1. 24 Nov, 2022 1 commit
    • Meir Shpilraien (Spielrein)'s avatar
      Module API to allow writes after key space notification hooks (#11199) · abc345ad
      Meir Shpilraien (Spielrein) authored
      ### Summary of API additions
      
      * `RedisModule_AddPostNotificationJob` - new API to call inside a key space
        notification (and on more locations in the future) and allow to add a post job as describe above.
      * New module option, `REDISMODULE_OPTIONS_ALLOW_NESTED_KEYSPACE_NOTIFICATIONS`,
        allows to disable Redis protection of nested key-space notifications.
      * `RedisModule_GetModuleOptionsAll` - gets the mask of all supported module options so a module
        will be able to check if a given option is supported by the current running Redis instance.
      
      ### Background
      
      The following PR is a proposal of handling write operations inside module key space notifications.
      After a lot of discussions we came to a conclusion that module should not perform any write
      operations on key space notification.
      
      Some examples of issues that such write operation can cause are describe on the following links:
      
      * Bad replication oreder - https://github.com/redis/redis/pull/10969
      * Used after free - https://github.com/redis/redis/pull/10969#issuecomment-1223771006
      * Used after free - https://github.com/redis/redis/pull/9406#issuecomment-1221684054
      
      
      
      There are probably more issues that are yet to be discovered. The underline problem with writing
      inside key space notification is that the notification runs synchronously, this means that the notification
      code will be executed in the middle on Redis logic (commands logic, eviction, expire).
      Redis **do not assume** that the data might change while running the logic and such changes
      can crash Redis or cause unexpected behaviour.
      
      The solution is to state that modules **should not** perform any write command inside key space
      notification (we can chose whether or not we want to force it). To still cover the use-case where
      module wants to perform a write operation as a reaction to key space notifications, we introduce
      a new API , `RedisModule_AddPostNotificationJob`, that allows to register a callback that will be
      called by Redis when the following conditions hold:
      
      * It is safe to perform any write operation.
      * The job will be called atomically along side the operation that triggers it (in our case, key
        space notification).
      
      Module can use this new API to safely perform any write operation and still achieve atomicity
      between the notification and the write.
      
      Although currently the API is supported on key space notifications, the API is written in a generic
      way so that in the future we will be able to use it on other places (server events for example).
      
      ### Technical Details
      
      Whenever a module uses `RedisModule_AddPostNotificationJob` the callback is added to a list
      of callbacks (called `modulePostExecUnitJobs`) that need to be invoke after the current execution
      unit ends (whether its a command, eviction, or active expire). In order to trigger those callback
      atomically with the notification effect, we call those callbacks on `postExecutionUnitOperations`
      (which was `propagatePendingCommands` before this PR). The new function fires the post jobs
      and then calls `propagatePendingCommands`.
      
      If the callback perform more operations that triggers more key space notifications. Those keys
      space notifications might register more callbacks. Those callbacks will be added to the end
      of `modulePostExecUnitJobs` list and will be invoke atomically after the current callback ends.
      This raises a concerns of entering an infinite loops, we consider infinite loops as a logical bug
      that need to be fixed in the module, an attempt to protect against infinite loops by halting the
      execution could result in violation of the feature correctness and so **Redis will make no attempt
      to protect the module from infinite loops**
      
      In addition, currently key space notifications are not nested. Some modules might want to allow
      nesting key-space notifications. To allow that and keep backward compatibility, we introduce a
      new module option called `REDISMODULE_OPTIONS_ALLOW_NESTED_KEYSPACE_NOTIFICATIONS`.
      Setting this option will disable the Redis key-space notifications nesting protection and will
      pass this responsibility to the module.
      
      ### Redis infrastructure
      
      This PR promotes the existing `propagatePendingCommands` to an "Execution Unit" concept,
      which is called after each atomic unit of execution,
      Co-authored-by: default avatarOran Agra <oran@redislabs.com>
      Co-authored-by: default avatarYossi Gottlieb <yossigo@gmail.com>
      Co-authored-by: default avatarMadelyn Olson <34459052+madolson@users.noreply.github.com>
      abc345ad