I cloned the mariadb repo today. It’s big (the repo for 10.2 was about 950MB with full revision history). I think I want to get my mind wrapped around the bigger parts first. I’ll start with MyISAM and Aria. I assume that if I don’t already have a good grasp on the details of any storage engine, starting with the simpler one might make sense.
The first place I started was
storage/myisam/NEWS, where some key departures from NISAM (where does this word exist apart from here?), and I think that comparing reading that file with the structures listed at mysql.com is a good start to get the main ‘header level’ details.
There seem to be several grouped files in the myisam directory, those with ft prefixing apply to the full-text index feature, ha is presumably high availability, but looking into the code, it might be ‘handler’ or similar, mi is MyISAM core functionality, rt is for RTree keys, and sp is for spatial.
The core myisam.h file is outside this directory, in the main include directory, and should also be considered part of the table code. The ha files seem to have the important (for a user) definition of the various state conditions during repair (if you take definition as “when the code is here at X, it sets state to Y”). The changelog for this code points back to 1999, so the myisam changes are either old, or per-engine changelogs aren’t being actively maintained. I’m still a little confused about the HA designation there. I’m fairly confident it’s handler in some respect, but there seems to be a lot of spillover of the concerns, and the prefixing in any one file doesn’t seem to map directly onto the contents (functions with mi_ are defined in ha_myiam.cc).
This post seems to have a high level critique and explanation of the coding style, which they term ‘C with classes’. Indeed, there is a redefinition of new() specifically intended to obviate the need to link against the c++ standard library, so the goal was class definitions in C rather than the full c++ language from the beginning.
The Internals Guide should also be considered as mandatory commentary on the structure and purpose of the code (in lieu of documentation in the source code).