[systemd-devel] [PATCH v2 2/2] nspawn: skip symlink to a combined cgroup hierarchy if it already exists

[Iago López Galeiras]

If a symlink to a combined cgroup hierarchy already exists and points to
the right path, skip it. This avoids an error when the cgroups are set
manually before calling nspawn.
---
 src/nspawn/nspawn.c | 10 ++++++++--
 src/shared/util.c   | 23 +++++++++++++++++++++++
 src/shared/util.h   |  1 +
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index f292c63..e9e703f 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -1365,8 +1365,14 @@ static int mount_cgroup(const char *dest) {
                         if (r < 0)
                                 return r;
 
-                        if (symlink(combined, target) < 0)
-                                return log_error_errno(errno, "Failed to create symlink for combined hierarchy: %m");
+                        r = symlink_idempotent(combined, target);
+                        if (r < 0) {
+                                if (r == -EINVAL) {
+                                        log_error("Invalid existing symlink for combined hierarchy");
+                                        return r;
+                                } else
+                                        return log_error_errno(r, "Failed to create symlink for combined hierarchy: %m");
+                        }
                 }
         }
 
diff --git a/src/shared/util.c b/src/shared/util.c
index 466dce4..22e00a6 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -2765,6 +2765,29 @@ int symlink_atomic(const char *from, const char *to) {
         return 0;
 }
 
+int symlink_idempotent(const char *from, const char *to) {
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(from);
+        assert(to);
+
+        if (symlink(from, to) < 0) {
+                if (errno == EEXIST) {
+                        r = readlink_malloc(to, &p);
+                        if (r < 0)
+                                return r;
+
+                        if (!streq(p, from)) {
+                                return -EINVAL;
+                        }
+                } else
+                        return -errno;
+        }
+
+        return 0;
+}
+
 int mknod_atomic(const char *path, mode_t mode, dev_t dev) {
         _cleanup_free_ char *t = NULL;
         int r;
diff --git a/src/shared/util.h b/src/shared/util.h
index 4a67d5c..8565fd6 100644
--- a/src/shared/util.h
+++ b/src/shared/util.h
@@ -404,6 +404,7 @@ bool machine_name_is_valid(const char *s) _pure_;
 
 char* strshorten(char *s, size_t l);
 
+int symlink_idempotent(const char *from, const char *to);
 
 int symlink_atomic(const char *from, const char *to);
 int mknod_atomic(const char *path, mode_t mode, dev_t dev);
-- 
2.4.0