MINOR: cpuset: add detection of online CPUs on Linux

This adds a generic function ha_cpuset_detect_online() which for now
only supports linux via /sys. It fills a cpuset with the list of online
CPUs that were detected (or returns a failure).

include/haproxy/cpuset.h

@@ -54,6 +54,12 @@ int ha_cpuset_size(void);
  */
 int ha_cpuset_detect_bound(struct hap_cpuset *set);
 
+/* Detects CPUs that are online on the system. It may rely on FS access (e.g.
+ * /sys on Linux). Returns the number of CPUs detected or 0 if the detection
+ * failed.
+ */
+int ha_cpuset_detect_online(struct hap_cpuset *set);
+
 /* Detects the CPUs that will be used based on the ones the process is bound to.
  * Returns non-zero on success, zero on failure. Note that it may not be
  * performed in the function above because some calls may rely on other items

src/cpuset.c

@@ -4,6 +4,7 @@
 
 #include <haproxy/api.h>
 #include <haproxy/cpuset.h>
+#include <haproxy/global.h>
 #include <haproxy/intops.h>
 #include <haproxy/tools.h>
 
@@ -176,6 +177,32 @@ int ha_cpuset_detect_bound(struct hap_cpuset *set)
 	return ha_cpuset_count(set);
 }
 
+/* Detects CPUs that are online on the system. It may rely on FS access (e.g.
+ * /sys on Linux). Returns the number of CPUs detected or 0 if the detection
+ * failed.
+ */
+int ha_cpuset_detect_online(struct hap_cpuset *set)
+{
+#if defined(__linux__)
+
+	ha_cpuset_zero(set);
+
+	/* contains a list of CPUs in the format <low>[-<high>][,...] */
+	if (read_line_to_trash("%s/cpu/online", NUMA_DETECT_SYSTEM_SYSFS_PATH) == 0) {
+		const char *parse_cpu_set_args[2] = { trash.area, "\0" };
+
+		if (parse_cpu_set(parse_cpu_set_args, set, NULL) != 0)
+			ha_cpuset_zero(set);
+	}
+
+#else // !__linux__
+
+	ha_cpuset_zero(set);
+
+#endif
+	return ha_cpuset_count(set);
+}
+
 /* Detects the CPUs that will be used based on the ones the process is bound to
  * at boot. The principle is the following: all CPUs from the boot cpuset will
  * be used since we don't know upfront how individual threads will be mapped to