reprocess command when client is unblocked on keys (#11012)

*TL;DR*
---------------------------------------
Following the discussion over the issue [#7551](https://github.com/redis/redis/issues/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>

1 files changed, 42 insertions, 43 deletions

diff --git a/src/server.h b/src/server.h
index 765b67d62..1201cab3b 100644
--- a/src/server.h
+++ b/src/server.h
@@ -359,7 +359,11 @@ extern int configOOMScoreAdjValuesDefaults[CONFIG_OOM_COUNT];
 #define CLIENT_LUA_DEBUG_SYNC (1<<26)  /* EVAL debugging without fork() */
 #define CLIENT_MODULE (1<<27) /* Non connected client used by some module. */
 #define CLIENT_PROTECTED (1<<28) /* Client should not be freed for now. */
-/* #define CLIENT_... (1<<29) currently unused, feel free to use in the future */
+#define CLIENT_EXECUTING_COMMAND (1<<29) /* Indicates that the client is currently in the process of handling
+                                          a command. usually this will be marked only during call()
+                                          however, blocked clients might have this flag kept until they
+                                          will try to reprocess the command. */
+
 #define CLIENT_PENDING_COMMAND (1<<30) /* Indicates the client has a fully
                                         * parsed command ready for execution. */
 #define CLIENT_TRACKING (1ULL<<31) /* Client enabled keys tracking in order to
@@ -388,15 +392,18 @@ extern int configOOMScoreAdjValuesDefaults[CONFIG_OOM_COUNT];
 
 /* Client block type (btype field in client structure)
  * if CLIENT_BLOCKED flag is set. */
-#define BLOCKED_NONE 0    /* Not blocked, no CLIENT_BLOCKED flag set. */
-#define BLOCKED_LIST 1    /* BLPOP & co. */
-#define BLOCKED_WAIT 2    /* WAIT for synchronous replication. */
-#define BLOCKED_MODULE 3  /* Blocked by a loadable module. */
-#define BLOCKED_STREAM 4  /* XREAD. */
-#define BLOCKED_ZSET 5    /* BZPOP et al. */
-#define BLOCKED_POSTPONE 6 /* Blocked by processCommand, re-try processing later. */
-#define BLOCKED_SHUTDOWN 7 /* SHUTDOWN. */
-#define BLOCKED_NUM 8      /* Number of blocked states. */
+typedef enum blocking_type {
+    BLOCKED_NONE,    /* Not blocked, no CLIENT_BLOCKED flag set. */
+    BLOCKED_LIST,    /* BLPOP & co. */
+    BLOCKED_WAIT,    /* WAIT for synchronous replication. */
+    BLOCKED_MODULE,  /* Blocked by a loadable module. */
+    BLOCKED_STREAM,  /* XREAD. */
+    BLOCKED_ZSET,    /* BZPOP et al. */
+    BLOCKED_POSTPONE, /* Blocked by processCommand, re-try processing later. */
+    BLOCKED_SHUTDOWN, /* SHUTDOWN. */
+    BLOCKED_NUM,      /* Number of blocked states. */
+    BLOCKED_END       /* End of enumeration */
+} blocking_type;
 
 /* Client request types */
 #define PROTO_REQ_INLINE 1
@@ -569,13 +576,11 @@ typedef enum {
 
 /* Command call flags, see call() function */
 #define CMD_CALL_NONE 0
-#define CMD_CALL_SLOWLOG (1<<0)
-#define CMD_CALL_STATS (1<<1)
-#define CMD_CALL_PROPAGATE_AOF (1<<2)
-#define CMD_CALL_PROPAGATE_REPL (1<<3)
+#define CMD_CALL_PROPAGATE_AOF (1<<0)
+#define CMD_CALL_PROPAGATE_REPL (1<<1)
 #define CMD_CALL_PROPAGATE (CMD_CALL_PROPAGATE_AOF|CMD_CALL_PROPAGATE_REPL)
-#define CMD_CALL_FULL (CMD_CALL_SLOWLOG | CMD_CALL_STATS | CMD_CALL_PROPAGATE)
-#define CMD_CALL_FROM_MODULE (1<<4)  /* From RM_Call */
+#define CMD_CALL_FULL (CMD_CALL_PROPAGATE)
+#define CMD_CALL_FROM_MODULE (1<<2)  /* From RM_Call */
 
 /* Command propagation flags, see propagateNow() function */
 #define PROPAGATE_NONE 0
@@ -992,27 +997,13 @@ typedef struct multiState {
  * The fields used depend on client->btype. */
 typedef struct blockingState {
     /* Generic fields. */
-    long count;             /* Elements to pop if count was specified (BLMPOP/BZMPOP), -1 otherwise. */
-    mstime_t timeout;       /* Blocking operation timeout. If UNIX current time
-                             * is > timeout then the operation timed out. */
-
-    /* BLOCKED_LIST, BLOCKED_ZSET and BLOCKED_STREAM */
-    dict *keys;             /* The keys we are waiting to terminate a blocking
-                             * operation such as BLPOP or XREAD. Or NULL. */
-    robj *target;           /* The key that should receive the element,
-                             * for BLMOVE. */
-    struct blockPos {
-        int wherefrom;      /* Where to pop from */
-        int whereto;        /* Where to push to */
-    } blockpos;              /* The positions in the src/dst lists/zsets
-                             * where we want to pop/push an element
-                             * for BLPOP, BRPOP, BLMOVE and BZMPOP. */
-
-    /* BLOCK_STREAM */
-    size_t xread_count;     /* XREAD COUNT option. */
-    robj *xread_group;      /* XREADGROUP group name. */
-    robj *xread_consumer;   /* XREADGROUP consumer name. */
-    int xread_group_noack;
+    blocking_type btype;                  /* Type of blocking op if CLIENT_BLOCKED. */
+    mstime_t timeout;           /* Blocking operation timeout. If UNIX current time
+                                 * is > timeout then the operation timed out. */
+    int unblock_on_nokey;       /* Whether to unblock the client when at least one of the keys
+                                   is deleted or does not exist anymore */
+    /* BLOCKED_LIST, BLOCKED_ZSET and BLOCKED_STREAM or any other Keys related blocking */
+    dict *keys;                 /* The keys we are blocked on */
 
     /* BLOCKED_WAIT */
     int numreplicas;        /* Number of replicas we are waiting for ACK. */
@@ -1029,7 +1020,7 @@ typedef struct blockingState {
  * operation such as B[LR]POP, but received new data in the context of the
  * last executed command.
  *
- * After the execution of every command or script, we run this list to check
+ * After the execution of every command or script, we iterate over this list to check
  * if as a result we should serve data to clients blocked, unblocking them.
  * Note that server.ready_keys will not have duplicates as there dictionary
  * also called ready_keys in every structure representing a Redis database,
@@ -1167,8 +1158,7 @@ typedef struct client {
     int slave_capa;         /* Slave capabilities: SLAVE_CAPA_* bitwise OR. */
     int slave_req;          /* Slave requirements: SLAVE_REQ_* */
     multiState mstate;      /* MULTI/EXEC state */
-    int btype;              /* Type of blocking op if CLIENT_BLOCKED. */
-    blockingState bpop;     /* blocking state */
+    blockingState bstate;     /* blocking state */
     long long woff;         /* Last write global replication offset. */
     list *watched_keys;     /* Keys WATCHED for MULTI/EXEC CAS */
     dict *pubsub_channels;  /* channels a client is interested in (SUBSCRIBE) */
@@ -2635,7 +2625,6 @@ int listTypeEqual(listTypeEntry *entry, robj *o);
 void listTypeDelete(listTypeIterator *iter, listTypeEntry *entry);
 robj *listTypeDup(robj *o);
 void listTypeDelRange(robj *o, long start, long stop);
-void unblockClientWaitingData(client *c);
 void popGenericCommand(client *c, int where);
 void listElementsRemoved(client *c, robj *key, int where, robj *o, long count, int signal, int *deleted);
 typedef enum {
@@ -2938,6 +2927,7 @@ int overMaxmemoryAfterAlloc(size_t moremem);
 uint64_t getCommandFlags(client *c);
 int processCommand(client *c);
 int processPendingCommandAndInputBuffer(client *c);
+int processCommandAndResetClient(client *c);
 void setupSignalHandlers(void);
 void removeSignalHandlers(void);
 int createSocketAcceptHandler(connListener *sfd, aeFileProc *accept_handler);
@@ -3166,6 +3156,7 @@ int objectSetLRUOrLFU(robj *val, long long lfu_freq, long long lru_idle,
 #define LOOKUP_NOSTATS (1<<2)  /* Don't update keyspace hits/misses counters. */
 #define LOOKUP_WRITE (1<<3)    /* Delete expired keys even in replicas. */
 #define LOOKUP_NOEXPIRE (1<<4) /* Avoid deleting lazy expired keys. */
+#define LOKKUP_NOEFFECTS (LOOKUP_NONOTIFY | LOOKUP_NOSTATS | LOOKUP_NOTOUCH | LOOKUP_NOEXPIRE) /* Avoid any effects from fetching the key */
 
 void dbAdd(redisDb *db, robj *key, robj *val);
 int dbAddRDBLoad(redisDb *db, sds key, robj *val);
@@ -3281,20 +3272,28 @@ typedef struct luaScript {
     robj *body;
 } luaScript;
 
-/* Blocked clients */
+/* Blocked clients API */
 void processUnblockedClients(void);
+void initClientBlockingState(client *c);
 void blockClient(client *c, int btype);
 void unblockClient(client *c);
+void unblockClientOnTimeout(client *c);
+void unblockClientOnError(client *c, const char *err_str);
 void queueClientForReprocessing(client *c);
 void replyToBlockedClientTimedOut(client *c);
 int getTimeoutFromObjectOrReply(client *c, robj *object, mstime_t *timeout, int unit);
 void disconnectAllBlockedClients(void);
 void handleClientsBlockedOnKeys(void);
 void signalKeyAsReady(redisDb *db, robj *key, int type);
+void blockForKeys(client *c, int btype, robj **keys, int numkeys, mstime_t timeout, int unblock_on_nokey);
+void blockClientShutdown(client *c);
+void blockPostponeClient(client *c);
+void blockForReplication(client *c, mstime_t timeout, long long offset, long numreplicas);
 void signalDeletedKeyAsReady(redisDb *db, robj *key, int type);
-void blockForKeys(client *c, int btype, robj **keys, int numkeys, long count, mstime_t timeout, robj *target, struct blockPos *blockpos, streamID *ids, int unblock_on_nokey);
 void updateStatsOnUnblock(client *c, long blocked_us, long reply_us, int had_errors);
 void scanDatabaseForDeletedKeys(redisDb *emptied, redisDb *replaced_with);
+void totalNumberOfBlockingKeys(unsigned long *blocking_keys, unsigned long *bloking_keys_on_nokey);
+
 
 /* timeout.c -- Blocked clients timeout and connections timeout. */
 void addClientToTimeoutTable(client *c);