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From: =?iso-8859-1?Q?Michael_Gr=F8nager?= <gronager@ceptacle.com>
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Date: Fri, 3 Feb 2012 10:52:22 +0100
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To: Pieter Wuille <pieter.wuille@gmail.com>
Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Announcement: libcoin
Precedence: list

> Hello Michael,
>=20
> I'm impressed by your refactorings, and hope some of them can make it =
into the Satoshi codebase.

Thanks:)

>  I am however not sure what you've said above is safe. In particular, =
how do you guarantee that no other thread modifies the blockchain =
structure while you are performing your query on it? Does the query code =
operate on a const copy of the structure, or is there guaranteed only =
one thread accessing it?

The BlockChain class encapsulates all access to the blockchain and only =
give you access to certain restricted queries on the chain. Actually =
that was the case already in the satoshi client, I have only tried to =
formalize and encapsulate these queries in the code instead of having =
all the code poking around directly in the database and the blockfile.

I should note that the database still keeps a mutex to protect reads =
from writes.

I agree that constness alone does not guarantee thread safety, it is one =
of the things to use to get there. Great care should be taken not to =
read a value that is being changed at the same time, at least if that =
will render the result unusable.

The list of allowed queries are the const public methods of BlockChain. =
Some examples :
1.    bool isSpent(Coin coin) const;
2.    int getNumSpent(uint256 hash) const ;
3.    uint256 spentIn(Coin coin) const;

    /// Check if the hash of a block belongs to a block in the main =
chain:
4.    bool isInMainChain(const uint256 hash) const;
   =20
    /// Get the best height
5.    int getBestHeight() const { return _bestIndex->nHeight; }

Or e.g.:
6.    void getBlock(const uint256 hash, Block& block) const;

1-3. can be used to check if a COutPoint (now Coin) has been spent =
etc... This will only generate sane results, even if the two threads are =
both active on the same data structures.
Same goes for 4. and 6. copies a block from the block file to the Block& =
provided so no issues here either.=20

I do, however, admit, that an extra review of all the public const =
methods would be wise, to ensure that I have not overlooked something. =
I'll open an issue on this and use a cold winter night on looking them =
over again.

Cheers,

Michael


>=20
> I've been thinking about moving to read-write locks that allow =
multiple threads reading the datastructure simultaneously, but removing =
the locking all together sounds wrong to me.
>=20
> --=20
> Pieter

Michael Gronager, PhD
Director, Ceptacle
Jens Juels Gade 33
2100 Copenhagen E
Mobile: +45 31 45 14 01
E-mail: gronager@ceptacle.com
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