tests/get_cpu_location: disable an assert on process-wide cpu-locatio…

…n when there are unexpected threads

qemu-user led us to add an env-var to disable some checks
(see commit de60938).

The actual problem is an expected threads in a process (launched by qemu-user).
Detect such cases on Linux by looking in /proc/self/task/ and auto-disable those asserts.
The env-var is still available for now, may be useful later if the same problem ever
occurs on !Linux.

Thanks to Ludovic Courtes for the help.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests/hwloc/hwloc_get_last_cpu_location.c

@@ -1,12 +1,15 @@
 /*
- * Copyright © 2011-2019 Inria.  All rights reserved.
+ * Copyright © 2011-2020 Inria.  All rights reserved.
  * Copyright © 2011 Université Bordeaux.  All rights reserved.
  * See COPYING in top-level directory.
  */
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
 #include <errno.h>
 
 #include "hwloc.h"
@@ -59,12 +62,29 @@ static int checkall(hwloc_const_cpuset_t set)
   return 0;
 }
 
+static int has_unexpected_threads(void)
+{
+#ifdef HWLOC_LINUX_SYS
+  struct stat stbuf;
+  int err = stat("/proc/self/task", &stbuf);
+  if (!err && stbuf.st_nlink > 3) {
+    printf("program has multiple threads, disabling process-wide binding/cpulocation checks.\n");
+    return 1;
+  }
+#endif
+  /* if the problem ever occurs on !Linux,
+   * we'll use HWLOC_TEST_DONTCHECK_PROC_CPULOCATION=1 until detecting it here
+   */
+  return 0;
+}
+
 int main(void)
 {
   unsigned depth;
   hwloc_obj_t obj;
 
-  checkprocincluded = (NULL == getenv("HWLOC_TEST_DONTCHECK_PROC_CPULOCATION"));
+  checkprocincluded = !has_unexpected_threads()
+    && getenv("HWLOC_TEST_DONTCHECK_PROC_CPULOCATION") == NULL;
 
   hwloc_topology_init(&topology);
   hwloc_topology_load(topology);