client: handle timeout more safely

Currently all wait methods are just cycling in a loop while timer is not
expired. At the beginning of each iteration, `timeout - timer.elapsed()`
expression is used to calculate timeout for `NetProvider::wait()`, and
that `wait` expects only non-negative timeouts (or `-1` for infinity).
So, for example, if the operating system will reschedule the process
right between `timer.isExpired()` and `timer.elapsed()` call, timer
can become expired after we are scheduled again, and we will suddenly
pass a negative value to the provider.

Let's replace `elapsed` method with more safe `timeLeft` which returns
zero when the timeout is expired. Also, such method will be handy in the
following commits.

Closes #128

Author: drewdzzz

src/Client/Connector.hpp

@@ -187,7 +187,7 @@ Connector<BUFFER, NetProvider>::wait(Connection<BUFFER, NetProvider> &conn,
 		return -1;
 	while (!conn.hasError() && !conn.futureIsReady(future) &&
 	       !timer.isExpired()) {
-		if (m_NetProvider.wait(timeout - timer.elapsed()) != 0) {
+		if (m_NetProvider.wait(timer.timeLeft()) != 0) {
 			conn.setError(std::string("Failed to poll: ") +
 				      strerror(errno), errno);
 			return -1;
@@ -227,7 +227,7 @@ Connector<BUFFER, NetProvider>::waitAll(Connection<BUFFER, NetProvider> &conn,
 	timer.start();
 	size_t last_not_ready = 0;
 	while (!conn.hasError() && !timer.isExpired()) {
-		if (m_NetProvider.wait(timeout - timer.elapsed()) != 0) {
+		if (m_NetProvider.wait(timer.timeLeft()) != 0) {
 			conn.setError(std::string("Failed to poll: ") +
 				      strerror(errno), errno);
 			return -1;
@@ -265,7 +265,7 @@ Connector<BUFFER, NetProvider>::waitAny(int timeout)
 	Timer timer{timeout};
 	timer.start();
 	while (m_ReadyToDecode.empty() && !timer.isExpired())
-		m_NetProvider.wait(timeout - timer.elapsed());
+		m_NetProvider.wait(timer.timeLeft());
 	if (m_ReadyToDecode.empty()) {
 		LOG_ERROR("wait() has been timed out! No responses are received");
 		return std::nullopt;
@@ -288,7 +288,7 @@ Connector<BUFFER, NetProvider>::waitCount(Connection<BUFFER, NetProvider> &conn,
 	timer.start();
 	size_t ready_futures = conn.getFutureCount();
 	while (!conn.hasError() && !timer.isExpired()) {
-		if (m_NetProvider.wait(timeout - timer.elapsed()) != 0) {
+		if (m_NetProvider.wait(timer.timeLeft()) != 0) {
 			conn.setError(std::string("Failed to poll: ") +
 				      strerror(errno), errno);
 			return -1;

src/Utils/Timer.hpp

@@ -29,6 +29,7 @@
  * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+#include <algorithm>
 #include <chrono>
 
 class Timer {
@@ -48,13 +49,22 @@ class Timer {
 			std::chrono::duration_cast<std::chrono::milliseconds>(end - m_Start);
 		return elapsed >= m_Timeout;
 	}
-	int elapsed() const
+	/**
+	 * The function to obtain amount of time left. Returns:
+	 * 1. `-1` if the initial timeout was `-1`.
+	 * 2. `0` if the timer has expired.
+	 * 3. Otherwise, amount of milliseconds left is returned.
+	 * NB: the function should not be used for expiration check - use `isExpired` instead.
+	 */
+	int timeLeft() const
 	{
 		if (m_Timeout == std::chrono::milliseconds{-1})
-			return 0;
+			return -1;
 		std::chrono::time_point<std::chrono::steady_clock> end =
 			std::chrono::steady_clock::now();
-		return std::chrono::duration_cast<std::chrono::milliseconds>(end - m_Start).count();
+		int timeLeft = m_Timeout.count() -
+			std::chrono::duration_cast<std::chrono::milliseconds>(end - m_Start).count();
+		return std::max(0, timeLeft);
 	}
 private:
 	std::chrono::milliseconds m_Timeout;