Let SSEClient::stop() interrupt a blocked read

The event loop reads through open_stream(), which takes the socket out
of the Client, so the client_.stop() in stop() could not reach it.
stop(), and the destructor with it, waited until the server sent
something or the read timeout passed, 300 seconds by default.

Shutting the socket down from stop() does not wake a read waiting in
WSAPoll on Windows, so 3744227 was reverted. A stream now takes an
interrupt check instead: while one is set, it waits for data in 100 ms
slices and gives up once the check returns true, as the server's
keep-alive wait does with its flag. SSEClient sets one that turns true
when stop() is called. Streams without one wait as before.

SSEIntegrationTest.ManualStop now ends its stream with the fixture's
flag. Its server outlives the test body, and the faster stop() let the
stream read a local of the body that had returned.

httplib.h

@@ -2020,6 +2020,12 @@ public:
   // each one. 0 lifts the bound.
   virtual void set_max_timeout(time_t msec) { (void)msec; }
 
+  // Lets a wait for data end early once `interrupted` returns true, so that
+  // another thread can stop a blocked read on every platform.
+  virtual void set_read_interrupt(std::function<bool()> interrupted) {
+    (void)interrupted;
+  }
+
   // Bytes already pulled off the socket and sitting in this stream's own
   // buffer. Exposing them lets a line reader scan for a terminator in one
   // pass instead of asking for a byte at a time. A stream that does no
@@ -5075,6 +5081,10 @@ inline void SSEClient::run_event_loop() {
     // bounded instead of the whole body.
     auto handle = client_.open_stream("GET", path_, {}, request_headers);
     handle.body_reader_.payload_max_length = 0;
+    // stop() ends a read that is waiting for the next event
+    if (handle.stream_) {
+      handle.stream_->set_read_interrupt([this] { return !running_.load(); });
+    }
     stream::Result result{std::move(handle)};
 
     // Connection error handling
@@ -6740,6 +6750,26 @@ inline ssize_t select_until(socket_t sock, short events,
                      static_cast<time_t>(left % 1000000));
 }
 
+// select_read(), but waits in slices and gives up once interrupted() returns
+// true. A local shutdown() would not wake the wait on Windows.
+inline ssize_t
+select_read_interruptible(socket_t sock, time_t sec, time_t usec,
+                          const std::function<bool()> &interrupted) {
+  if (!interrupted) { return select_read(sock, sec, usec); }
+
+  const auto slice = std::chrono::microseconds(100000);
+  auto deadline = deadline_after(sec, usec);
+  while (!interrupted()) {
+    auto left = std::chrono::duration_cast<std::chrono::microseconds>(
+        deadline - std::chrono::steady_clock::now());
+    auto wait =
+        (std::max)((std::min)(left, slice), std::chrono::microseconds(0));
+    auto ret = select_impl(sock, POLLIN, 0, static_cast<time_t>(wait.count()));
+    if (ret != 0 || wait < slice) { return ret; }
+  }
+  return 0;
+}
+
 inline Error wait_until_socket_is_ready(socket_t sock, time_t sec,
                                         time_t usec) {
   struct pollfd pfd_read;
@@ -6798,6 +6828,7 @@ public:
   time_t duration() const override;
   void set_read_timeout(time_t sec, time_t usec = 0) override;
   void set_max_timeout(time_t msec) override;
+  void set_read_interrupt(std::function<bool()> interrupted) override;
   const char *buffered_data(size_t &size) const override;
   void consume_buffered(size_t size) override;
 
@@ -6819,6 +6850,7 @@ private:
   time_t write_timeout_usec_;
   time_t max_timeout_msec_;
   std::chrono::time_point<std::chrono::steady_clock> start_time_;
+  std::function<bool()> read_interrupted_;
 
   std::vector<char> read_buff_;
   size_t read_buff_off_ = 0;
@@ -10679,6 +10711,7 @@ public:
   time_t duration() const override;
   void set_read_timeout(time_t sec, time_t usec = 0) override;
   void set_max_timeout(time_t msec) override;
+  void set_read_interrupt(std::function<bool()> interrupted) override;
 
   // See SocketStream::set_readable_hint().
   void set_readable_hint() { readable_hint_ = true; }
@@ -10698,6 +10731,7 @@ private:
   time_t write_timeout_usec_;
   time_t max_timeout_msec_;
   std::chrono::time_point<std::chrono::steady_clock> start_time_;
+  std::function<bool()> read_interrupted_;
   bool readable_hint_ = false;
   tls::TlsError tls_error_;
 };
@@ -12610,7 +12644,8 @@ inline bool SocketStream::is_readable() const {
 
 inline bool SocketStream::wait_readable() const {
   if (max_timeout_msec_ <= 0) {
-    return select_read(sock_, read_timeout_sec_, read_timeout_usec_) > 0;
+    return select_read_interruptible(sock_, read_timeout_sec_,
+                                     read_timeout_usec_, read_interrupted_) > 0;
   }
 
   time_t read_timeout_sec;
@@ -12618,7 +12653,8 @@ inline bool SocketStream::wait_readable() const {
   calc_actual_timeout(max_timeout_msec_, duration(), read_timeout_sec_,
                       read_timeout_usec_, read_timeout_sec, read_timeout_usec);
 
-  return select_read(sock_, read_timeout_sec, read_timeout_usec) > 0;
+  return select_read_interruptible(sock_, read_timeout_sec, read_timeout_usec,
+                                   read_interrupted_) > 0;
 }
 
 inline bool SocketStream::wait_writable() const {
@@ -12749,6 +12785,11 @@ inline void SocketStream::set_max_timeout(time_t msec) {
   start_time_ = std::chrono::steady_clock::now();
 }
 
+inline void
+SocketStream::set_read_interrupt(std::function<bool()> interrupted) {
+  read_interrupted_ = std::move(interrupted);
+}
+
 // Buffer stream implementation
 inline bool BufferStream::is_readable() const { return true; }
 
@@ -13174,7 +13215,8 @@ inline bool SSLSocketStream::is_readable() const {
 
 inline bool SSLSocketStream::wait_readable() const {
   if (max_timeout_msec_ <= 0) {
-    return select_read(sock_, read_timeout_sec_, read_timeout_usec_) > 0;
+    return select_read_interruptible(sock_, read_timeout_sec_,
+                                     read_timeout_usec_, read_interrupted_) > 0;
   }
 
   time_t read_timeout_sec;
@@ -13182,7 +13224,8 @@ inline bool SSLSocketStream::wait_readable() const {
   calc_actual_timeout(max_timeout_msec_, duration(), read_timeout_sec_,
                       read_timeout_usec_, read_timeout_sec, read_timeout_usec);
 
-  return select_read(sock_, read_timeout_sec, read_timeout_usec) > 0;
+  return select_read_interruptible(sock_, read_timeout_sec, read_timeout_usec,
+                                   read_interrupted_) > 0;
 }
 
 inline bool SSLSocketStream::wait_writable() const {
@@ -13309,6 +13352,11 @@ inline void SSLSocketStream::set_max_timeout(time_t msec) {
   start_time_ = std::chrono::steady_clock::now();
 }
 
+inline void
+SSLSocketStream::set_read_interrupt(std::function<bool()> interrupted) {
+  read_interrupted_ = std::move(interrupted);
+}
+
 inline WebSocketSSLStream::WebSocketSSLStream(socket_t sock,
                                               tls::session_t session,
                                               time_t read_timeout_sec,