4feaa28728 refactor: Rely on returned value of GetCoin instead of parameter (Lőrinc)
46dfbf169b refactor: Return optional of Coin in GetCoin (Lőrinc)
e31bfb26c2 refactor: Remove unrealistic simulation state (Lőrinc)
Pull request description:
While reviewing [the removal of the unreachable combinations from the Coin cache logic](https://github.com/bitcoin/bitcoin/pull/30673#discussion_r1721727681), we've noticed that the related tests often [reflect impossible states](https://github.com/bitcoin/bitcoin/pull/30673/files#r1740154464).
Browsing the Coin cache refactoring history revealed that migrating `bool GetCoin` to `optional<Coin> GetCoin` was [already proposed a few times before](https://github.com/bitcoin/bitcoin/pull/18746#issuecomment-842393167).
This refactor makes certain invalid states impossible, reducing the possibility of errors and making the code easier to understand. This will let us remove test code that exercises the impossible states as well.
The PR is done in multiple small steps, first swapping the new `optional` return value, slowly strangling out the usages of the return parameter, followed by the removal of the parameter.
Most of the invalid test states were still kept, except for https://github.com/bitcoin/bitcoin/pull/30673/files#r1748087322, where the new design prohibits invalid usage and https://github.com/bitcoin/bitcoin/pull/30673/files#r1749350258 was just marked with a TODO, will be removed in a follow-up PR.
ACKs for top commit:
andrewtoth:
re-ACK 4feaa28728
achow101:
ACK 4feaa28728
laanwj:
Code review ACK 4feaa28728
theStack:
Code-review ACK 4feaa28728
Tree-SHA512: 818d60b2e97f58c489a61120fe761fb67a08dffbefe7a3fce712d362fc9eb8c2cced23074f1bec55fe71c616a3561b5a8737919ad6ffb2635467ec4711683df7
31cc5006c3 init: Return fatal failure on snapshot validation failure (Martin Zumsande)
8f1246e833 init: Improve chainstate init db error messages (TheCharlatan)
cd093049dd init: Remove incorrect comment about shutdown condition (MarcoFalke)
635e9f85d7 init: Remove misleading log line when user chooses not to retry (TheCharlatan)
720ce880a3 init: Improve comment describing chainstate load retry behaviour (Martin Zumsande)
baea842ff1 init: Remove unneeded argument for mempool_opts checks (stickies-v)
Pull request description:
These are mostly followups from #30968, making the code, log lines, error messages, and comments more consistent.
The last commit is an attempt at improving the error reporting when loading the chainstate. It aims to more cleanly distinguish between errors arising from a specific database, and errors where the culprit may be less clear.
ACKs for top commit:
achow101:
ACK 31cc5006c3
mzumsande:
Code Review / lightly tested ACK 31cc5006c3
BrandonOdiwuor:
Code Review ACK 31cc5006c3.
stickies-v:
ACK 31cc5006c3
Tree-SHA512: 59fba4845ee45a3d91bf55807ae6b1c81458463b96bf664c8b1badfac503f6b01efd52a915fc399294e68a3f69985362a5a10a3844fa23f7707145ebe9ad349b
525e9dcba0 Add submitSolution to BlockTemplate interface (Sjors Provoost)
47b4875ef0 Add getCoinbaseMerklePath() to Mining interface (Sjors Provoost)
63d6ad7c89 Move BlockMerkleBranch back to merkle.{h,cpp} (Sjors Provoost)
Pull request description:
The new `BlockTemplate` interface introduced in #30440 allows for a more efficient way for a miner to submit the block solution. Instead of having the send the full block, it only needs to provide the nonce, timestamp, version fields and coinbase transaction.
This PR introduces `submitSolution()` for that. It's currently unused.
#29432 and https://github.com/Sjors/bitcoin/pull/48 use it to process the Stratum v2 message [SubmitSolution](https://github.com/stratum-mining/sv2-spec/blob/main/07-Template-Distribution-Protocol.md#77-submitsolution-client---server). The method should be sufficiently generic to work with alternative mining protocols (none exist that I'm aware off).
This PR also introduces `getCoinbaseMerklePath()`, which is needed in Stratum v2 to construct the `merkle_path` field of the `NewTemplate` message (see [spec](https://github.com/stratum-mining/sv2-spec/blob/main/07-Template-Distribution-Protocol.md#72-newtemplate-server---client)). The coinbase merkle path is also used in Stratum "v1", see e.g. https://bitcoin.stackexchange.com/questions/109820/questions-on-merkle-root-hashing-for-stratum-pools
This last function uses `BlockMerkleBranch` which was moved to the test code in #13191. The reason back then for moving it was that it was no longer used. This PR moves it back.
This PR does not change behaviour since both methods are unused.
ACKs for top commit:
achow101:
ACK 525e9dcba0
itornaza:
Code review ACK 525e9dcba0
tdb3:
Code review and light test ACK 525e9dcba0
ryanofsky:
Code review ACK 525e9dcba0. Left minor suggestions but none are important, and looks like this could be merged as-is
Tree-SHA512: 2a6a8f5d409ff4926643193cb67702240c7c687615414371e53383d2c13c485807f65e21e8ed98515b5456eca3d9fca13cec04675814a4081467d88b849c5653
Instead of having a single NodeContext::shutdown member that is used both to
request shutdowns and check if they have been requested, use separate members
for each. Benefits of this change:
1. Should make code a little clearer and easier to search because it is easier
to see which parts of code are triggering shutdowns and which parts are just
checking to see if they were triggered.
2. Makes it possible for init.cpp to specify additional code to run when a
shutdown is requested, like signalling the m_tip_block_cv condition variable.
Motivation for this change was to remove hacky NodeContext argument and
m_tip_block_cv access from the StopRPC function, so StopRPC can just be
concerned with RPC functionality, not other node functionality.
This is not strictly required, but all places using m_tip_block_cv
(except shutdown) already take the lock. The annotation makes it easier
to catch potential deadlocks before review.
Adding the missing lock to the shutdown sequence is a bugfix.
An alternative would be to take the lock and release it before
notifying, see
https://github.com/bitcoin/bitcoin/pull/30967#discussion_r1778899716
5c7cacf649 ci: Remove natpmp build option and libnatpmp dependency (laanwj)
7e7ec984da doc: Remove mention of natpmp build options (laanwj)
061c3e32a2 depends: Drop natpmp and associated option from depends (laanwj)
20a18bf6aa build: Drop libnatpmp from build system (laanwj)
7b04709862 qt: Changes for built-in PCP+NAT-PMP (laanwj)
52f8ef66c6 net: Replace libnatpmp with built-in NATPMP+PCP implementation in mapport (laanwj)
97c97177cd net: Add PCP and NATPMP implementation (laanwj)
d72df63d16 net: Use GetLocalAddresses in Discover (laanwj)
e02030432b net: Add netif utility (laanwj)
754e425438 crypto: Add missing WriteBE16 function (laanwj)
Pull request description:
Continues #30005. Closes #17012..
This PR adds PCP (Port Control Protocol) from [RFC6887](https://datatracker.ietf.org/doc/html/rfc6887). This adds, in addition to the existing IPv4 port mapping (which now uses PCP, with fallback to NAT-PMP), support for IPv6 pinholing-that is, opening a port on the firewall to make it reachable.
PCP, like NAT-PMP is a simple UDP-based protocol, and the implementation is self-contained, so this gets rid of lthe libnatpnp dependency without adding a new one. It should otherwise be a drop-in replacement. NAT-PMP fallback is implemented so this will not make router support worse.
For now it is disabled by default, though in the future (not in this PR) we could consider enable it by default to increase the number of connectable nodes without adding significant attack surface.
To test:
```bash
bitcoind -regtest -natpmp=1 -debug=net
```
(most of the changes in this PR are, ironically, removing the libnatpmp dependency and associated build system and build docs)
## TODO
- [x] Default gateway discovery on Linux / FreeBSD
- [x] Default gateway discovery on Windows
- [x] Default gateway discovery on MacOS
- [x] Either solve FreeBSD compile issue (probably upstream issue) or remove FreeBSD support
## Things to consider for follow-up PRs
- https://github.com/bitcoin/bitcoin/pull/30043#discussion_r1658764974 avoid unreachable nets (not given to -onlynet=)
- https://github.com/bitcoin/bitcoin/pull/30043#discussion_r1658949236 could announce an addr:port where we do not listen (no -bind)
- https://github.com/bitcoin/bitcoin/pull/30043#discussion_r1684368824 could announce the wrong port because it uses GetListenPort()
- https://github.com/bitcoin/bitcoin/pull/30043#discussion_r1679709347 if we requested one port but another was assigned, then which one to use in the renewal?
- https://github.com/bitcoin/bitcoin/pull/30043#discussion_r1772017020 Use `GetAdapterAddresses` to discover local addresses for Windows
ACKs for top commit:
Sjors:
ACK 5c7cacf649
achow101:
ACK 5c7cacf649
vasild:
ACK 5c7cacf649
Tree-SHA512: e35b69e56d5f5449a3d48a799f9b7b65107c65eeb3e245c2c1e9d42221e469ca5ead90afae423160601cd664dd553a51c859e04f4492f335b064aae3bf23e3bc
7942951e3f Remove unused g_best_block (Ryan Ofsky)
e3a560ca68 rpc: use waitTipChanged for longpoll (Ryan Ofsky)
460687a09c Remove unused CRPCSignals (Sjors Provoost)
dca923150e Replace RPCNotifyBlockChange with waitTipChanged() (Sjors Provoost)
2a40ee1121 rpc: check for negative timeout arg in waitfor* (Sjors Provoost)
de7c855b3a rpc: recommend -rpcclienttimeout=0 for waitfor* (Sjors Provoost)
77ec072925 rpc: fix waitfornewblock description (Sjors Provoost)
285fe9fb51 rpc: add test for waitforblock and waitfornewblock (Sjors Provoost)
b94b27cf05 Add waitTipChanged to Mining interface (Sjors Provoost)
7eccdaf160 node: Track last block that received a blockTip notification (Sjors Provoost)
ebb8215f23 Rename getTipHash() to getTip() and return BlockRef (Sjors Provoost)
89a8f74bbb refactor: rename BlockKey to BlockRef (Sjors Provoost)
Pull request description:
This continues the work in #30200 so that a future Stratum v2 Template Provider (see #29432) can avoid accessing node internals. It needs to know when a new block arrives in order to push new templates to connected clients.
`waitTipChanged()` uses a new kernel notification `notifications().m_tip_block_mutex`, which this PR also introduces (a previous version used `g_best_block`).
In order to ensure the new method works as intended, the `waitfornewblock`, `waitforblock` and `waitforblockheight` RPC methods are refactored to use it. This allows removing `RPCNotifyBlockChange`.
There's a commit to add (direct) tests for the methods that are about to be refactored:
- `waitfornewblock` was already implicitly tested by `feature_shutdown.py`.
- `waitforblockheight` by `feature_coinstatsindex.py` and `example_test.py`
This PR renames `getTipHash()` to `getTip()` and returns a `BlockRef` (renamed from `BlockKey`) so that callers can use either the height or hash.
The later commits make trivial improvements to the `waitfor*` RPC calls (not needed for this PR).
The `waitTipChanged()` method could probably also be used for the longpoll functionality in `getblocktemplate`, but I'm a bit reluctant to touch that.
`RPCServer::OnStarted` no longer does anything and `RPCServer::OnStopped` merely prints a log statement. They were added in #5711 as a refactor. This PR drops them entirely.
Finally `g_best_block` is also dropped.
ACKs for top commit:
achow101:
ACK 7942951e3f
ryanofsky:
Code review ACK 7942951e3f. Just rebased since last review
TheCharlatan:
Re-ACK 7942951e3f
Tree-SHA512: a5559446b4000c95e07aad33284b7ee2e57aafd87e1ae778b3825d59689566d047a8047e47a10f76e6e341e7dc72fd265a65afbc0a9c011d17c4cafd55031837
8466329127 chain: simplify `deleteRwSettings` code and improve it's doc (ismaelsadeeq)
f8d91f49c7 chain: dont check for null settings value in `overwriteRwSetting` (ismaelsadeeq)
df601993f2 chain: ensure `updateRwSetting` doesn't update to a null settings (ismaelsadeeq)
c8e2eeeffb chain: uniformly use `SettingsAction` enum in settings methods (ismaelsadeeq)
1e9e735670 chain: move new settings safely in `overwriteRwSetting` (ismaelsadeeq)
1c409004c8 test: remove wallet context from `write_wallet_settings_concurrently` (ismaelsadeeq)
Pull request description:
This PR addresses the remaining review comments from #30697
1. Disallowed overwriting settings values with a `null` value.
2. Uniformly used the `SettingsAction` enum in all settings methods instead of a boolean parameter.
3. Updated `overwriteRwSetting` to receive the `common::SettingsValue` parameter by value, enabling it to be moved safely.
4. Removed wallet context from the `write_wallet_settings_concurrently` unit test, as it is not needed.
ACKs for top commit:
achow101:
ACK 8466329127
ryanofsky:
Code review ACK 8466329127. Looks good, thanks for taking suggestions and applying them to the right commits. Only changes since last review were documentation improvements and simplifying delete method.
furszy:
Code review ACK 8466329127
Tree-SHA512: baf2f59ed5aac4a4bda0c84fb6554a466a40d1f7b52b61dc2ff293d83ae60e82b925b7003237b633fecb65eba3a4c108e69166046895d1295809fbe0de67b052
Also signal m_tip_block_cv when StopRPC is called, for
consistency with g_best_block_cv. This is handled in
StopRPC instead of OnRPCStopped() because the latter
is deleted in a later commit.
Co-authored-by: TheCharlatan <seb.kung@gmail.com>
Co-authored-by: Ryan Ofsky <ryan@ofsky.org>
a240e150e8 streams: remove AutoFile::Get() entirely (Pieter Wuille)
e624a9bef1 streams: cache file position within AutoFile (Pieter Wuille)
Pull request description:
Fixes #30833.
Instead of relying on frequent `ftell` calls (which appear to cause a significant slowdown on some systems) in XOR-enabled `AutoFile`s, cache the file position within `AutoFile` itself.
ACKs for top commit:
achow101:
ACK a240e150e8
davidgumberg:
untested reACK a240e150e8
theStack:
Code-review ACK a240e150e8
Tree-SHA512: fd3681edc018afaf955dc7a41a0c953ca80d46c1129e3c5b306c87c95aae93b2fe7b900794eb8b6f10491f9211645e7939918a28838295e6873eb226fca7006f
bb3b980dfd validation: drop maximum -dbcache (Sjors Provoost)
Pull request description:
Due to recent UTXO set growth, the current maximum value for `-dbcache` of 16GB is ~just months away from being~ insufficient (for those who wish to complete IBD with the UTXO set held in RAM).
This drops the limit. It also adds a warning that it's up to users to check that they have enough RAM.
Fixes #28249.
---
A previous version of this PR increased the maximum to 64GB. It also made startup abort if the value provided is too high, rather than quietly round it down. But this didn't get much support.
ACKs for top commit:
achow101:
ACK bb3b980dfd
tdb3:
ACK bb3b980dfd
BenWestgate:
crACK bb3b980dfd.
Tree-SHA512: 8515fff468c2387a0b04bd9523ab1df46d6325738588b7550fabddbb8624817a583d95b95ea246407f9f0ff3e43e760cf7334621bec6af79710176328528a3ef
a93c171faa Drop unneeded nullptr check from CreateNewBlock() (Sjors Provoost)
dd87b6dff3 Have createNewBlock return BlockTemplate interface (Sjors Provoost)
Pull request description:
Suggested in https://github.com/bitcoin/bitcoin/pull/29432#issuecomment-2225337100
An external program that uses the Mining interface may need quick access to some information in the block template, while it can wait a bit longer for the full raw transaction data.
This would be the case for a Stratum v2 Template Provider which needs to send a [NewTemplate](https://github.com/stratum-mining/sv2-spec/blob/main/07-Template-Distribution-Protocol.md#72-newtemplate-server---client) message message (which doesn't include transactions) as quickly as possible. It does not include the serialized block transactions.
ACKs for top commit:
achow101:
ACK a93c171faa
ryanofsky:
Code review ACK a93c171faa. Since last review, just rebased with no changes or conflicts
itornaza:
Code review ACK a93c171faa
TheCharlatan:
Re-ACK a93c171faa
Tree-SHA512: 17cb61eb5548e9d4a69e34dd732f68a06cde2ad3d82c8339efee704c7860d5de144d93b23d6ecd6ee4ec205844e5560ad0f6d3917822fa75bb8e640c5f51af9a
The thread handle is never used by the ChainstateManager, so move it out
and into the node context. Users of the kernel library now no longer
have to manually join the thread when destructing the ChainstateManager.
An external program that uses the Mining interface may need quick access to some information in the block template, while it can wait a bit longer for the full raw transaction data.
This would be the case for a Stratum v2 Template Provider which needs to send a NewTemplate message (which doesn't include transactions) as quickly as possible.
b31a0cd037 log: expand BCLog::LogFlags (categories) to 64 bits (Larry Ruane)
Pull request description:
Increase the maximum number of logging categories from 32 to 64.
We're currently using 29 of the 32 available logging categories (there are only 3 remaining). It would be good to increase the limit soon; the fourth PR to be merged that adds a new logging category will be blocked until something like this is done.
This PR also adds a `TEST` category that uses the new range (`1ULL << 63`) in case there's a hidden assumption somewhere that the `BCLog::LogFlags` type is 32 bits. (Also added a test for this test category.) It also provides an example showing that the expression must be `1ULL << <shift>` for shift value 31 and beyond.
ACKs for top commit:
achow101:
ACK b31a0cd037
vasild:
ACK b31a0cd037
ryanofsky:
Code review ACK b31a0cd037, just dropping mask_bit constant since last review. I still think
theStack:
Code-review ACK b31a0cd037
Tree-SHA512: de422dbeb479848d370aed42d415f42461457ab0eda62b245dc7ff9f0e111626e7d4c0d62ff13082ec664d05fbb0db04c71eb4b6f22eb8f19198826a67c4035e
a2955f0979 validation: Use span for ImportBlocks paths (TheCharlatan)
20515ea3f5 validation: Use span for CalculateClaimedHeadersWork (TheCharlatan)
52575e96e7 validation: Use span for ProcessNewBlockHeaders (TheCharlatan)
Pull request description:
Makes it friendlier for potential future users of the kernel library if they do not store the headers in a std::vector, but can guarantee contiguous memory.
Take this opportunity to also change the argument of ImportBlocks previously taking a `std::vector` to a `std::span`.
ACKs for top commit:
stickies-v:
re-ACK a2955f0979 - no changes except further walking the ~file~ path of modernizing variable names.
maflcko:
ACK a2955f0979🕑
achow101:
ACK a2955f0979
danielabrozzoni:
ACK a2955f0979
Tree-SHA512: 8b07f4ad26e270b65600d1968cd78847b85caca5bfbb83fd9860389f26656b1d9a40b85e0990339f50403d18cedcd2456990054f3b8b0bedce943e50222d2709
Makes it friendlier for potential future users of the kernel library if
they do not store the headers in a std::vector, but can guarantee
contiguous memory.
18d65d2772 test: use uint256::FromUserHex for RANDOM_CTX_SEED (stickies-v)
6819e5a329 node: use uint256::FromUserHex for -assumevalid parsing (stickies-v)
2e58fdb544 util: remove unused IsHexNumber (stickies-v)
8a44d7d3c1 node: use uint256::FromUserHex for -minimumchainwork parsing (stickies-v)
70e2c87737 refactor: add uint256::FromUserHex helper (stickies-v)
85b7cbfcbe test: unittest chainstatemanager_args (stickies-v)
Pull request description:
Since fad2991ba0, `uint256S` has been [deprecated](fad2991ba0 (diff-800776e2dda39116e889839f69409571a5d397de048a141da7e4003bc099e3e2R138)) because it is less robust than the `base_blob::FromHex()` introduced in [the same PR](https://github.com/bitcoin/bitcoin/pull/30482). Specifically, it tries to recover from length-mismatches, recover from untrimmed whitespace, 0x-prefix and garbage at the end, instead of simply requiring exactly 64 hex-only characters. _(see also #30532)_
This PR carves out the few `uint256S` callsites that may potentially prove a bit more controversial to change because they deal with user input and backwards incompatible behaviour change.
The main behaviour change introduced in this PR is:
- `-minimumchainwork` will raise an error when input is longer than 64 hex digits
- `-assumevalid` will raise an error when input contains invalid hex characters, or when it is longer than 64 hex digits
- test: the optional RANDOM_CTX_SEED env var will now cause tests to abort when it contains invalid hex characters, or when it is longer than 64 hex digits
After this PR, the remaining work to remove `uint256S` completely is almost entirely mechanical and/or test related. I will open that PR once #30560 is merged because it builds on that.
ACKs for top commit:
hodlinator:
re-ACK 18d65d2772
l0rinc:
ACK 18d65d2772
achow101:
ACK 18d65d2772
ryanofsky:
Code review ACK 18d65d2772. Very nice change that cleans up the API, adds checking for invalid values, makes parsing of values more consistent, and adds test coverage.
Tree-SHA512: ec118ea3d56e1dfbc4c79acdbfc797f65c4d2107b0ca9577c848b4ab9b7cb8d05db9f3c7fe8441a09291aca41bf671572431f4eddc59be8fb53abbea76bbbf86
1b41d45d46 wallet: bugfix: ensure atomicity in settings updates (ismaelsadeeq)
Pull request description:
This PR fixes #30620.
As outlined in the issue, creating two wallets with `load_on_startup=true` simultaneously results in only one wallet being added to the startup file.
The current issue arises because the wallet settings update process involves:
1. Obtaining the settings value while acquiring the settings lock.
2. Modifying the settings value.
3. Overwriting the settings value while acquiring the settings lock again.
This sequence is not thread-safe. Different threads could modify the same base value simultaneously, overwriting data from other workers without realizing it.
The PR attempts to fix this by modifying the chain interface's `updateRwSetting` method to accept a function that will be called with the settings reference. This function will either update or delete the setting and return an enum indicating whether the settings need to be overwritten in this or not.
Additionally, this PR introduces two new methods to the chain interface:
- `overwriteRwSetting`: This method replaces the setting with a new value.
Used in `VerifyWallets`
- `deleteRwSettings`: This method completely erases a specified setting.
This method is currently used only in `overwriteRwSetting`.
These changes ensure that updates are race-free across all clients.
ACKs for top commit:
achow101:
ACK 1b41d45d46
furszy:
self-code-ACK 1b41d45d46
Tree-SHA512: 50cda612b782aeb5e03e2cf63cc44779a013de1c535b883b57af4de22f24b0de80b4edecbcda235413baec0a12bdf0e5750fb6731c9e67d32e742d8c63f08c13
- Settings updates were not thread-safe, as they were executed in
three separate steps:
1) Obtain settings value while acquiring the settings lock.
2) Modify settings value.
3) Overwrite settings value while acquiring the settings lock.
This approach allowed concurrent threads to modify the same base value
simultaneously, leading to data loss. When this occurred, the final
settings state would only reflect the changes from the last thread
that completed the operation, overwriting updates from other threads.
Fix this by making the settings update operation atomic.
- Add test coverage for this behavior.
Co-authored-by: furszy <matiasfurszyfer@protonmail.com>
Removes dependency on unsafe and deprecated uint256S.
This makes parsing more strict, by returning an error
when the input contains non-hex characters, or when it
contains more than 64 hex digits.
Also make feature_assumevalid.py more robust by using CBlock.hash
which is guaranteed to be 64 characters long, as opposed to the
variable-length hex(CBlock.sha256)
Removes dependency on unsafe and deprecated uint256S.
This makes parsing more strict, by returning an error
when the input contains more than 64 hex digits.
00618e8745 assumeutxo: Drop block height from metadata (Fabian Jahr)
Pull request description:
Fixes https://github.com/bitcoin/bitcoin/issues/30514 which has more context and shows how the issue can be reproduced. Since the value in question is removed, there is no test to add to reproduce anything.
This is an alternative approach to #30516 with much of the [code being suggested there](https://github.com/bitcoin/bitcoin/pull/30516#discussion_r1689146902).
ACKs for top commit:
maflcko:
re-ACK 00618e8745🎌
achow101:
ACK 00618e8745
theStack:
Code-review ACK 00618e8745
ismaelsadeeq:
Re-ACK 00618e8745
mzumsande:
ACK 00618e8745
Tree-SHA512: db9575247bae838ad7742a27a216faaf55bb11e022f9afdd05752bb09bbf9614717d0ad64304ff5722a16bf41d8dea888af544e4ae26dcaa528c1add0269a4a8
2925bd537c refactor: use c++20 std::views::reverse instead of reverse_iterator.h (stickies-v)
Pull request description:
C++20 introduces [`std::ranges::views::reverse`](https://en.cppreference.com/w/cpp/ranges/reverse_view), which allows us to drop our own `reverse_iterator.h` implementation and also makes it easier to chain views (even though I think we currently don't use this).
ACKs for top commit:
achow101:
ACK 2925bd537c
maflcko:
ACK 2925bd537c🎷
Tree-SHA512: 567666ec44af5d1beb7a271836bcc89c4c577abc77f522fcc18bc6d4de516ae9b0df766d0bfa6dd217569e6878331c2aee1d9815620860375e3510dad7fed476
