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reprocess command when client is unblocked on keys (redis#11012)
*TL;DR* --------------------------------------- Following the discussion over the issue [redis#7551](redis#7551) We decided to refactor the client blocking code to eliminate some of the code duplications and to rebuild the infrastructure better for future key blocking cases. *In this PR* --------------------------------------- 1. reprocess the command once a client becomes unblocked on key (instead of running custom code for the unblocked path that's different than the one that would have run if blocking wasn't needed) 2. eliminate some (now) irrelevant code for handling unblocking lists/zsets/streams etc... 3. modify some tests to intercept the error in cases of error on reprocess after unblock (see details in the notes section below) 4. replace '$' on the client argv with current stream id. Since once we reprocess the stream XREAD we need to read from the last msg and not wait for new msg in order to prevent endless block loop. 5. Added statistics to the info "Clients" section to report the: * `total_blocking_keys` - number of blocking keys * `total_blocking_keys_on_nokey` - number of blocking keys which have at least 1 client which would like to be unblocked on when the key is deleted. 6. Avoid expiring unblocked key during unblock. Previously we used to lookup the unblocked key which might have been expired during the lookup. Now we lookup the key using NOTOUCH and NOEXPIRE to avoid deleting it at this point, so propagating commands in blocked.c is no longer needed. 7. deprecated command flags. We decided to remove the CMD_CALL_STATS and CMD_CALL_SLOWLOG and make an explicit verification in the call() function in order to decide if stats update should take place. This should simplify the logic and also mitigate existing issues: for example module calls which are triggered as part of AOF loading might still report stats even though they are called during AOF loading. *Behavior changes* --------------------------------------------------- 1. As this implementation prevents writing dedicated code handling unblocked streams/lists/zsets, since we now re-process the command once the client is unblocked some errors will be reported differently. The old implementation used to issue ``UNBLOCKED the stream key no longer exists`` in the following cases: - The stream key has been deleted (ie. calling DEL) - The stream and group existed but the key type was changed by overriding it (ie. with set command) - The key not longer exists after we swapdb with a db which does not contains this key - After swapdb when the new db has this key but with different type. In the new implementation the reported errors will be the same as if the command was processed after effect: **NOGROUP** - in case key no longer exists, or **WRONGTYPE** in case the key was overridden with a different type. 2. Reprocessing the command means that some checks will be reevaluated once the client is unblocked. For example, ACL rules might change since the command originally was executed and will fail once the client is unblocked. Another example is OOM condition checks which might enable the command to run and block but fail the command reprocess once the client is unblocked. 3. One of the changes in this PR is that no command stats are being updated once the command is blocked (all stats will be updated once the client is unblocked). This implies that when we have many clients blocked, users will no longer be able to get that information from the command stats. However the information can still be gathered from the client list. **Client blocking** --------------------------------------------------- the blocking on key will still be triggered the same way as it is done today. in order to block the current client on list of keys, the call to blockForKeys will still need to be made which will perform the same as it is today: * add the client to the list of blocked clients on each key * keep the key with a matching list node (position in the global blocking clients list for that key) in the client private blocking key dict. * flag the client with CLIENT_BLOCKED * update blocking statistics * register the client on the timeout table **Key Unblock** --------------------------------------------------- Unblocking a specific key will be triggered (same as today) by calling signalKeyAsReady. the implementation in that part will stay the same as today - adding the key to the global readyList. The reason to maintain the readyList (as apposed to iterating over all clients blocked on the specific key) is in order to keep the signal operation as short as possible, since it is called during the command processing. The main change is that instead of going through a dedicated code path that operates the blocked command we will just call processPendingCommandsAndResetClient. **ClientUnblock (keys)** --------------------------------------------------- 1. Unblocking clients on keys will be triggered after command is processed and during the beforeSleep 8. the general schema is: 9. For each key *k* in the readyList: ``` For each client *c* which is blocked on *k*: in case either: 1. *k* exists AND the *k* type matches the current client blocking type OR 2. *k* exists and *c* is blocked on module command OR 3. *k* does not exists and *c* was blocked with the flag unblock_on_deleted_key do: 1. remove the client from the list of clients blocked on this key 2. remove the blocking list node from the client blocking key dict 3. remove the client from the timeout list 10. queue the client on the unblocked_clients list 11. *NEW*: call processCommandAndResetClient(c); ``` *NOTE:* for module blocked clients we will still call the moduleUnblockClientByHandle which will queue the client for processing in moduleUnblockedClients list. **Process Unblocked clients** --------------------------------------------------- The process of all unblocked clients is done in the beforeSleep and no change is planned in that part. The general schema will be: For each client *c* in server.unblocked_clients: * remove client from the server.unblocked_clients * set back the client readHandler * continue processing the pending command and input buffer. *Some notes regarding the new implementation* --------------------------------------------------- 1. Although it was proposed, it is currently difficult to remove the read handler from the client while it is blocked. The reason is that a blocked client should be unblocked when it is disconnected, or we might consume data into void. 2. While this PR mainly keep the current blocking logic as-is, there might be some future additions to the infrastructure that we would like to have: - allow non-preemptive blocking of client - sometimes we can think that a new kind of blocking can be expected to not be preempt. for example lets imagine we hold some keys on disk and when a command needs to process them it will block until the keys are uploaded. in this case we will want the client to not disconnect or be unblocked until the process is completed (remove the client read handler, prevent client timeout, disable unblock via debug command etc...). - allow generic blocking based on command declared keys - we might want to add a hook before command processing to check if any of the declared keys require the command to block. this way it would be easier to add new kinds of key-based blocking mechanisms. Co-authored-by: Oran Agra <oran@redislabs.com> Signed-off-by: Ran Shidlansik <ranshid@amazon.com>
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