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, 66 insertions, 0 deletions

diff --git a/src/repo/s6rc_repo_setcopy.c b/src/repo/s6rc_repo_setcopy.c
new file mode 100644
index 0000000..30cb042
--- /dev/null
+++ b/src/repo/s6rc_repo_setcopy.c
@@ -0,0 +1,66 @@
+/* ISC license. */
+
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include <skalibs/posixplz.h>
+#include <skalibs/djbunix.h>
+#include <skalibs/unix-transactional.h>
+
+#include <s6-rc/repo.h>
+
+static void cleanup (char const *s)
+{
+  int e = errno ;
+  rm_rf(s) ;
+  errno = e ;
+}
+
+int s6rc_repo_setcopy (char const *repo, char const *srcname, char const *dstname, unsigned int flags)
+{
+  size_t repolen = strlen(repo) ;
+  size_t srclen = strlen(srcname) ;
+  size_t dstlen = strlen(dstname) ;
+  size_t r ;
+  char src[repolen + srclen + 10] ;
+  char dst[repolen + dstlen + 10] ;
+  char realsrc[repolen + srclen + 18] ;
+  char realdst[repolen + dstlen + 18] ;
+  char olddst[repolen + dstlen + 18] ;
+  memcpy(src, repo, repolen) ;
+  memcpy(src + repolen, "/sources/", 9) ;
+  memcpy(src + repolen + 9, srcname, srclen + 1) ;
+  memcpy(dst, src, repolen + 9) ;
+  memcpy(dst + repolen + 9, dstname, dstlen + 1) ;
+  memcpy(olddst, dst, repolen + 9) ;
+  olddst[repolen + 9] = 0 ;
+  if (access(dst, F_OK) == -1)
+  {
+    if (errno != ENOENT) return errno ;
+  }
+  else if (!(flags & 1)) return -1 ;
+
+  memcpy(realsrc, src, repolen + 9) ;
+  r = readlink(src, realsrc + repolen + 9, srclen + 9) ;
+  if (r == -1) return errno ;
+  if (r != srclen + 8) return -2 ;
+  realsrc[repolen + srclen + 17] = 0 ;
+  memcpy(realdst, dst, repolen + 9) ;
+  realdst[repolen + 9] = '.' ;
+  memcpy(realdst + repolen + 10, dstname, dstlen) ;
+  memcpy(realdst + repolen + 10 + dstlen, ":XXXXXX", 8) ;
+  if (mkntemp(realdst) == -1) return errno ;
+  if (!hiercopy(realsrc, realdst))
+  {
+    cleanup(realdst) ;
+    return errno ;
+  }
+  if (!atomic_symlink4(realdst + repolen + 9, dst, olddst + repolen + 9, dstlen + 9))
+  {
+    cleanup(realdst) ;
+    return errno ;
+  }
+  if (olddst[repolen + 9]) cleanup(olddst) ;
+  return 0 ;
+}