base: fdReady(): Improve accuracy and simplify code.

This is done by reusing the existing cross-platform
`getProcessElapsedTime()` function, which already provides nanosecond
monotonic clocks, and fallback for platforms that don't have those.

To do this, `getProcessElapsedTime()` had to be moved from a private RTS
symbol into the public interface.

Accuracy is improved in 2 ways:

* Use of the monotonic clock where available
* Measuring the total time spent waiting instead of a sum
  of intervals (between which there are small gaps)

Reviewers: bgamari, austin, hvr, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3953

1 files changed, 10 insertions, 21 deletions

diff --git a/libraries/base/cbits/inputReady.c b/libraries/base/cbits/inputReady.c
index e27851a2c7..dbfdb28b70 100644
--- a/libraries/base/cbits/inputReady.c
+++ b/libraries/base/cbits/inputReady.c
@@ -7,9 +7,9 @@
 /* select and supporting types is not Posix */
 /* #include "PosixSource.h" */
 #include "HsBase.h"
+#include "Rts.h"
 #if !defined(_WIN32)
 #include <poll.h>
-#include <sys/time.h>
 #endif
 
 /*
@@ -25,37 +25,26 @@ fdReady(int fd, int write, int msecs, int isSock)
 #if !defined(_WIN32)
     struct pollfd fds[1];
 
-    // if we need to track the then record the current time in case we are
+    // if we need to track the time then record the end time in case we are
     // interrupted.
-    struct timeval tv0;
+    Time endTime = 0;
     if (msecs > 0) {
-        if (gettimeofday(&tv0, NULL) != 0) {
-            fprintf(stderr, "fdReady: gettimeofday failed: %s\n",
-                    strerror(errno));
-            abort();
-        }
+        endTime = getProcessElapsedTime() + MSToTime(msecs);
     }
 
     fds[0].fd = fd;
     fds[0].events = write ? POLLOUT : POLLIN;
     fds[0].revents = 0;
 
+    Time remaining = MSToTime(msecs);
+
     int res;
-    while ((res = poll(fds, 1, msecs)) < 0) {
+    while ((res = poll(fds, 1, TimeToMS(remaining))) < 0) {
         if (errno == EINTR) {
             if (msecs > 0) {
-                struct timeval tv;
-                if (gettimeofday(&tv, NULL) != 0) {
-                    fprintf(stderr, "fdReady: gettimeofday failed: %s\n",
-                            strerror(errno));
-                    abort();
-                }
-
-                int elapsed = 1000 * (tv.tv_sec - tv0.tv_sec)
-                            + (tv.tv_usec - tv0.tv_usec) / 1000;
-                msecs -= elapsed;
-                if (msecs <= 0) return 0;
-                tv0 = tv;
+                Time now = getProcessElapsedTime();
+                if (now >= endTime) return 0;
+                remaining = endTime - now;
             }
         } else {
             return (-1);