Merge bitcoin#20685: Add I2P support using I2P SAM

a701fcf net: Do not skip the I2P network from GetNetworkNames() (Vasil Dimov)
0181e24 net: recognize I2P from ParseNetwork() so that -onlynet=i2p works (Vasil Dimov)
b905363 net: accept incoming I2P connections from CConnman (Vasil Dimov)
0635233 net: make outgoing I2P connections from CConnman (Vasil Dimov)
9559bd1 net: add I2P to the reachability map (Vasil Dimov)
76c35c6 init: introduce I2P connectivity options (Vasil Dimov)
c22daa2 net: implement the necessary parts of the I2P SAM protocol (Vasil Dimov)
5bac7e4 net: extend Sock with a method to check whether connected (Vasil Dimov)
42c779f net: extend Sock with methods for robust send & read until terminator (Vasil Dimov)
ea18453 net: extend Sock::Wait() to report a timeout (Vasil Dimov)
78fdfbe net: dedup MSG_NOSIGNAL and MSG_DONTWAIT definitions (Vasil Dimov)
34bcfab net: move the constant maxWait out of InterruptibleRecv() (Vasil Dimov)
cff65c4 net: extend CNetAddr::SetSpecial() to support I2P (Vasil Dimov)
f6c267d net: avoid unnecessary GetBindAddress() call (Vasil Dimov)
7c224fd net: isolate the protocol-agnostic part of CConnman::AcceptConnection() (Vasil Dimov)
1f75a65 net: get the bind address earlier in CConnman::AcceptConnection() (Vasil Dimov)
2560589 net: check for invalid socket earlier in CConnman::AcceptConnection() (Vasil Dimov)
545bc5f util: fix WriteBinaryFile() claiming success even if error occurred (Vasil Dimov)
8b6e4b3 util: fix ReadBinaryFile() returning partial contents (Vasil Dimov)
4cba2fd util: extract {Read,Write}BinaryFile() to its own files (Vasil Dimov)

Pull request description:

  Add I2P support by using the [I2P SAM](https://geti2p.net/en/docs/api/samv3) protocol. Unlike Tor, for incoming connections we get the I2P address of the peer (and they also receive ours when we are the connection initiator).

  Two new options are added:

  ```
    -i2psam=<ip:port>
         I2P SAM proxy to reach I2P peers and accept I2P connections (default:
         none)

    -i2pacceptincoming
         If set and -i2psam is also set then incoming I2P connections are
         accepted via the SAM proxy. If this is not set but -i2psam is set
         then only outgoing connections will be made to the I2P network.
         Ignored if -i2psam is not set. Notice that listening for incoming
         I2P connections is done through the SAM proxy, not by binding to
         a local address and port (default: true)
  ```

  # Overview of the changes

  ## Make `ReadBinary()` and `WriteBinary()` reusable

  We would need to dump the I2P private key to a file and read it back later. Move those two functions out of `torcontrol.cpp`.

  ```
  util: extract {Read,Write}BinaryFile() to its own files
  util: fix ReadBinaryFile() returning partial contents
  util: fix WriteBinaryFile() claiming success even if error occurred
  ```

  ## Split `CConnman::AcceptConnection()`

  Most of `CConnman::AcceptConnection()` is agnostic of how the socket was accepted. The other part of it deals with the details of the `accept(2)` system call. Split those so that the protocol-agnostic part can be reused if we accept a socket by other means.

  ```
  net: check for invalid socket earlier in CConnman::AcceptConnection()
  net: get the bind address earlier in CConnman::AcceptConnection()
  net: isolate the protocol-agnostic part of CConnman::AcceptConnection()
  net: avoid unnecessary GetBindAddress() call
  ```

  ## Implement the I2P [SAM](https://geti2p.net/en/docs/api/samv3) protocol (not all of it)

  Just the parts that would enable us to make outgoing and accept incoming I2P connections.

  ```
  net: extend CNetAddr::SetSpecial() to support I2P
  net: move the constant maxWait out of InterruptibleRecv()
  net: dedup MSG_NOSIGNAL and MSG_DONTWAIT definitions
  net: extend Sock::Wait() to report a timeout
  net: extend Sock with methods for robust send & read until terminator
  net: extend Sock with a method to check whether connected
  net: implement the necessary parts of the I2P SAM protocol
  ```

  ## Use I2P SAM to connect to and accept connections from I2P peers

  Profit from all of the preceding commits.

  ```
  init: introduce I2P connectivity options
  net: add I2P to the reachability map
  net: make outgoing I2P connections from CConnman
  net: accept incoming I2P connections from CConnman
  net: recognize I2P from ParseNetwork() so that -onlynet=i2p works
  net: Do not skip the I2P network from GetNetworkNames()
  ```

ACKs for top commit:
  laanwj:
    re-ACK a701fcf
  jonatack:
    re-ACK a701fcf reviewed diff per `git range-diff ad89812 2a7bb34 a701fcf`, debug built and launched bitcoind with i2pd v2.35 running a dual I2P+Torv3 service with the I2P config settings listed below (did not test `onlynet=i2p`); operation appears nominal (same as it has been these past weeks), and tested the bitcoind help outputs grepping for `-i i2p` and the rpc getpeerinfo and getnetworkinfo helps

Tree-SHA512: de42090c9c0bf23b43b5839f5b4fc4b3a2657bde1e45c796b5f3c7bf83cb8ec6ca4278f8a89e45108ece92f9b573cafea3b42a06bc09076b40a196c909b6610e

Author: laanwj

doc/files.md

@@ -64,6 +64,7 @@ Subdirectory       | File(s)               | Description
 `./`               | `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](bitcoin-conf.md). 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

src/Makefile.am

@@ -148,6 +148,7 @@ BITCOIN_CORE_H = \
   fs.h \
   httprpc.h \
   httpserver.h \
+  i2p.h \
   index/base.h \
   index/blockfilterindex.h \
   index/disktxpos.h \
@@ -242,6 +243,7 @@ BITCOIN_CORE_H = \
   util/message.h \
   util/moneystr.h \
   util/rbf.h \
+  util/readwritefile.h \
   util/ref.h \
   util/settings.h \
   util/sock.h \
@@ -314,6 +316,7 @@ libbitcoin_server_a_SOURCES = \
   flatfile.cpp \
   httprpc.cpp \
   httpserver.cpp \
+  i2p.cpp \
   index/base.cpp \
   index/blockfilterindex.cpp \
   index/txindex.cpp \
@@ -572,6 +575,7 @@ libbitcoin_util_a_SOURCES = \
   util/message.cpp \
   util/moneystr.cpp \
   util/rbf.cpp \
+  util/readwritefile.cpp \
   util/settings.cpp \
   util/threadnames.cpp \
   util/spanparsing.cpp \

src/compat.h

@@ -44,13 +44,22 @@ typedef unsigned int SOCKET;
 #define WSAEINVAL           EINVAL
 #define WSAEALREADY         EALREADY
 #define WSAEWOULDBLOCK      EWOULDBLOCK
+#define WSAEAGAIN           EAGAIN
 #define WSAEMSGSIZE         EMSGSIZE
 #define WSAEINTR            EINTR
 #define WSAEINPROGRESS      EINPROGRESS
 #define WSAEADDRINUSE       EADDRINUSE
 #define WSAENOTSOCK         EBADF
 #define INVALID_SOCKET      (SOCKET)(~0)
 #define SOCKET_ERROR        -1
+#else
+#ifndef WSAEAGAIN
+#ifdef EAGAIN
+#define WSAEAGAIN EAGAIN
+#else
+#define WSAEAGAIN WSAEWOULDBLOCK
+#endif
+#endif
 #endif
 
 #ifdef WIN32
@@ -96,4 +105,14 @@ bool static inline IsSelectableSocket(const SOCKET& s) {
 #endif
 }
 
+// MSG_NOSIGNAL is not available on some platforms, if it doesn't exist define it as 0
+#if !defined(MSG_NOSIGNAL)
+#define MSG_NOSIGNAL 0
+#endif
+
+// MSG_DONTWAIT is not available on some platforms, if it doesn't exist define it as 0
+#if !defined(MSG_DONTWAIT)
+#define MSG_DONTWAIT 0
+#endif
+
 #endif // BITCOIN_COMPAT_H