cae6d895f8a8cf5f57e05519536fda5d62b10841 fuzz: add target for CoinsViewOverlay (Andrew Toth)
86eda88c8e486eb1db724e60948f71349d050e1d fuzz: move backend mutating block to end of coins_view (Andrew Toth)
89824fb27b228a12d5c2f63106c2a4d793e73107 fuzz: pass coins_view_cache to TestCoinsView in coins_view (Andrew Toth)
73e99a59665551243d6dbe03a0e9baa9cab046b9 coins: don't mutate main cache when connecting block (Andrew Toth)
67c0d1798e6147f48d4bafc2c9e5ff30f2a62340 coins: introduce CoinsViewOverlay (Andrew Toth)
69b01af0eb9017a6ae7ca3134c9dcf89e74dbfa8 coins: add PeekCoin() (Andrew Toth)
Pull request description:
This is a slightly modified version of the first few commits of #31132, which can be merged as an independent change. It has a small benefit on its own, but will help in moving the parent PR forward.
When accessing coins via the `CCoinsViewCache`, methods like `GetCoin` can call `FetchCoin` which actually mutate `cacheCoins` internally to cache entries when they are pulled from the backing db. This is generally a performance improvement for single threaded access patterns, but it precludes us from accessing entries in a `CCoinsViewCache` from multiple threads without a lock.
Another aspect is that when we use the resettable `CCoinsViewCache` view backed by the main cache for use in `ConnectBlock()`, we will insert entries into the main cache even if the block is determined to be invalid. This is not the biggest concern, since an invalid block requires proof-of-work. But, an attacker could craft multiple invalid blocks to fill the main cache. This would make us `Flush` the cache more often than necessary. Obviously this would be very expensive to do on mainnet.
Introduce `CoinsViewOverlay`, a `CCoinsViewCache` subclass that reads coins without mutating the underlying cache via `FetchCoin()`.
Add `PeekCoin()` to look up a Coin through a stack of `CCoinsViewCache` layers without populating parent caches. This prevents the main cache from caching inputs pulled from disk for a block that has not yet been fully validated. Once `Flush()` is called on the view, these inputs will be added as spent to `coinsCache` in the main cache via `BatchWrite()`.
This is the foundation for async input fetching, where worker threads must not mutate shared state.
ACKs for top commit:
l0rinc:
ACK cae6d895f8a8cf5f57e05519536fda5d62b10841
sipa:
reACK cae6d895f8a8cf5f57e05519536fda5d62b10841
sedited:
Re-ACK cae6d895f8a8cf5f57e05519536fda5d62b10841
willcl-ark:
ACK cae6d895f8a8cf5f57e05519536fda5d62b10841
vasild:
Cursory ACK cae6d895f8a8cf5f57e05519536fda5d62b10841
ryanofsky:
Code review ACK cae6d895f8a8cf5f57e05519536fda5d62b10841. PR is basically back to the form I had acked the first time, implementing `PeekCoin()` by calling `GetCoin()`. This is not ideal because `PeekCoin()` is not supposed to modify caches and `GetCoin()` does that, but it at least avoids problems of the subsequent approach tried where `GetCoin()` calls `PeekCoin` and would result in bugs when subclasses implement `GetCoin` forgetting to override `PeekCoin`. Hopefully #34124 can clean all of this by making relevant methods pure virtual.
Tree-SHA512: a81a98e60ca9e47454933ad879840cc226cb3b841bc36a4b746c34b350e07c546cdb5ddc55ec1ff66cf65d1ec503d22201d3dc12d4e82a8f4d386ccc52ba6441
ee2065fdea