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doc: add comments clarifying how local services are advertised

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Recent questions have come up regarding dynamic service registration
(see https://github.com/bitcoin/bitcoin/pull/16442#discussion_r308702676
and the assumeutxo project, which needs to dynamically flip NODE_NETWORK).

While investigating how dynamic service registration might work, I was
confused about how we convey local services to peers. This adds some
documentation that hopefully clarifies this process.
This commit is contained in:
James O'Beirne 2019-09-10 13:09:12 -04:00
parent 1985c4efda
commit 82e53f37e1
2 changed files with 38 additions and 2 deletions
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37
src/net.h
View file

@ -282,6 +282,12 @@ public:
bool DisconnectNode(const CNetAddr& addr);
bool DisconnectNode(NodeId id);
//! Used to convey which local services we are offering peers during node
//! connection.
//!
//! The data returned by this is used in CNode construction,
//! which is used to advertise which services we are offering
//! that peer during `net_processing.cpp:PushNodeVersion()`.
ServiceFlags GetLocalServices() const;
//!set the max outbound target in bytes
@ -413,7 +419,18 @@ private:
std::atomic<NodeId> nLastNodeId{0};
unsigned int nPrevNodeCount{0};
/** Services this instance offers */
/**
* Services this instance offers.
*
* This data is replicated in each CNode instance we create during peer
* connection (in ConnectNode()) under a member also called
* nLocalServices.
*
* This data is not marked const, but after being set it should not
* change. See the note in CNode::nLocalServices documentation.
*
* \sa CNode::nLocalServices
*/
ServiceFlags nLocalServices;
std::unique_ptr<CSemaphore> semOutbound;
@ -786,8 +803,24 @@ public:
private:
const NodeId id;
const uint64_t nLocalHostNonce;
// Services offered to this peer
//! Services offered to this peer.
//!
//! This is supplied by the parent CConnman during peer connection
//! (CConnman::ConnectNode()) from its attribute of the same name.
//!
//! This is const because there is no protocol defined for renegotiating
//! services initially offered to a peer. The set of local services we
//! offer should not change after initialization.
//!
//! An interesting example of this is NODE_NETWORK and initial block
//! download: a node which starts up from scratch doesn't have any blocks
//! to serve, but still advertises NODE_NETWORK because it will eventually
//! fulfill this role after IBD completes. P2P code is written in such a
//! way that it can gracefully handle peers who don't make good on their
//! service advertisements.
const ServiceFlags nLocalServices;
const int nMyStartingHeight;
int nSendVersion{0};
NetPermissionFlags m_permissionFlags{ PF_NONE };

3
src/net_processing.cpp
View file

@ -415,6 +415,9 @@ static void UpdatePreferredDownload(CNode* node, CNodeState* state) EXCLUSIVE_LO
static void PushNodeVersion(CNode *pnode, CConnman* connman, int64_t nTime)
{
// Note that pnode->GetLocalServices() is a reflection of the local
// services we were offering when the CNode object was created for this
// peer.
ServiceFlags nLocalNodeServices = pnode->GetLocalServices();
uint64_t nonce = pnode->GetLocalNonce();
int nNodeStartingHeight = pnode->GetMyStartingHeight();