First part of the format-upgrade support in repo

 In order to work across major bumps, we need to add a call to
s6-rc-format-upgrade before s6-rc-update in the s6-rc-set-install
sequence of commands.
 s6-rc-format-upgrade only works when the db is exactly the same
across s6-rc versions. That means we need to have the source version
of the current live set at all times, in order to compile it with
the new s6-rc version before upgrading and then updating.
 So:
 1. we take a snapshot of the source version of a set every time
we commit.
 2. we refuse to install if we don't have the snapshot.
 3. we add s6-rc-format-upgrade to the processing chain
 4. for cleanup: after installing, we delete all the snapshots of
the set except the one that corresponds to the newly
installed live set.

 This commit implements 1: taking the snapshot. It's a regular
s6-rc-set-copy, using an internal name. We refactor
s6-rc-set-copy and s6-rc-set-delete to have primitives we can
use internally.

 Wait for later commits before using repo, else you'll fill
your repository with snapshots that won't get cleaned up.

 TBD: how to store the s6-rc version in the compiled db. We
probably need to add a small file in there.

1 files changed, 30 insertions, 0 deletions

diff --git a/src/repo/s6rc_repo_setdelete.c b/src/repo/s6rc_repo_setdelete.c
new file mode 100644
index 0000000..a372d93
--- /dev/null
+++ b/src/repo/s6rc_repo_setdelete.c
@@ -0,0 +1,30 @@
+/* ISC license. */
+
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include <skalibs/posixplz.h>
+#include <skalibs/djbunix.h>
+
+#include <s6-rc/repo.h>
+
+void s6rc_repo_setdelete (char const *repo, char const *set)
+{
+  size_t repolen = strlen(repo) ;
+  size_t setlen = strlen(set) ;
+  ssize_t r ;
+  char real[repolen + setlen + 18] ;
+  char fn[repolen + setlen + 11] ;
+  memcpy(fn, repo, repolen) ;
+  memcpy(fn + repolen, "/sources/", 9) ;
+  memcpy(fn + repolen + 9, set, setlen + 1) ;
+  if (access(fn, W_OK) == -1 && errno != ENOENT) return ;
+  memcpy(real, repo, repolen) ;
+  memcpy(real + repolen, "/sources/", 9) ;
+  r = readlink(fn, real + repolen + 9, setlen + 9) ;
+  if (r == -1 || r != setlen + 8) return ;
+  real[repolen + setlen + 17] = 0 ;
+  unlink_void(fn) ;
+  rm_rf(real) ;
+}