In #10025 we added a mechanism for flagging certain properties for Redis Functions.
This lead us to think we'd like to "port" this mechanism to Redis Scripts (`EVAL`) as well.
One good reason for this, other than the added functionality is because it addresses the
poor behavior we currently have in `EVAL` in case the script performs a (non DENY_OOM) write operation
during OOM state. See #8478 (And a previous attempt to handle it via #10093) for details.
Note that in Redis Functions **all** write operations (including DEL) will return an error during OOM state
unless the function is flagged as `allow-oom` in which case no OOM checking is performed at all.
This PR:
- Enables setting `EVAL` (and `SCRIPT LOAD`) script flags as defined in #10025.
- Provides a syntactical framework via [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) for
additional script annotations and even engine selection (instead of just lua) for scripts.
- Provides backwards compatibility so scripts without the new annotations will behave as they did before.
- Appropriate tests.
- Changes `EVAL[SHA]/_RO` to be flagged as `STALE` commands. This makes it possible to flag individual
scripts as `allow-stale` or not flag them as such. In backwards compatibility mode these commands will
return the `MASTERDOWN` error as before.
- Changes `SCRIPT LOAD` to be flagged as a `STALE` command. This is mainly to make it logically
compatible with the change to `EVAL` in the previous point. It enables loading a script on a stale server
which is technically okay it doesn't relate directly to the server's dataset. Running the script does, but that
won't work unless the script is explicitly marked as `allow-stale`.
Note that even though the LUA syntax doesn't support hash tag comments `.lua` files do support a shebang
tag on the top so they can be executed on Unix systems like any shell script. LUA's `luaL_loadfile` handles
this as part of the LUA library. In the case of `luaL_loadbuffer`, which is what Redis uses, I needed to fix the
input script in case of a shebang manually. I did this the same way `luaL_loadfile` does, by replacing the
first line with a single line feed character.
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