In issue #2948 a crash was reported in processCommand(). Later Oran Agra
(@oranagra) traced the bug (in private chat) in the following sequence
of events:
1. Some maxmemory is set.
2. The slave is the currently active client and is executing PING or
REPLCONF or whatever a slave can send to its master.
3. freeMemoryIfNeeded() is called since maxmemory is set.
4. flushSlavesOutputBuffers() is called by freeMemoryIfNeeded().
5. During slaves buffers flush, a write error could be encoutered in
writeToClient() or sendReplyToClient() depending on the version of
Redis. This will trigger freeClient() against the currently active
client, so a segmentation fault will likely happen in
processCommand() immediately after the call to freeMemoryIfNeeded().
There are different possible fixes:
1. Add flags to writeToClient() (recent versions code base) so that
we can ignore the write errors, and use this flag in
flushSlavesOutputBuffers(). However this is not simple to do in older
versions of Redis.
2. Use freeClientAsync() during write errors. This works but changes the
current behavior of releasing clients ASAP when possible. Normally
we write to clients during the normal event loop processing, in the
writable client, where there is no active client, so no care must be
taken.
3. The fix of this commit: to detect that the current client is no
longer valid. This fix is a bit "ad-hoc", but works across all the
versions and has the advantage of not changing the remaining
behavior. Only alters what happens during this race condition,
hopefully.
* Function to test for slave handshake renamed slaveIsInHandshakeState.
* Function no longer accepts arguments since it always tests the
same global state.
* Test for state translated to a range test since defines are guaranteed
to stay in order in the future.
* Use the new function in the ROLE command implementation as well.
This change allows a slave to properly time out a dead master during
the extended asynchronous synchronization state machine. Now, slaves
will record their last interaction with the master and apply the
replication timeout before a response to the PSYNC request is received.
The code was broken and resulted in redis-cli --pipe to, most of the
times, writing everything received in the standard input to the Redis
connection socket without ever reading back the replies, until all the
content to write was written.
This means that Redis had to accumulate all the output in the output
buffers of the client, consuming a lot of memory.
Fixed thanks to the original report of anomalies in the behavior
provided by Twitter user @fsaintjacques.
HINCRBY* tests later used the value "tmp" that was sometimes generated
by the random key generation function. The result was ovewriting what
Tcl expected to be inside Redis with another value, causing the next
HSTRLEN test to fail.
MOVE was not able to move the TTL: when a key was moved into a different
database number, it became persistent like if PERSIST was used.
In some incredible way (I guess almost nobody uses Redis MOVE) this bug
remained unnoticed inside Redis internals for many years.
Finally Andy Grunwald discovered it and opened an issue.
This commit fixes the bug and adds a regression test.
Close#2765.
As Oran Agra suggested, in startBgsaveForReplication() when the BGSAVE
attempt returns an error, we scan the list of slaves in order to remove
them since there is no way to serve them currently.
However we check for the replication state BGSAVE_START, which was
modified by rdbSaveToSlaveSockets() before forking(). So when fork fails
the state of slaves remain BGSAVE_END and no cleanup is performed.
This commit fixes the problem by making rdbSaveToSlavesSockets() able to
undo the state change on fork failure.
We have a check to rewrite the config properly when a failover is in
progress, in order to add the current (already failed over) master as
slave, and don't include in the slave list the promoted slave itself.
However there was an issue, the variable with the right address was
computed but never used when the code was modified, and no tests are
available for this feature for two reasons:
1. The Sentinel unit test currently does not test Sentinel ability to
persist its state at all.
2. It is a very hard to trigger state since it lasts for little time in
the context of the testing framework.
However this feature should be covered in the test in some way.
The bug was found by @badboy using the clang static analyzer.
Effects of the bug on safety of Sentinel
===
This bug results in severe issues in the following case:
1. A Sentinel is elected leader.
2. During the failover, it persists a wrong config with a known-slave
entry listing the master address.
3. The Sentinel crashes and restarts, reading invalid configuration from
disk.
4. It sees that the slave now does not obey the logical configuration
(should replicate from the current master), so it sends a SLAVEOF
command to the master (since the slave master is the same) creating a
replication loop (attempt to replicate from itself) which Redis is
currently unable to detect.
5. This means that the master is no longer available because of the bug.
However the lack of availability should be only transient (at least
in my tests, but other states could be possible where the problem
is not recovered automatically) because:
6. Sentinels treat masters reporting to be slaves as failing.
7. A new failover is triggered, and a slave is promoted to master.
Bug lifetime
===
The bug is there forever. Commit 16237d78 actually tried to fix the bug
but in the wrong way (the computed variable was never used! My fault).
So this bug is there basically since the start of Sentinel.
Since the bug is hard to trigger, I remember little reports matching
this condition, but I remember at least a few. Also in automated tests
where instances were stopped and restarted multiple times automatically
I remember hitting this issue, however I was not able to reproduce nor
to determine with the information I had at the time what was causing the
issue.
Using chained replication where C is slave of B which is in turn slave of
A, if B reconnects the replication link with A but discovers it is no
longer possible to PSYNC, slaves of B must be disconnected and PSYNC
not allowed, since the new B dataset may be completely different after
the synchronization with the master.
Note that there are varius semantical differences in the way this is
handled now compared to the past. In the past the semantics was:
1. When a slave lost connection with its master, disconnected the chained
slaves ASAP. Which is not needed since after a successful PSYNC with the
master, the slaves can continue and don't need to resync in turn.
2. However after a failed PSYNC the replication backlog was not reset, so a
slave was able to PSYNC successfully even if the instance did a full
sync with its master, containing now an entirely different data set.
Now instead chained slaves are not disconnected when the slave lose the
connection with its master, but only when it is forced to full SYNC with
its master. This means that if the slave having chained slaves does a
successful PSYNC all its slaves can continue without troubles.
See issue #2694 for more details.
Talking with @oranagra we had to reason a little bit to understand if
this function could ever flush the output buffers of the wrong slaves,
having online state but actually not being ready to receive writes
before the first ACK is received from them (this happens with diskless
replication).
Next time we'll just read this comment.
Before this commit, after triggering a BGSAVE it was up to the caller of
startBgsavForReplication() to handle slaves in WAIT_BGSAVE_START in
order to update them accordingly. However when the replication target is
the socket, this is not possible since the process of updating the
slaves and sending the FULLRESYNC reply must be coupled with the process
of starting an RDB save (the reason is, we need to send the FULLSYNC
command and spawn a child that will start to send RDB data to the slaves
ASAP).
This commit moves the responsibility of handling slaves in
WAIT_BGSAVE_START to startBgsavForReplication() so that for both
diskless and disk-based replication we have the same chain of
responsiblity. In order accomodate such change, the syncCommand() also
needs to put the client in the slave list ASAP (just after the initial
checks) and not at the end, so that startBgsavForReplication() can find
the new slave alrady in the list.
Another related change is what happens if the BGSAVE fails because of
fork() or other errors: we now remove the slave from the list of slaves
and send an error, scheduling the slave connection to be terminated.
As a side effect of this change the following errors found by
Oran Agra are fixed (thanks!):
1. rdbSaveToSlavesSockets() on failed fork will get the slaves cleaned
up, otherwise they remain in a wrong state forever since we setup them
for full resync before actually trying to fork.
2. updateSlavesWaitingBgsave() with replication target set as "socket"
was broken since the function changed the slaves state from
WAIT_BGSAVE_START to WAIT_BGSAVE_END via
replicationSetupSlaveForFullResync(), so later rdbSaveToSlavesSockets()
will not find any slave in the right state (WAIT_BGSAVE_START) to feed.
It is simpler if removing the read event handler from the FD is up to
slaveTryPartialResynchronization, after all it is only called in the
context of syncWithMaster.
This commit also makes sure that on error all the event handlers are
removed from the socket before closing it.
Add the concept of slaves capabilities to Redis, the slave now presents
to the Redis master with a set of capabilities in the form:
REPLCONF capa SOMECAPA capa OTHERCAPA ...
This has the effect of setting slave->slave_capa with the corresponding
SLAVE_CAPA macros that the master can test later to understand if it
the slave will understand certain formats and protocols of the
replication process. This makes it much simpler to introduce new
replication capabilities in the future in a way that don't break old
slaves or masters.
This patch was designed and implemented together with Oran Agra
(@oranagra).
Our function to read a line with a timeout handles newlines as requests
to refresh the timeout, however the code kept subtracting the buffer
size left every time a newline was received, for a bug in the loop
logic. Fixed by this commit.
For PINGs we use the period configured by the user, but for the newlines
of slaves waiting for an RDB to be created (including slaves waiting for
the FULLRESYNC reply) we need to ping with frequency of 1 second, since
the timeout is fixed and needs to be refreshed.
In previous commits we moved the FULLRESYNC to the moment we start the
BGSAVE, so that the offset we provide is the right one. However this
also means that we need to re-emit the SELECT statement every time a new
slave starts to accumulate the changes.
To obtian this effect in a more clean way, the function that sends the
FULLRESYNC reply was overloaded with a more important role of also doing
this and chanigng the slave state. So it was renamed to
replicationSetupSlaveForFullResync() to better reflect what it does now.
Salvatore Sanfilippo