Reviewing Lightning memory-mapped database librarytries++
I decided to resume my attempt to figure out the LMDB codebase. The previous attempt ended when my eyes started bleeding a little, but I think that I can get a bit further now.
One thing that I figured out already is that because this is C code, memory management is really painful. I noted previous how good it was from leveldb to use std::string for most allocations, since it frees both caller and lib from having to worry about the memory. LMDB only shows how important that was. Take a look at the following method:
Quick, what do you think it does. As it turn out, what is does is update the key to point to the key that the cursor is currently pointing at. The reason for that is that the library doesn’t want to own the memory for key (since then it would have to provide a way to free it).
Now, to be fair, this appears to be an internal method, only used by the lib itself, but such things are pretty pervasive in the codebase, and it is really annoying that the code is all in one spot, with no real structure, since I am still trying to figure out some sort of hierarchy for the code so I could get a grip on what is actually going on.
I decided to just go to the start and figure out how it opens a database:
It is interesting that you need a transaction to open the database. However, the opened database survives the transaction closing, and from the docs, if you open a db in a transaction, that is the only thing you can do in that transaction.
I think that I just figured out something, the code in mdb.c is actually written in reverse depth order. So the closer to the end of the file a function is, the more visible it is.
Okay, it appears that there is the notion of multiple dbs, but there is also the notion of the main db. It looks like the main db is also used for tracking things for child dbs. In particular, when opening a named db, we have the following:
We search for the db information in the main db. As you can see, mdb_cursor_set is actually setting the data on the cursor to the value we passed in. More interesting is what happens if the db is not there. It appears to merely create the db data and set it in the main db. Okay, looking at this function docs, it appears that this is just about allocating a database handle, not about opening the actual database.
I am not really sure that I understand the reasoning behind it, but a new database is actually created in mdb_cursor_put. So only after the first time you create a value, will the db actually be created.
I guess that explains why this function goes on for over 400 lines of code and has no less than 10 goto(!) and 7 goto sections!
I get that this is C, but come on, seriously!
More posts in "Reviewing Lightning memory-mapped database library" series:
- (08 Aug 2013) MVCC
- (07 Aug 2013) What about free pages?
- (06 Aug 2013) Transactions & commits
- (05 Aug 2013) A thoughtful hiatus
- (02 Aug 2013) On page splits and other painful things
- (30 Jul 2013) On disk data
- (25 Jul 2013) Stepping through make everything easier
- (24 Jul 2013) going deeper
- (15 Jul 2013) Because, damn it!
- (12 Jul 2013) tries++
- (09 Jul 2013) Partial