[lttng-dev] [PATCH lttng-ust] compat: work around broken _SC_NPROCESSORS_CONF on MUSL libc

[Michael Jeanson]

On MUSL libc the _SC_NPROCESSORS_CONF sysconf will report the number of
CPUs allocated to the task based on the affinity mask instead of the
total number of CPUs configured on the system.

Signed-off-by: Michael Jeanson <mjeanson at efficios.com>
---
 libringbuffer/smp.c | 66 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/libringbuffer/smp.c b/libringbuffer/smp.c
index 9e7114be..656a75da 100644
--- a/libringbuffer/smp.c
+++ b/libringbuffer/smp.c
@@ -2,6 +2,7 @@
  * libringbuffer/smp.c
  *
  * Copyright (C) 2011-2012 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
+ * Copyright (C) 2019 Michael Jeanson <mjeanson at efficios.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -26,6 +27,7 @@
 
 int __num_possible_cpus;
 
+#if (defined(__GLIBC__) || defined( __UCLIBC__))
 void _get_num_possible_cpus(void)
 {
 	int result;
@@ -43,3 +45,67 @@ void _get_num_possible_cpus(void)
 		return;
 	__num_possible_cpus = result;
 }
+
+#else
+
+/*
+ * The MUSL libc implementation of the _SC_NPROCESSORS_CONF sysconf does not
+ * return the number of configured CPUs in the system but relies on the cpu
+ * affinity mask of the current task.
+ *
+ * So instead we use a strategy similar to GLIBC's, counting the cpu
+ * directories in "/sys/devices/system/cpu" and fallback on the value from
+ * sysconf if it fails.
+ */
+
+#include <dirent.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+
+#define __max(a,b) ((a)>(b)?(a):(b))
+
+void _get_num_possible_cpus(void)
+{
+	int result, count = 0;
+	DIR *cpudir;
+	struct dirent *entry;
+
+	cpudir = opendir("/sys/devices/system/cpu");
+	if (cpudir == NULL)
+		goto end;
+
+	/*
+	 * Count the number of directories named "cpu" followed by and
+	 * integer. This is the same strategy as glibc uses.
+	 */
+	while ((entry = readdir(cpudir))) {
+		if (entry->d_type == DT_DIR &&
+			strncmp(entry->d_name, "cpu", 3) == 0) {
+
+			char *endptr;
+			unsigned long cpu_num;
+
+			cpu_num = strtoul(entry->d_name + 3, &endptr, 10);
+			if ((cpu_num < ULONG_MAX) && (endptr != entry->d_name + 3)
+					&& (*endptr == '\0')) {
+				count++;
+			}
+		}
+	}
+
+end:
+	/*
+	 * Get the sysconf value as a fallback. Keep the highest number.
+	 */
+	result = __max(sysconf(_SC_NPROCESSORS_CONF), count);
+
+	/*
+	 * If both methods failed, don't store the value.
+	 */
+	if (result < 1)
+		return;
+	__num_possible_cpus = result;
+}
+#endif
-- 
2.17.1