Core: ngx_current_msec now uses monotonic time if available.

When clock_gettime(CLOCK_MONOTONIC) (or faster variants, _FAST on FreeBSD,
and _COARSE on Linux) is available, we now use it for ngx_current_msec.
This should improve handling of timers if system time changes (ticket #189).

auto/unix

diff --git a/auto/unix b/auto/unix
index 3be8b27..90b5894 100644
--- a/auto/unix
+++ b/auto/unix
@@ -791,6 +791,30 @@
 . auto/feature
 
 
+ngx_feature="clock_gettime(CLOCK_MONOTONIC)"
+ngx_feature_name="NGX_HAVE_CLOCK_MONOTONIC"
+ngx_feature_run=no
+ngx_feature_incs="#include <time.h>"
+ngx_feature_path=
+ngx_feature_libs=
+ngx_feature_test="struct timespec ts; clock_gettime(CLOCK_MONOTONIC, &ts)"
+. auto/feature
+
+
+if [ $ngx_found = no ]; then
+
+    # Linux before glibc 2.17, notably CentOS 6
+
+    ngx_feature="clock_gettime(CLOCK_MONOTONIC) in librt"
+    ngx_feature_libs="-lrt"
+    . auto/feature
+
+    if [ $ngx_found = yes ]; then
+        CORE_LIBS="$CORE_LIBS -lrt"
+    fi
+fi
+
+
 ngx_feature="posix_memalign()"
 ngx_feature_name="NGX_HAVE_POSIX_MEMALIGN"
 ngx_feature_run=no

src/core/ngx_times.c

diff --git a/src/core/ngx_times.c b/src/core/ngx_times.c
index b2edf1a..7964b00 100644
--- a/src/core/ngx_times.c
+++ b/src/core/ngx_times.c
@@ -9,6 +9,9 @@
 #include <ngx_core.h>
 
 
+static ngx_msec_t ngx_monotonic_time(time_t sec, ngx_uint_t msec);
+
+
 /*
  * The time may be updated by signal handler or by several threads.
  * The time update operations are rare and require to hold the ngx_time_lock.
@@ -93,7 +96,7 @@
     sec = tv.tv_sec;
     msec = tv.tv_usec / 1000;
 
-    ngx_current_msec = (ngx_msec_t) sec * 1000 + msec;
+    ngx_current_msec = ngx_monotonic_time(sec, msec);
 
     tp = &cached_time[slot];
 
@@ -189,6 +192,31 @@
 }
 
 
+static ngx_msec_t
+ngx_monotonic_time(time_t sec, ngx_uint_t msec)
+{
+#if (NGX_HAVE_CLOCK_MONOTONIC)
+    struct timespec  ts;
+
+#if defined(CLOCK_MONOTONIC_FAST)
+    clock_gettime(CLOCK_MONOTONIC_FAST, &ts);
+
+#elif defined(CLOCK_MONOTONIC_COARSE)
+    clock_gettime(CLOCK_MONOTONIC_COARSE, &ts);
+
+#else
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+#endif
+
+    sec = ts.tv_sec;
+    msec = ts.tv_nsec / 1000000;
+
+#endif
+
+    return (ngx_msec_t) sec * 1000 + msec;
+}
+
+
 #if !(NGX_WIN32)
 
 void