http: Introduce HTTPServer class and implement binding to listening socket

Introduce a new low-level socket managing class `HTTPServer`.

BindAndStartListening() was copied from CConnMan's BindListenPort()
in net.cpp and modernized.

Unit-test it with a new class `SocketTestingSetup` which mocks
`CreateSock()` and will enable mock client I/O in future commits.

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

src/httpserver.cpp

@@ -2,6 +2,8 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <bitcoin-build-config.h> // IWYU pragma: keep
+
 #include <httpserver.h>
 
 #include <chainparamsbase.h>
@@ -15,6 +17,7 @@
 #include <sync.h>
 #include <util/check.h>
 #include <util/signalinterrupt.h>
+#include <util/sock.h>
 #include <util/strencodings.h>
 #include <util/threadnames.h>
 #include <util/threadpool.h>
@@ -46,6 +49,9 @@
 
 #include <support/events.h>
 
+//! Explicit alias for setting socket option methods.
+static constexpr int SOCKET_OPTION_TRUE{1};
+
 using common::InvalidPortErrMsg;
 using http_libevent::HTTPRequest;
 
@@ -889,4 +895,85 @@ bool HTTPRequest::LoadBody(LineReader& reader)
 
     return true;
 }
+
+util::Expected<void, std::string> HTTPServer::BindAndStartListening(const CService& to)
+{
+    // Create socket for listening for incoming connections
+    sockaddr_storage storage;
+    auto sa = reinterpret_cast<sockaddr*>(&storage);
+    socklen_t len{sizeof(storage)};
+    if (!to.GetSockAddr(sa, &len)) {
+        return util::Unexpected{strprintf("Bind address family for %s not supported", to.ToStringAddrPort())};
+    }
+
+    std::unique_ptr<Sock> sock{CreateSock(to.GetSAFamily(), SOCK_STREAM, IPPROTO_TCP)};
+    if (!sock) {
+        return util::Unexpected{strprintf("Cannot create %s listen socket: %s",
+                                          to.ToStringAddrPort(),
+                                          NetworkErrorString(WSAGetLastError()))};
+    }
+
+    // Allow binding if the port is still in TIME_WAIT state after
+    // the program was closed and restarted.
+    if (sock->SetSockOpt(SOL_SOCKET, SO_REUSEADDR, &SOCKET_OPTION_TRUE, sizeof(SOCKET_OPTION_TRUE)) == SOCKET_ERROR) {
+        LogDebug(BCLog::HTTP,
+                 "Cannot set SO_REUSEADDR on %s listen socket: %s, continuing anyway",
+                 to.ToStringAddrPort(),
+                 NetworkErrorString(WSAGetLastError()));
+    }
+
+    // some systems don't have IPV6_V6ONLY but are always v6only; others do have the option
+    // and enable it by default or not. Try to enable it, if possible.
+    if (to.IsIPv6()) {
+#ifdef IPV6_V6ONLY
+        if (sock->SetSockOpt(IPPROTO_IPV6, IPV6_V6ONLY, &SOCKET_OPTION_TRUE, sizeof(SOCKET_OPTION_TRUE)) == SOCKET_ERROR) {
+            LogDebug(BCLog::HTTP,
+                     "Cannot set IPV6_V6ONLY on %s listen socket: %s, continuing anyway",
+                     to.ToStringAddrPort(),
+                     NetworkErrorString(WSAGetLastError()));
+        }
+#endif
+#ifdef WIN32
+        int prot_level{PROTECTION_LEVEL_UNRESTRICTED};
+        if (sock->SetSockOpt(IPPROTO_IPV6,
+                             IPV6_PROTECTION_LEVEL,
+                             &prot_level,
+                             sizeof(prot_level)) == SOCKET_ERROR) {
+            LogDebug(BCLog::HTTP,
+                     "Cannot set IPV6_PROTECTION_LEVEL on %s listen socket: %s, continuing anyway",
+                     to.ToStringAddrPort(),
+                     NetworkErrorString(WSAGetLastError()));
+        }
+#endif
+    }
+
+    if (sock->Bind(sa, len) == SOCKET_ERROR) {
+        const int err{WSAGetLastError()};
+        if (err == WSAEADDRINUSE) {
+            return util::Unexpected{strprintf("Unable to bind to %s on this computer. %s is probably already running.",
+                                              to.ToStringAddrPort(),
+                                              CLIENT_NAME)};
+        } else {
+            return util::Unexpected{strprintf("Unable to bind to %s on this computer (bind returned error %s)",
+                                              to.ToStringAddrPort(),
+                                              NetworkErrorString(err))};
+        }
+    }
+
+    // Listen for incoming connections
+    if (sock->Listen(SOMAXCONN) == SOCKET_ERROR) {
+        return util::Unexpected{strprintf("Cannot listen on %s: %s",
+                                          to.ToStringAddrPort(),
+                                          NetworkErrorString(WSAGetLastError()))};
+    }
+
+    m_listen.emplace_back(std::move(sock));
+
+    return {};
+}
+
+void HTTPServer::StopListening()
+{
+    m_listen.clear();
+}
 } // namespace http_bitcoin