dfcef536d0 blockstorage: do not flush block to disk if it is already there (Matthew Zipkin)
Pull request description:
Closes https://github.com/bitcoin/bitcoin/issues/2039
When reindexing from flat-file block storage there is no need to write anything back to disk, since the block data is already there. This PR skips flushing to disk those blocks that already have a known position in the datastore. Skipping this means that users can write-protect the `blk` files on disk which may be useful for security or even safely sharing that data between multiple bitcoind instances.
`FindBlockPos()` may also flush the undo data file, but again this is skipped if the corresponding block position is known, like during the initial stage of a reindex when block data is being indexed. Once the block index is complete the validation mechanism will call `ConnectBlock()` which will save undo data at that time.
The call stack looks like this:
```
init()
ThreadImport() <-- process fReindex flag
LoadExternalBlockFile()
AcceptBlock()
SaveBlockToDisk()
FindBlockPos()
FlushBlockFile() <-- unnecessary if block is already on disk
```
A larger refactor of this part of the code was started by mzumsande here: https://github.com/mzumsande/bitcoin/tree/202207_refactor_findblockpos including this fix, reviewers can let me know if the changes should be combined.
ACKs for top commit:
sipa:
utACK dfcef536d0
mzumsande:
re-ACK dfcef536d0
achow101:
ACK dfcef536d0
furszy:
Rebase diff ACK dfcef53.
Tree-SHA512: 385c5ac1288b325135398d0ddd3ab788fa98cc0ca19bd2474c74039f2ce70d5088c1d1c9d4dd10aefcbd4c757767ec5805d07ba8cee9289a66f96e6f9eaa5279
The src/node/ directory contains code that needs to access node state
(state in CChain, CBlockIndex, CCoinsView, CTxMemPool, and similar
classes).
Code in src/node/ is meant to be segregated from code in
src/wallet/ and src/qt/, to ensure wallet and GUI
code changes don't interfere with node operation, to allow wallet and GUI code
to run in separate processes, and to perhaps eventually allow wallet and GUI
code to be maintained in separate source repositories.
As a rule of thumb, code in one of the src/node/,
src/wallet/, or src/qt/ directories should avoid
calling code in the other directories directly, and only invoke it indirectly
through the more limited src/interfaces/ classes.
This directory is at the moment
sparsely populated. Eventually more substantial files like
src/validation.cpp and
src/txmempool.cpp might be moved there.
69ddee6f39