Retry poll calls on EINTR

When signals are delievered to the process, calls here to poll may be
interrupted and return with a spurious failure.  The call instead should be
restarted.

Author: johnwheffner

sdk/core/azure-core/src/http/curl/curl.cpp

@@ -14,6 +14,7 @@
 #include "curl_session_private.hpp"
 
 #if defined(AZ_PLATFORM_POSIX)
+#include <chrono>
 #include <poll.h> // for poll()
 #include <sys/socket.h> // for socket shutdown
 #elif defined(AZ_PLATFORM_WINDOWS)
@@ -102,17 +103,38 @@ int pollSocketUntilEventOrTimeout(
     interval = timeout;
   }
   int result = 0;
-  for (long counter = 0; counter < timeout && result == 0; counter = counter + interval)
+  bool poll_interrupted = false; // deadline and now are only valid if poll_interrupted
+  long deadline{};
+  long now{};
+  for (long counter = 0; (poll_interrupted ? now < deadline : counter < timeout) && result == 0;
+       counter = counter + interval)
   {
     // check cancelation
     context.ThrowIfCancelled();
 #if defined(AZ_PLATFORM_POSIX)
     result = poll(&poller, 1, interval);
+    if (result < 0 && EINTR == errno)
+    {
+      // Only make extra syscalls to get the time in the (rare) event that poll
+      // is interrupted. We jump through these hoops because the poll call does
+      // not tell us how long it waited before being interrupted.  For the first
+      // interruption only, assume no time has passed during the interrupted
+      // call.
+      now = std::chrono::duration_cast<std::chrono::milliseconds>(
+                std::chrono::steady_clock::now().time_since_epoch())
+                .count();
+      if (!poll_interrupted)
+      {
+        poll_interrupted = true;
+        deadline = now + timeout - counter;
+        result = 0;
+      }
+    }
 #elif defined(AZ_PLATFORM_WINDOWS)
     result = WSAPoll(&poller, 1, interval);
 #endif
   }
-  // result can be either 0 (timeout) or > 1 (socket ready)
+  // result can be 0 (timeout), > 0 (socket ready), or < 0 (error)
   return result;
 }