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, 23 insertions, 0 deletions

diff --git a/src/repo/s6rc_repo_compile.c b/src/repo/s6rc_repo_compile.c
index 64f8879..83fa744 100644
--- a/src/repo/s6rc_repo_compile.c
+++ b/src/repo/s6rc_repo_compile.c
@@ -22,6 +22,7 @@ int s6rc_repo_compile (char const *repo, char const *set, char const *const *rxs
   size_t setlen = strlen(set) ;
   size_t totrxlen = 0 ;
   int needprefix = strcmp(set, ".ref") ;
+  char snap[repolen + setlen + 37] ;
   char newc[repolen + setlen + 45] ;
   memcpy(newc, repo, repolen) ;
   memcpy(newc + repolen, "/compiled/.", 11) ;
@@ -89,6 +90,26 @@ int s6rc_repo_compile (char const *repo, char const *set, char const *const *rxs
       return (WEXITSTATUS(wstat) == 1) - 1 ;
     }
   }
+
+  if (needprefix)
+  {
+    int e ;
+    memcpy(snap, ".snapshot:", 10) ;
+    memcpy(snap + 10, newc + repolen + 11, setlen + 26) ;
+    snap[repolen + setlen + 36] = 0 ;
+    e = s6rc_repo_setcopy(repo, set, snap, 1) ;
+    switch (e)
+    {
+      case -2 :
+        strerr_warnf("internal layout error while taking a snapshot of set ", set) ;
+        return (errno = EILSEQ, -1) ;
+      case 0 : break ;
+      default :
+        strerr_warnfusys("take a snapshot of set ", set) ;
+        return (errno = e, -1) ;
+    }
+  }
+
   {
     char fn[repolen + setlen + 11] ;
     memcpy(fn, newc, repolen + 10) ;
@@ -97,10 +118,12 @@ int s6rc_repo_compile (char const *repo, char const *set, char const *const *rxs
     {
       int e = errno ;
       rm_rf(newc) ;
+      s6rc_repo_setdelete(repo, snap) ;
       errno = e ;
       strerr_warnfu4sys("atomically symlink ", newc + repolen + 10, " to ", fn) ;
       return -1 ;
     }
   }
+
   return 1 + !!oldc[repolen + 10] ;
 }