Add s6_svc_lock_take() and s6_svc_lock_release()

 Allows fixing a race condition in s6-rc-init.
 s6-supervise also creates event/ before supervise/ so that
locking the servicedir ensures event/ can be used.

2 files changed, 29 insertions, 0 deletions

diff --git a/src/libs6/deps-lib/s6 b/src/libs6/deps-lib/s6
index ace5a05..5acb672 100644
--- a/src/libs6/deps-lib/s6
+++ b/src/libs6/deps-lib/s6
@@ -26,6 +26,7 @@ s6_accessrules_uidgid_cdb.o
 s6_accessrules_uidgid_fs.o
 s6_supervise_lock.o
 s6_supervise_lock_mode.o
+s6_svc_lock_take.o
 s6_svc_ok.o
 s6_svc_write.o
 s6_svc_writectl.o
diff --git a/src/libs6/s6_svc_lock_take.c b/src/libs6/s6_svc_lock_take.c
new file mode 100644
index 0000000..57830c0
--- /dev/null
+++ b/src/libs6/s6_svc_lock_take.c
@@ -0,0 +1,28 @@
+/* ISC license. */
+
+#include <sys/stat.h>
+#include <string.h>
+#include <errno.h>
+#include <skalibs/djbunix.h>
+#include <s6/s6-supervise.h>
+
+int s6_svc_lock_take (char const *dir)
+{
+  size_t dirlen = strlen(dir) ;
+  int fdlock ;
+  char lock[dirlen + sizeof(S6_SUPERVISE_CTLDIR) + 6] ;
+  memcpy(lock, dir, dirlen) ;
+  memcpy(lock + dirlen, "/" S6_SUPERVISE_CTLDIR, sizeof(S6_SUPERVISE_CTLDIR) + 1) ;
+  if ((mkdir(lock, S_IRWXU) < 0) && (errno != EEXIST)) return -1 ;
+  memcpy(lock + dirlen + sizeof(S6_SUPERVISE_CTLDIR), "/lock", 6) ;
+  fdlock = open_create(lock) ;
+  if (fdlock < 0) return -1 ;
+  if (coe(fdlock) < 0 || lock_ex(fdlock) < 0)
+  {
+    int e = errno ;
+    fd_close(fdlock) ;
+    errno = e ;
+    return -1 ;
+  }
+  return fdlock ;
+}