unix: fix uv_uptime() on linux

First check `/proc/uptime`, then fallback to `clock_gettime()`.

Fixes: #3068
Co-authored-by: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-by: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

src/unix/linux-core.c

@@ -602,22 +602,53 @@ int uv_resident_set_memory(size_t* rss) {
   return UV_EINVAL;
 }
 
+static int uv__slurp(const char* filename, char* buf, size_t len) {
+  ssize_t n;
+  int fd;
+
+  assert(len > 0);
+
+  fd = uv__open_cloexec(filename, O_RDONLY);
+  if (fd < 0)
+    return fd;
+
+  do
+    n = read(fd, buf, len - 1);
+  while (n == -1 && errno == EINTR);
+
+  if (uv__close_nocheckstdio(fd))
+    abort();
+
+  if (n < 0)
+    return UV__ERR(errno);
+
+  buf[n] = '\0';
+
+  return 0;
+}
 
 int uv_uptime(double* uptime) {
   static volatile int no_clock_boottime;
+  char buf[128];
   struct timespec now;
   int r;
 
+  /* Try /proc/uptime first, then fallback to clock_gettime(). */
+
+  if (0 == uv__slurp("/proc/uptime", buf, sizeof(buf)))
+    if (1 == sscanf(buf, "%f", uptime))
+      return 0;
+
   /* Try CLOCK_BOOTTIME first, fall back to CLOCK_MONOTONIC if not available
    * (pre-2.6.39 kernels). CLOCK_MONOTONIC doesn't increase when the system
    * is suspended.
    */
   if (no_clock_boottime) {
-    retry: r = clock_gettime(CLOCK_MONOTONIC, &now);
+    retry_clock_gettime: r = clock_gettime(CLOCK_MONOTONIC, &now);
   }
   else if ((r = clock_gettime(CLOCK_BOOTTIME, &now)) && errno == EINVAL) {
     no_clock_boottime = 1;
-    goto retry;
+    goto retry_clock_gettime;
   }
 
   if (r)
@@ -1025,32 +1056,6 @@ void uv__set_process_title(const char* title) {
 }
 
 
-static int uv__slurp(const char* filename, char* buf, size_t len) {
-  ssize_t n;
-  int fd;
-
-  assert(len > 0);
-
-  fd = uv__open_cloexec(filename, O_RDONLY);
-  if (fd < 0)
-    return fd;
-
-  do
-    n = read(fd, buf, len - 1);
-  while (n == -1 && errno == EINTR);
-
-  if (uv__close_nocheckstdio(fd))
-    abort();
-
-  if (n < 0)
-    return UV__ERR(errno);
-
-  buf[n] = '\0';
-
-  return 0;
-}
-
-
 static uint64_t uv__read_proc_meminfo(const char* what) {
   uint64_t rc;
   char* p;