Adding module api for processing commands during busy jobs and allow flagging the commands that should be handled at this status (#9963)

Some modules might perform a long-running logic in different stages of Redis lifetime, for example:
* command execution
* RDB loading
* thread safe context

During this long-running logic Redis is not responsive.

This PR offers 
1. An API to process events while a busy command is running (`RM_Yield`)
2. A new flag (`ALLOW_BUSY`) to mark the commands that should be handled during busy
  jobs which can also be used by modules (`allow-busy`)
3. In slow commands and thread safe contexts, this flag will start rejecting commands with -BUSY only
  after `busy-reply-threshold`
4. During loading (`rdb_load` callback), it'll process events right away (not wait for `busy-reply-threshold`),
  but either way, the processing is throttled to the server hz rate.
5. Allow modules to Yield to redis background tasks, but not to client commands

* rename `script-time-limit` to `busy-reply-threshold` (an alias to the pre-7.0 `lua-time-limit`)

Co-authored-by: Oran Agra <oran@redislabs.com>

1 files changed, 19 insertions, 13 deletions

diff --git a/redis.conf b/redis.conf
index ca3d4d6ad..a433e4fc4 100644
--- a/redis.conf
+++ b/redis.conf
@@ -1485,23 +1485,29 @@ aof-timestamp-enabled no
 #
 # shutdown-timeout 10
 
-################################ LUA SCRIPTING  ###############################
+################ NON-DETERMINISTIC LONG BLOCKING COMMANDS #####################
 
-# Max execution time of a Lua script in milliseconds.
+# Maximum time in milliseconds for EVAL scripts, functions and in some cases
+# modules' commands before Redis can start processing or rejecting other clients.
 #
-# If the maximum execution time is reached Redis will log that a script is
-# still in execution after the maximum allowed time and will start to
-# reply to queries with an error.
+# If the maximum execution time is reached Redis will start to reply to most
+# commands with a BUSY error.
 #
-# When a long running script exceeds the maximum execution time only the
-# SCRIPT KILL and SHUTDOWN NOSAVE commands are available. The first can be
-# used to stop a script that did not yet call any write commands. The second
-# is the only way to shut down the server in the case a write command was
-# already issued by the script but the user doesn't want to wait for the natural
-# termination of the script.
+# In this state Redis will only allow a handful of commands to be executed.
+# For instance, SCRIPT KILL, FUNCTION KILL, SHUTDOWN NOSAVE and possibly some
+# module specific 'allow-busy' commands.
 #
-# Set it to 0 or a negative value for unlimited execution without warnings.
-lua-time-limit 5000
+# SCRIPT KILL and FUNCTION KILL will only be able to stop a script that did not
+# yet call any write commands, so SHUTDOWN NOSAVE may be the only way to stop
+# the server in the case a write command was already issued by the script when
+# the user doesn't want to wait for the natural termination of the script.
+#
+# The default is 5 seconds. It is possible to set it to 0 or a negative value
+# to disable this mechanism (uninterrupted execution). Note that in the past
+# this config had a different name, which is now an alias, so both of these do
+# the same:
+# lua-time-limit 5000
+# busy-reply-threshold 5000
 
 ################################ REDIS CLUSTER  ###############################