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Merge bitcoin/bitcoin#29775 : Testnet4 including PoW difficulty adjustment fix

6bfa26048d testnet: Add timewarp attack prevention for Testnet4 (Fabian Jahr)
0100907ca1 testnet: Add Testnet4 difficulty adjustment rules fix (Fabian Jahr)
74a04f9e7a testnet: Introduce Testnet4 (Fabian Jahr)

Pull request description:

  To supplement the [ongoing conceptual discussion about a testnet reset](https://groups.google.com/g/bitcoindev/c/9bL00vRj7OU/m/9yCPo3uUBwAJ) I have drafted a move to v4 including a fix to the difficulty adjustment mechanism, which was part of the motivation that started the discussion.

  Conceptual considerations:
  - The conceptual discussion about doing a testnet4 or softforking the fix into testnet3 is outside of the scope of this PR and I would ask reviewers to contribute their opinions on this on the ML instead. However, I am happy to adapt this PR to a softfork change on testnet3 if there is consensus for that instead.
  - The difficulty adjustment fix suggested here touches the `CalculateNextWorkRequired` function and uses the same logic used in `GetNextWorkRequired` to find the last previous block that was not mined with difficulty 1 under the exceptionf. An alternative fix briefly mentioned on the mailing list by Jameson Lopp would be to "restrict the special testnet minimum difficulty rule so that it can't be triggered on the block right before a difficulty retarget". That would also fix the issue but I find my suggestion here a bit more elegant.

ACKs for top commit:
  jsarenik:
    tACK 6bfa26048d
  achow101:
    ACK 6bfa26048d
  murchandamus:
    tACK 6bfa26048d

Tree-SHA512: 0b8b69a621406a944da5be551b863d065358ba94d85dd3b80d83c412660e230ee93b27316081fbee9b4851cc4ff8585db64c7dfa26cb5148ac835663f2712c3d

2024-08-07 13:05:04 -04:00

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Bitcoin Core file system

Contents

Data directory location
Data directory layout
Multi-wallet environment
- Berkeley DB database based wallets
- SQLite database based wallets
GUI settings
Legacy subdirectories and files
Notes

Data directory location

The data directory is the default location where the Bitcoin Core files are stored.

The default data directory paths for supported platforms are:

Platform	Data directory path
Linux	`$HOME/.bitcoin/`
macOS	`$HOME/Library/Application Support/Bitcoin/`
Windows	`%LOCALAPPDATA%\Bitcoin\` ^[1]

A custom data directory path can be specified with the -datadir option.
All content of the data directory, except for bitcoin.conf file, is chain-specific. This means the actual data directory paths for non-mainnet cases differ:

Chain option	Data directory path
`-chain=main` (default)	path_to_datadir`/`
`-chain=test` or `-testnet`	path_to_datadir`/testnet3/`
`-chain=testnet4` or `-testnet4`	path_to_datadir`/testnet4/`
`-chain=signet` or `-signet`	path_to_datadir`/signet/`
`-chain=regtest` or `-regtest`	path_to_datadir`/regtest/`

Data directory layout

Subdirectory	File(s)	Description
`blocks/`		Blocks directory; can be specified by `-blocksdir` option (except for `blocks/index/`)
`blocks/index/`	LevelDB database	Block index; `-blocksdir` option does not affect this path
`blocks/`	`blkNNNNN.dat`^[2]	Actual Bitcoin blocks (dumped in network format, 128 MiB per file)
`blocks/`	`revNNNNN.dat`^[2]	Block undo data (custom format)
`blocks/`	`xor.dat`	Rolling XOR pattern for block and undo data files
`chainstate/`	LevelDB database	Blockchain state (a compact representation of all currently unspent transaction outputs (UTXOs) and metadata about the transactions they are from)
`indexes/txindex/`	LevelDB database	Transaction index; optional, used if `-txindex=1`
`indexes/blockfilter/basic/db/`	LevelDB database	Blockfilter index LevelDB database for the basic filtertype; optional, used if `-blockfilterindex=basic`
`indexes/blockfilter/basic/`	`fltrNNNNN.dat`^[2]	Blockfilter index filters for the basic filtertype; optional, used if `-blockfilterindex=basic`
`indexes/coinstats/db/`	LevelDB database	Coinstats index; optional, used if `-coinstatsindex=1`
`wallets/`		Contains wallets; can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, wallets reside in the data directory
`./`	`anchors.dat`	Anchor IP address database, created on shutdown and deleted at startup. Anchors are last known outgoing block-relay-only peers that are tried to re-connect to on startup
`./`	`banlist.json`	Stores the addresses/subnets of banned nodes.
`./`	`bitcoin.conf`	User-defined configuration settings for `bitcoind` or `bitcoin-qt`. File is not written to by the software and must be created manually. Path can be specified by `-conf` option
`./`	`bitcoind.pid`	Stores the process ID (PID) of `bitcoind` or `bitcoin-qt` while running; created at start and deleted on shutdown; can be specified by `-pid` option
`./`	`debug.log`	Contains debug information and general logging generated by `bitcoind` or `bitcoin-qt`; can be specified by `-debuglogfile` option
`./`	`fee_estimates.dat`	Stores statistics used to estimate minimum transaction fees required for confirmation
`./`	`guisettings.ini.bak`	Backup of former GUI settings after `-resetguisettings` option is used
`./`	`ip_asn.map`	IP addresses to Autonomous System Numbers (ASNs) mapping used for bucketing of the peers; path can be specified with the `-asmap` option
`./`	`mempool.dat`	Dump of the mempool's transactions
`./`	`onion_v3_private_key`	Cached Tor onion service private key for `-listenonion` option
`./`	`i2p_private_key`	Private key that corresponds to our I2P address. When `-i2psam=` is specified the contents of this file is used to identify ourselves for making outgoing connections to I2P peers and possibly accepting incoming ones. Automatically generated if it does not exist.
`./`	`peers.dat`	Peer IP address database (custom format)
`./`	`settings.json`	Read-write settings set through GUI or RPC interfaces, augmenting manual settings from bitcoin.conf. File is created automatically if read-write settings storage is not disabled with `-nosettings` option. Path can be specified with `-settings` option
`./`	`.cookie`	Session RPC authentication cookie; if used, created at start and deleted on shutdown; can be specified by `-rpccookiefile` option
`./`	`.lock`	Data directory lock file

Multi-wallet environment

Wallets are Berkeley DB (BDB) or SQLite databases.

Each user-defined wallet named "wallet_name" resides in the wallets/wallet_name/ subdirectory.
The default (unnamed) wallet resides in wallets/ subdirectory; if the latter does not exist, the wallet resides in the data directory.
A wallet database path can be specified with the -wallet option.
wallet.dat files must not be shared across different node instances, as that can result in key-reuse and double-spends due the lack of synchronization between instances.
Any copy or backup of the wallet should be done through a backupwallet call in order to update and lock the wallet, preventing any file corruption caused by updates during the copy.

Berkeley DB database based wallets

Subdirectory	File(s)	Description
`database/`	BDB logging files	Part of BDB environment; created at start and deleted on shutdown; a user must keep it as safe as personal wallet `wallet.dat`
`./`	`db.log`	BDB error file
`./`	`wallet.dat`	Personal wallet (a BDB database) with keys and transactions
`./`	`.walletlock`	BDB wallet lock file

SQLite database based wallets

Subdirectory	File	Description
`./`	`wallet.dat`	Personal wallet (a SQLite database) with keys and transactions
`./`	`wallet.dat-journal`	SQLite Rollback Journal file for `wallet.dat`. Usually created at start and deleted on shutdown. A user must keep it as safe as the `wallet.dat` file.

GUI settings

bitcoin-qt uses QSettings class; this implies platform-specific locations where application settings are stored.

Legacy subdirectories and files

These subdirectories and files are no longer used by Bitcoin Core:

Path	Description	Repository notes
`banlist.dat`	Stores the addresses/subnets of banned nodes; superseded by `banlist.json` in 22.0 and completely ignored in 23.0	PR #20966, PR #22570
`blktree/`	Blockchain index; replaced by `blocks/index/` in 0.8.0	PR #2231, `8fdc94cc`
`coins/`	Unspent transaction output database; replaced by `chainstate/` in 0.8.0	PR #2231, `8fdc94cc`
`blkindex.dat`	Blockchain index BDB database; replaced by {`chainstate/`, `blocks/index/`, `blocks/revNNNNN.dat`^[2]} in 0.8.0	PR #1677
`blk000?.dat`	Block data (custom format, 2 GiB per file); replaced by `blocks/blkNNNNN.dat`^[2] in 0.8.0	PR #1677
`addr.dat`	Peer IP address BDB database; replaced by `peers.dat` in 0.7.0	PR #1198, `928d3a01`
`onion_private_key`	Cached Tor onion service private key for `-listenonion` option. Was used for Tor v2 services; replaced by `onion_v3_private_key` in 0.21.0	PR #19954

Notes

1. The / (slash, U+002F) is used as the platform-independent path component separator in this document.

2. NNNNN matches [0-9]{5} regex.

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