the past multiple times, and always refused by me.

The reason why it was asked: Imagine you have clients operating in DB 0.
At the same time, you create a new version of the dataset in DB 1.
When the new version of the dataset is available, you immediately want
to swap the two views, so that the clients will transparently use the
new version of the data. At the same time you'll likely destroy the
DB 1 dataset (that contains the old data) and start to build a new
version, to repeat the process.

This is an interesting pattern, but the reason why I always opposed to
implement this, was that FLUSHDB was a blocking command in Redis before
Redis 4.0 improvements. Now we have FLUSHDB ASYNC that releases the
old data in O(1) from the point of view of the client, to reclaim memory
incrementally in a different thread.

At this point, the pattern can really be supported without latency
spikes, so I'm providing this implementation for the users to comment.
In case a very compelling argument will be made against this new command
it may be removed.

BEHAVIOR WITH BLOCKING OPERATIONS
---

If a client is blocking for a list in a given DB, after the swap it will
still be blocked in the same DB ID, since this is the most logical thing
to do: if I was blocked for a list push to list "foo", even after the
swap I want still a LPUSH to reach the key "foo" in the same DB in order
to unblock.

However an interesting thing happens when a client is, for instance,
blocked waiting for new elements in list "foo" of DB 0. Then the DB
0 and 1 are swapped with SWAPDB. However the DB 1 happened to have
a list called "foo" containing elements. When this happens, this
implementation can correctly unblock the client.

It is possible that there are subtle corner cases that are not covered
in the implementation, but since the command is self-contained from the
POV of the implementation and the Redis core, it cannot cause anything
bad if not used.

Tests and documentation are yet to be provided.

Just a draft to align the main ideas, never executed code. Compiles.

It was noted by @dvirsky that it is not possible to use string functions
when writing the AOF file. This sometimes is critical since the command
rewriting may need to be built in the context of the AOF callback, and
without access to the context, and the limited types that the AOF
production functions will accept, this can be an issue.

Moreover there are other needs that we can't anticipate regarding the
ability to use Redis Modules APIs using the context in order to build
representations to emit AOF / RDB.

Because of this a new API was added that allows the user to get a
temporary context from the IO context. The context is auto released
if obtained when the RDB / AOF callback returns.

Calling multiple time the function to get the context, always returns
the same one, since it is invalid to have more than a single context.