Merge bitcoin/bitcoin#24356: refactor: replace CConnman::SocketEvents() with mockable Sock::WaitMany()

6e68ccbefea6509c61fc4405a391a517c6057bb0 net: use Sock::WaitMany() instead of CConnman::SocketEvents() (Vasil Dimov)
ae263460bab9e6aa112dc99790c8ef06a56ec838 net: introduce Sock::WaitMany() (Vasil Dimov)
cc74459768063a923fb6220a4f420eaf211aee7b net: also wait for exceptional events in Sock::Wait() (Vasil Dimov)

Pull request description:

  _This is a piece of #21878, chopped off to ease review._

  `Sock::Wait()` waits for IO events on one socket. Introduce a similar `virtual` method `WaitMany()` that waits simultaneously for IO events on more than one socket.

  Use `WaitMany()` instead of `CConnman::SocketEvents()` (and ditch the latter). Given that the former is a `virtual` method, it can be mocked by unit and fuzz tests. This will help to make bigger parts of `CConnman` testable (unit and fuzz).

ACKs for top commit:
  laanwj:
    Code review ACK 6e68ccbefea6509c61fc4405a391a517c6057bb0
  jonatack:
    re-ACK 6e68ccbefea6509c61fc4405a391a517c6057bb0 per `git range-diff e18fd47 6747729 6e68ccb`, and verified rebase to master and debug build

Tree-SHA512: 917fb6ad880d64d3af1ebb301c06fbd01afd8ff043f49e4055a088ebed6affb7ffe1dcf59292d822f10de5f323b6d52d557cb081dd7434634995f9148efcf08f

2 files changed, 141 insertions, 42 deletions

diff --git a/src/util/sock.cpp b/src/util/sock.cpp
index 3579af4458..7d5069423a 100644
--- a/src/util/sock.cpp
+++ b/src/util/sock.cpp
@@ -113,63 +113,103 @@ int Sock::SetSockOpt(int level, int opt_name, const void* opt_val, socklen_t opt
 
 bool Sock::Wait(std::chrono::milliseconds timeout, Event requested, Event* occurred) const
 {
-#ifdef USE_POLL
-    pollfd fd;
-    fd.fd = m_socket;
-    fd.events = 0;
-    if (requested & RECV) {
-        fd.events |= POLLIN;
-    }
-    if (requested & SEND) {
-        fd.events |= POLLOUT;
-    }
+    // We need a `shared_ptr` owning `this` for `WaitMany()`, but don't want
+    // `this` to be destroyed when the `shared_ptr` goes out of scope at the
+    // end of this function. Create it with a custom noop deleter.
+    std::shared_ptr<const Sock> shared{this, [](const Sock*) {}};
+
+    EventsPerSock events_per_sock{std::make_pair(shared, Events{requested})};
 
-    if (poll(&fd, 1, count_milliseconds(timeout)) == SOCKET_ERROR) {
+    if (!WaitMany(timeout, events_per_sock)) {
         return false;
     }
 
     if (occurred != nullptr) {
-        *occurred = 0;
-        if (fd.revents & POLLIN) {
-            *occurred |= RECV;
+        *occurred = events_per_sock.begin()->second.occurred;
+    }
+
+    return true;
+}
+
+bool Sock::WaitMany(std::chrono::milliseconds timeout, EventsPerSock& events_per_sock) const
+{
+#ifdef USE_POLL
+    std::vector<pollfd> pfds;
+    for (const auto& [sock, events] : events_per_sock) {
+        pfds.emplace_back();
+        auto& pfd = pfds.back();
+        pfd.fd = sock->m_socket;
+        if (events.requested & RECV) {
+            pfd.events |= POLLIN;
         }
-        if (fd.revents & POLLOUT) {
-            *occurred |= SEND;
+        if (events.requested & SEND) {
+            pfd.events |= POLLOUT;
         }
     }
 
-    return true;
-#else
-    if (!IsSelectableSocket(m_socket)) {
+    if (poll(pfds.data(), pfds.size(), count_milliseconds(timeout)) == SOCKET_ERROR) {
         return false;
     }
 
-    fd_set fdset_recv;
-    fd_set fdset_send;
-    FD_ZERO(&fdset_recv);
-    FD_ZERO(&fdset_send);
-
-    if (requested & RECV) {
-        FD_SET(m_socket, &fdset_recv);
+    assert(pfds.size() == events_per_sock.size());
+    size_t i{0};
+    for (auto& [sock, events] : events_per_sock) {
+        assert(sock->m_socket == static_cast<SOCKET>(pfds[i].fd));
+        events.occurred = 0;
+        if (pfds[i].revents & POLLIN) {
+            events.occurred |= RECV;
+        }
+        if (pfds[i].revents & POLLOUT) {
+            events.occurred |= SEND;
+        }
+        if (pfds[i].revents & (POLLERR | POLLHUP)) {
+            events.occurred |= ERR;
+        }
+        ++i;
     }
 
-    if (requested & SEND) {
-        FD_SET(m_socket, &fdset_send);
+    return true;
+#else
+    fd_set recv;
+    fd_set send;
+    fd_set err;
+    FD_ZERO(&recv);
+    FD_ZERO(&send);
+    FD_ZERO(&err);
+    SOCKET socket_max{0};
+
+    for (const auto& [sock, events] : events_per_sock) {
+        const auto& s = sock->m_socket;
+        if (!IsSelectableSocket(s)) {
+            return false;
+        }
+        if (events.requested & RECV) {
+            FD_SET(s, &recv);
+        }
+        if (events.requested & SEND) {
+            FD_SET(s, &send);
+        }
+        FD_SET(s, &err);
+        socket_max = std::max(socket_max, s);
     }
 
-    timeval timeout_struct = MillisToTimeval(timeout);
+    timeval tv = MillisToTimeval(timeout);
 
-    if (select(m_socket + 1, &fdset_recv, &fdset_send, nullptr, &timeout_struct) == SOCKET_ERROR) {
+    if (select(socket_max + 1, &recv, &send, &err, &tv) == SOCKET_ERROR) {
         return false;
     }
 
-    if (occurred != nullptr) {
-        *occurred = 0;
-        if (FD_ISSET(m_socket, &fdset_recv)) {
-            *occurred |= RECV;
+    for (auto& [sock, events] : events_per_sock) {
+        const auto& s = sock->m_socket;
+        events.occurred = 0;
+        if (FD_ISSET(s, &recv)) {
+            events.occurred |= RECV;
+        }
+        if (FD_ISSET(s, &send)) {
+            events.occurred |= SEND;
         }
-        if (FD_ISSET(m_socket, &fdset_send)) {
-            *occurred |= SEND;
+        if (FD_ISSET(s, &err)) {
+            events.occurred |= ERR;
         }
     }
 
diff --git a/src/util/sock.h b/src/util/sock.h
index dd2913a66c..3245820995 100644
--- a/src/util/sock.h
+++ b/src/util/sock.h
@@ -12,6 +12,7 @@
 #include <chrono>
 #include <memory>
 #include <string>
+#include <unordered_map>
 
 /**
  * Maximum time to wait for I/O readiness.
@@ -130,26 +131,84 @@ public:
     /**
      * If passed to `Wait()`, then it will wait for readiness to read from the socket.
      */
-    static constexpr Event RECV = 0b01;
+    static constexpr Event RECV = 0b001;
 
     /**
      * If passed to `Wait()`, then it will wait for readiness to send to the socket.
      */
-    static constexpr Event SEND = 0b10;
+    static constexpr Event SEND = 0b010;
+
+    /**
+     * Ignored if passed to `Wait()`, but could be set in the occurred events if an
+     * exceptional condition has occurred on the socket or if it has been disconnected.
+     */
+    static constexpr Event ERR = 0b100;
 
     /**
      * Wait for readiness for input (recv) or output (send).
      * @param[in] timeout Wait this much for at least one of the requested events to occur.
      * @param[in] requested Wait for those events, bitwise-or of `RECV` and `SEND`.
-     * @param[out] occurred If not nullptr and `true` is returned, then upon return this
-     * indicates which of the requested events occurred. A timeout is indicated by return
-     * value of `true` and `occurred` being set to 0.
-     * @return true on success and false otherwise
+     * @param[out] occurred If not nullptr and the function returns `true`, then this
+     * indicates which of the requested events occurred (`ERR` will be added, even if
+     * not requested, if an exceptional event occurs on the socket).
+     * A timeout is indicated by return value of `true` and `occurred` being set to 0.
+     * @return true on success (or timeout, if `occurred` of 0 is returned), false otherwise
      */
     [[nodiscard]] virtual bool Wait(std::chrono::milliseconds timeout,
                                     Event requested,
                                     Event* occurred = nullptr) const;
 
+    /**
+     * Auxiliary requested/occurred events to wait for in `WaitMany()`.
+     */
+    struct Events {
+        explicit Events(Event req) : requested{req}, occurred{0} {}
+        Event requested;
+        Event occurred;
+    };
+
+    struct HashSharedPtrSock {
+        size_t operator()(const std::shared_ptr<const Sock>& s) const
+        {
+            return s ? s->m_socket : std::numeric_limits<SOCKET>::max();
+        }
+    };
+
+    struct EqualSharedPtrSock {
+        bool operator()(const std::shared_ptr<const Sock>& lhs,
+                        const std::shared_ptr<const Sock>& rhs) const
+        {
+            if (lhs && rhs) {
+                return lhs->m_socket == rhs->m_socket;
+            }
+            if (!lhs && !rhs) {
+                return true;
+            }
+            return false;
+        }
+    };
+
+    /**
+     * On which socket to wait for what events in `WaitMany()`.
+     * The `shared_ptr` is copied into the map to ensure that the `Sock` object
+     * is not destroyed (its destructor would close the underlying socket).
+     * If this happens shortly before or after we call `poll(2)` and a new
+     * socket gets created under the same file descriptor number then the report
+     * from `WaitMany()` will be bogus.
+     */
+    using EventsPerSock = std::unordered_map<std::shared_ptr<const Sock>, Events, HashSharedPtrSock, EqualSharedPtrSock>;
+
+    /**
+     * Same as `Wait()`, but wait on many sockets within the same timeout.
+     * @param[in] timeout Wait this long for at least one of the requested events to occur.
+     * @param[in,out] events_per_sock Wait for the requested events on these sockets and set
+     * `occurred` for the events that actually occurred.
+     * @return true on success (or timeout, if all `what[].occurred` are returned as 0),
+     * false otherwise
+     */
+    [[nodiscard]] virtual bool WaitMany(std::chrono::milliseconds timeout,
+                                        EventsPerSock& events_per_sock) const;
+
     /* Higher level, convenience, methods. These may throw. */
 
     /**