peer: Refactor peer code into its own package.

This commit introduces package peer which contains peer related features
refactored from peer.go.

The following is an overview of the features the package provides:

- Provides a basic concurrent safe bitcoin peer for handling bitcoin
  communications via the peer-to-peer protocol
- Full duplex reading and writing of bitcoin protocol messages
- Automatic handling of the initial handshake process including protocol
  version negotiation
- Automatic periodic keep-alive pinging and pong responses
- Asynchronous message queueing of outbound messages with optional
  channel for notification when the message is actually sent
- Inventory message batching and send trickling with known inventory
  detection and avoidance
- Ability to wait for shutdown/disconnect
- Flexible peer configuration
  - Caller is responsible for creating outgoing connections and listening
    for incoming connections so they have flexibility to establish
    connections as they see fit (proxies, etc.)
  - User agent name and version
  - Bitcoin network
  - Service support signalling (full nodes, bloom filters, etc.)
  - Maximum supported protocol version
  - Ability to register callbacks for handling bitcoin protocol messages
- Proper handling of bloom filter related commands when the caller does
  not specify the related flag to signal support
  - Disconnects the peer when the protocol version is high enough
  - Does not invoke the related callbacks for older protocol versions
- Snapshottable peer statistics such as the total number of bytes read
  and written, the remote address, user agent, and negotiated protocol
  version
- Helper functions for pushing addresses, getblocks, getheaders, and
  reject messages
  - These could all be sent manually via the standard message output
    function, but the helpers provide additional nice functionality such
    as duplicate filtering and address randomization
- Full documentation with example usage
- Test coverage

In addition to the addition of the new package, btcd has been refactored
to make use of the new package by extending the basic peer it provides to
work with the blockmanager and server to act as a full node.  The
following is a broad overview of the changes to integrate the package:

- The server is responsible for all connection management including
  persistent peers and banning
- Callbacks for all messages that are required to implement a full node
  are registered
- Logic necessary to serve data and behave as a full node is now in the
  callback registered with the peer

Finally, the following peer-related things have been improved as a part
of this refactor:

- Don't log or send reject message due to peer disconnects
- Remove trace logs that aren't particularly helpful
- Finish an old TODO to switch the queue WaitGroup over to a channel
- Improve various comments and fix some code consistency cases
- Improve a few logging bits
- Implement a most-recently-used nonce tracking for detecting self
  connections and generate a unique nonce for each peer

config.go

@@ -62,6 +62,15 @@ var (
 // to parse and execute service commands specified via the -s flag.
 var runServiceCommand func(string) error
 
+// minUint32 is a helper function to return the minimum of two uint32s.
+// This avoids a math import and the need to cast to floats.
+func minUint32(a, b uint32) uint32 {
+	if a < b {
+		return a
+	}
+	return b
+}
+
 // config defines the configuration options for btcd.
 //
 // See loadConfig for details on the configuration load process.

docs/README.md

@@ -203,6 +203,8 @@ information.
 	  for the underlying JSON-RPC command and return values
     * [wire](https://github.com/btcsuite/btcd/tree/master/wire) - Implements the
 	  Bitcoin wire protocol
+    * [peer](https://github.com/btcsuite/btcd/tree/master/peer) -
+	  Provides a common base for creating and managing Bitcoin network peers.
     * [blockchain](https://github.com/btcsuite/btcd/tree/master/blockchain) -
 	  Implements Bitcoin block handling and chain selection rules
     * [txscript](https://github.com/btcsuite/btcd/tree/master/txscript) -

log.go

@@ -11,9 +11,9 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/addrmgr"
-
 	"github.com/btcsuite/btcd/blockchain"
 	"github.com/btcsuite/btcd/database"
+	"github.com/btcsuite/btcd/peer"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btclog"
@@ -118,6 +118,7 @@ func useLogger(subsystemID string, logger btclog.Logger) {
 
 	case "PEER":
 		peerLog = logger
+		peer.UseLogger(logger)
 
 	case "RPCS":
 		rpcsLog = logger