• Meir Shpilraien (Spielrein)'s avatar
    Unified MULTI, LUA, and RM_Call with respect to blocking commands (#8025) · d87a0d02
    Meir Shpilraien (Spielrein) authored
    
    
    Blocking command should not be used with MULTI, LUA, and RM_Call. This is because,
    the caller, who executes the command in this context, expects a reply.
    
    Today, LUA and MULTI have a special (and different) treatment to blocking commands:
    
    LUA   - Most commands are marked with no-script flag which are checked when executing
    and command from LUA, commands that are not marked (like XREAD) verify that their
    blocking mode is not used inside LUA (by checking the CLIENT_LUA client flag).
    MULTI - Command that is going to block, first verify that the client is not inside
    multi (by checking the CLIENT_MULTI client flag). If the client is inside multi, they
    return a result which is a match to the empty key with no timeout (for example blpop
    inside MULTI will act as lpop)
    For modules that perform RM_Call with blocking command, the returned results type is
    REDISMODULE_REPLY_UNKNOWN and the caller can not really know what happened.
    
    Disadvantages of the current state are:
    
    No unified approach, LUA, MULTI, and RM_Call, each has a different treatment
    Module can not safely execute blocking command (and get reply or error).
    Though It is true that modules are not like LUA or MULTI and should be smarter not
    to execute blocking commands on RM_Call, sometimes you want to execute a command base
    on client input (for example if you create a module that provides a new scripting
    language like javascript or python).
    While modules (on modules command) can check for REDISMODULE_CTX_FLAGS_LUA or
    REDISMODULE_CTX_FLAGS_MULTI to know not to block the client, there is no way to
    check if the command came from another module using RM_Call. So there is no way
    for a module to know not to block another module RM_Call execution.
    
    This commit adds a way to unify the treatment for blocking clients by introducing
    a new CLIENT_DENY_BLOCKING client flag. On LUA, MULTI, and RM_Call the new flag
    turned on to signify that the client should not be blocked. A blocking command
    verifies that the flag is turned off before blocking. If a blocking command sees
    that the CLIENT_DENY_BLOCKING flag is on, it's not blocking and return results
    which are matches to empty key with no timeout (as MULTI does today).
    
    The new flag is checked on the following commands:
    
    List blocking commands: BLPOP, BRPOP, BRPOPLPUSH, BLMOVE,
    Zset blocking commands: BZPOPMIN, BZPOPMAX
    Stream blocking commands: XREAD, XREADGROUP
    SUBSCRIBE, PSUBSCRIBE, MONITOR
    In addition, the new flag is turned on inside the AOF client, we do not want to
    block the AOF client to prevent deadlocks and commands ordering issues (and there
    is also an existing assert in the code that verifies it).
    
    To keep backward compatibility on LUA, all the no-script flags on existing commands
    were kept untouched. In addition, a LUA special treatment on XREAD and XREADGROUP was kept.
    
    To keep backward compatibility on MULTI (which today allows SUBSCRIBE, and PSUBSCRIBE).
    We added a special treatment on those commands to allow executing them on MULTI.
    
    The only backward compatibility issue that this PR introduces is that now MONITOR
    is not allowed inside MULTI.
    
    Tests were added to verify blocking commands are not blocking the client on LUA, MULTI,
    or RM_Call. Tests were added to verify the module can check for CLIENT_DENY_BLOCKING flag.
    Co-authored-by: default avatarOran Agra <oran@redislabs.com>
    Co-authored-by: default avatarItamar Haber <itamar@redislabs.com>
    d87a0d02
multi.c 14 KB