Added get_ordered_blocks using bitcoind index

[asutoshpalai]

The LevelDB index of bitcoind contains details about the height of the block, file and offset of the block in the file. This can be used to extract the block ordered by height.

Fixes #13

[itoonx]

Awesome

[brannondorsey]

In it's current state, I am finding that this implementation seems to corrupt the .bitcoin/blocks/index such that it requires bitcoin-core to re-index the entire blockchain each time this code is run. Its a great approach to the solution, but I just want to warn folks who might come across it that this may occur.

[alecalve]

I tried this branch and some variant of it and both resulted in the corruption of the index. bitcoind's error message (LevelDB read failure: Corruption: corrupted compressed block contents) indicates some issue with Snappy compression.

I tried using plyvel 1.0.4 which should have support for snappy, but I got the same error.

A simple fix to this issue is to require to copy the index to a different directory than blocks/index and use this copy to read blocks position. While it would most likely work, it makes using this feature cumbersome.

[asutoshpalai]

After @brannondorsey reported it, I tried to look into it. When I was working on this, my Bitcoin blockchain was offline. Since I didn't turn the daemon back on, I didn't face this issue.

Thanks, @alecalve for looking into this. I was hoping shifting to a modern LevelDB library would have fixed this issue. Unfortunately, I have lost access to the bitcoin blockchain server I was using and currently don't possess the capability to set up one in near future. This makes me incapable of working on this issue.

If anyone wants to continue this and find some fix in this or any other PR/fork, I will be glad. I hope this work doesn't get wasted.

[brannondorsey]

Perhaps this is related to this issue. They have the same problem with snappy compression corrupting the LevelDB chainstate. It seems that opening the LevelDB database without snappy compression worked for them.

# Open the LevelDB
db = plyvel.DB(".bitcoin/chainstate", compression=None)

@alecalve, when you ran your tests with Plyvel, did you try disabling snappy compression?

[alecalve]

I didn't, I ran it with the defaults args, which make use of snappy.

[brannondorsey]

Ok, I'm going to take another look at this.

[asutoshpalai]

My sincere gratitude to @alecalve and @brannondorsey for their efforts in making this work!