Isolation: wired up cgroup to build system.

This commit enables the building of the cgroup code. This is only built
when the cgroupv2 filesystem is found.

If cgroupv2 support is found then

  cgroupv2: .................. YES

will be printed by ./configure

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

auto/cgroup

@@ -0,0 +1,22 @@
+# Copyright (C) Andrew Clayton
+# Copyright (C) F5, Inc.
+
+NXT_HAVE_CGROUP=NO
+
+if [ -f "/proc/mounts" ]; then
+    CGROUP=$(grep cgroup2 /proc/mounts | head -n 1 | cut -d " " -f 2)
+
+    if [ "$CGROUP" ]; then
+        NXT_HAVE_CGROUP=YES
+
+        cat << END >> $NXT_AUTO_CONFIG_H
+
+#ifndef NXT_HAVE_CGROUP
+#define NXT_HAVE_CGROUP  1
+#define NXT_CGROUP_ROOT  "$CGROUP"
+#endif
+
+END
+
+    fi
+fi

auto/sources

@@ -304,6 +304,11 @@ if [ "$NXT_HAVE_CLONE" = "YES" ]; then
 fi
 
 
+if [ "$NXT_HAVE_CGROUP" = "YES" ]; then
+    NXT_LIB_SRCS="$NXT_LIB_SRCS src/nxt_cgroup.c"
+fi
+
+
 if [ "$NXT_TEST_BUILD" = "YES" ]; then
     NXT_LIB_SRCS="$NXT_LIB_SRCS $NXT_TEST_BUILD_SRCS"
 fi

auto/summary

@@ -31,6 +31,7 @@ Unit configuration summary:
   NJS support: ............... $NXT_NJS
 
   process isolation: ......... $NXT_ISOLATION
+  cgroupv2: .................. $NXT_HAVE_CGROUP
 
   debug logging: ............. $NXT_DEBUG
 

configure

@@ -129,6 +129,7 @@ if [ $NXT_REGEX = YES ]; then
     . auto/pcre
 fi
 
+. auto/cgroup
 . auto/isolation
 . auto/capability