linux: fix cpu count

When libuv is running inside container - eg. lxc container, cpu number
is not obvious. Linux control groups (cgroups) may limit numer of cpus.

As a result of different number cpu cores inside container and
outside container, libuv is crashing.

sysconf(_SC_NPROCESSORS_ONLN) - returns num of host cpus (eg. 32)
`/proc/stat` - sees only cpus limited by cgroups (eg. 4)

When libuv is trying to operate at both numbers and they're different
it's crashing with current test:

run-tests: ../src/unix/linux-core.c:766: read_times: Assertion `num ==
numcpus' failed.

Count the number of cpus based on `/proc/stat` instead.

PR-URL: https://github.com/libuv/libuv/pull/735
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/linux-core.c

@@ -69,7 +69,7 @@
 #endif
 
 static int read_models(unsigned int numcpus, uv_cpu_info_t* ci);
-static int read_times(unsigned int numcpus, uv_cpu_info_t* ci);
+static int read_times(FILE* statfile_fp, unsigned int numcpus, uv_cpu_info_t* ci);
 static void read_speeds(unsigned int numcpus, uv_cpu_info_t* ci);
 static unsigned long read_cpufreq(unsigned int cpunum);
 
@@ -552,15 +552,48 @@ int uv_uptime(double* uptime) {
 }
 
 
+static int uv__cpu_num(FILE* statfile_fp, unsigned int* numcpus) {
+  unsigned int num;
+  char buf[1024];
+
+  if (!fgets(buf, sizeof(buf), statfile_fp))
+    abort();
+
+  num = 0;
+  while (fgets(buf, sizeof(buf), statfile_fp)) {
+    if (strncmp(buf, "cpu", 3))
+      break;
+    num++;
+  }
+
+  *numcpus = num;
+  return 0;
+}
+
+
 int uv_cpu_info(uv_cpu_info_t** cpu_infos, int* count) {
   unsigned int numcpus;
   uv_cpu_info_t* ci;
   int err;
+  int statfile_fd;
+  FILE* statfile_fp;
 
   *cpu_infos = NULL;
   *count = 0;
 
-  numcpus = sysconf(_SC_NPROCESSORS_ONLN);
+  err = uv__open_cloexec("/proc/stat", O_RDONLY);
+  if (err < 0)
+    return err;
+  statfile_fd = err;
+
+  statfile_fp = fdopen(statfile_fd, "r");
+  if (statfile_fp == NULL)
+    return -errno;
+
+  err = uv__cpu_num(statfile_fp, &numcpus);
+  if (err < 0)
+    return err;
+
   assert(numcpus != (unsigned int) -1);
   assert(numcpus != 0);
 
@@ -570,7 +603,10 @@ int uv_cpu_info(uv_cpu_info_t** cpu_infos, int* count) {
 
   err = read_models(numcpus, ci);
   if (err == 0)
-    err = read_times(numcpus, ci);
+    rewind(statfile_fp);
+    err = read_times(statfile_fp, numcpus, ci);
+
+  uv__close(statfile_fd);
 
   if (err) {
     uv_free_cpu_info(ci, numcpus);
@@ -696,7 +732,7 @@ static int read_models(unsigned int numcpus, uv_cpu_info_t* ci) {
 }
 
 
-static int read_times(unsigned int numcpus, uv_cpu_info_t* ci) {
+static int read_times(FILE* statfile_fp, unsigned int numcpus, uv_cpu_info_t* ci) {
   unsigned long clock_ticks;
   struct uv_cpu_times_s ts;
   unsigned long user;
@@ -708,22 +744,19 @@ static int read_times(unsigned int numcpus, uv_cpu_info_t* ci) {
   unsigned int num;
   unsigned int len;
   char buf[1024];
-  FILE* fp;
 
   clock_ticks = sysconf(_SC_CLK_TCK);
   assert(clock_ticks != (unsigned long) -1);
   assert(clock_ticks != 0);
 
-  fp = fopen("/proc/stat", "r");
+  rewind(statfile_fp);
-  if (fp == NULL)
-    return -errno;
 
-  if (!fgets(buf, sizeof(buf), fp))
+  if (!fgets(buf, sizeof(buf), statfile_fp))
     abort();
 
   num = 0;
 
-  while (fgets(buf, sizeof(buf), fp)) {
+  while (fgets(buf, sizeof(buf), statfile_fp)) {
     if (num >= numcpus)
       break;
 
@@ -762,7 +795,6 @@ static int read_times(unsigned int numcpus, uv_cpu_info_t* ci) {
     ts.irq  = clock_ticks * irq;
     ci[num++].cpu_times = ts;
   }
-  fclose(fp);
   assert(num == numcpus);
 
   return 0;