btcd

netsync: add TestStartSyncBlockFallback for block-only sync path

Verify that startSync skips header download and directly requests
blocks when the header chain is already caught up to the peer's
height but the block chain lags behind.

netsync: add TestSyncStateMachine for end-to-end IBD sync flow

Tests the full IBD state machine: startSync → fetchHeaders →
handleHeadersMsg → fetchHeaderBlocks → handleBlockMsg → IBD complete.

Three sub-cases exercise different paths:
- fresh IBD from genesis
- node restart with partially synced blocks
- stalled sync peer recovery via handleStallSample

netsync: change startSync to be based off of processed headers

ince we now utilize ProcessBlockHeaders, we change the startSync
function to utilize the block index for downloading blocks/headers
instead of using the headerList in SyncManager.

netsync: update handleBlockMsg

handleBlockMsg is updated to not be based off of the headerList and
sm.nextCheckpoint when doing operations related to checkpoints and
headers.

netsync: rename headersFirstMode to ibdMode

ibdMode is a more fitting name than headersFirstMode since all blocks
are downloaded headers-first during the initial block download.

netsync: remove unused headerList and checkpoint code

Since we no longer utilize the headerList for doing headers-first
download and checkpoint tracking, we remove related code.

netsync: require peer argument in fetchHeaderBlocks

Refactor fetchHeaderBlocks and buildBlockRequest to take an explicit
peer parameter instead of implicitly using sm.syncPeer. This makes the
caller responsible for choosing which peer to fetch from and adds a nil
guard to prevent a panic if the sync peer has been cleared.

integration: add reorg regression test for fetchHeaderBlocks fork point

This tests that after two nodes diverge and reconnect, the shorter
node downloads blocks starting from the fork point rather than its
current height. It verifies the reorg completes, both nodes
converge, and the orphaned fork appears as a side chain via
getchaintips.

netsync: change fetchHeaderBlocks to be based on the processed block
headers

We introduce buildBlockRequest that'll create a getdata message based
off of the block index instead of the headerList in SyncManager.

The new fetchHeaderBlocks utilizes buildBlockRequest and now will create
fetch requests based on the processed block headers.

netsync: add fetchHeaders

fetchHeaders picks a random peer at a higher advertised block and
requests headers from them.

netsync: use ProcessBlockHeader in handleBlockHeader

Instead of the old headerList based header processing, we make use of
the new ProcessBlockHeader function.

blockchain: add BestChainHeaderForkHeight to return the fork point
between the best chain and the best header chain

netsync: add isInIBDMode

isInIBDMode returns if the SyncManager needs to download blocks and sync
to the latest chain tip.

It determines if it's in ibd mode by checking if the blockchain thinks
we're current and if we don't have peers that are at higher advertised
blocks.

netsync: add fetchHigherPeers

fetchHigherPeers provides a convenient function to get peers that are
sync candidates and are at a higher advertised height than the passed in
height.

netsync: add checkHeadersList

checkHeadersList takes in a blockhash and returns if it's a checkpointed
block and the correct behavior flags for the verification of the block.

blockchain: change HaveBlock to also check for block data availability

In block processing and block downloading, HaveBlock is used to check if
the block data already exists. It was ok to just check for the
existence of the blockNode but we now we also need to check if the data
exists as the blockNode may be present for just the block header.

blockchain: reuse existing header node in maybeAcceptBlock

maybeAcceptBlock unconditionally created a new blockNode, overwriting the
index entry. If maybeAcceptBlockHeader had already processed the header,
the pointer held by bestHeader's chainView became orphaned, breaking
bestHeader.Contains and downstream checks like IsValidHeader.

Check for an existing node first and upgrade its status to
statusDataStored rather than replacing it.

blockchain: add exports methods based on block header tip

The added exported methods on BlockChain provide access to the block
header tip like fetching block hashes and heights from the headers
chain.

blockchain: don't flush blockNodes that we don't have the data for

On flushes to the database, we check that the blockNodes we have for
the downloaded block headers are not flushed to the disk unless the
block data is stored as well for backwards compatibility.

With older btcd clients, they rely on the fact that the blockNode is
present to check if the block data is also present. Since we now
store blockNodes for just the block headers, this is no longer true.

Because of this, we don't flush the blockNodes if there's no
accompanying block data for it. This results in downloading and
verifying the headers again if the node were to restart but since the
header data is small and the verification is quick, it's not a big
downside.

As an optimization, flushToDB now skips opening a write transaction
entirely when every dirty node is header-only. This avoids a no-op
write transaction on every ProcessBlockHeader call during header sync.

blockchain: add ProcessBlockHeader

ProcessBlockHeader performs chain selection and context-free &
contextual validation for the given block header. The function allows
a header-first downloading of blocks even without checkpoints.

blockchain: add ErrKnownInvalidBlock error code

Distinguish between a block that has itself failed validation
versus one that is only invalid due to an ancestor.

blockchain: add statusHeaderStored for blockNode status

Since we may now have blockNodes with just the block header stored
without the data, we add a new status to account for this.

blockchain: add maybeAcceptHeader

maybeAcceptHeader performs checks to accept block headers into the
header chain. This function allows for a true headers-first download
where we only accept block headers for downloading new blocks.

blockchain: add bestHeader to BlockChain

We add a chainview of bestHeaders so that we'll be able to keep track of
headers separately from the bestChain. This is needed as we're getting
headers for new block annoucements instead of invs.

server: address review feedback on peer lifecycle handling

Prioritize verAckCh in peerLifecycleHandler select to avoid
nondeterministic peerAdd skipping when both channels are ready.

Guard OnVerAck against double-close by checking the channel before
closing, logging an error instead of panicking.

Adjust peerLifecycleEvent comment to reflect that peerAdd may be
skipped when the peer disconnects before or concurrently with verack.

Fix verAckCh field comment formatting.