confd: dagger: Reduce memory usage

When reconfiguring the system many times (seen during regression
testing), the RAM usage of keeping all generations around starts to
add up.

A tmpfs file, no matter how small, will always consume at least one
full page (4k). So one generation of tiny setup scripts and log files
can end up locking a 1 MB (!) of memory.

Therefore: Once we're done with a generation, archive it in a
compressed tarball, which typically fits inside a single 4k
page. Limit the total number of archives to 10. This should give full
visibility into the history of a system in all normal situations,
while still allowing hammering systems with reconfig cycles in testing
scenarios.

src/confd/bin/dagger

@@ -104,7 +104,9 @@ do_abandon()
     [ "$next" ] && [ -d "$basedir/$next" ] \
 	|| abort "Next generation does not exist"
 
-    mv "$basedir/$next" "$basedir/$next-ABANDONED-$(date +%F-%T)"
+    tar caf "$basedir/$next-ABANDONED-$(date +%F-%t).tar.gz" "$basedir/$next"
+    rm -rf "$basedir/$next"
+
     rm "$basedir/next"
     inform info "Abandoned generation $next"
 }
@@ -128,6 +130,19 @@ do_evolve()
 
     mv "$basedir/next" "$basedir/current"
     inform info "Evolved to generation $next"
+
+    if [ "$current" ]; then
+       (cd "$basedir" && tar caf "$current.tar.gz" "$current")
+       rm -rf "$basedir/$current"
+    fi
+}
+
+do_prune()
+{
+    local keep=${1:-10}
+
+    cd "$basedir"
+    rm -f old=$(ls -rv *.tar.gz | tail "+$((keep + 1))")
 }
 
 usage()
@@ -158,6 +173,10 @@ Commands:
   exec <action>
     Run the specified action of the current generation.
 
+  prune [<num-generations>]
+    Remove all but the last <num-generations>, or 10 by default,
+    archived generations.
+
   help
     Show this message.
 
@@ -201,6 +220,9 @@ case $cmd in
     "exec")
 	do_exec "$@"
 	;;
+    "prune")
+	do_prune "$@"
+	;;
 
     *)
 	usage && exit 1

src/confd/src/dagger.c

@@ -114,11 +114,23 @@ int dagger_evolve(struct dagger *d)
 	return exitcode;
 }
 
+static int dagger_prune(struct dagger *d)
+{
+	int exitcode;
+
+	exitcode = systemf("dagger -C %s prune", d->path);
+	DEBUG("dagger(%d->%d): prune: exitcode=%d\n",
+	      d->current, d->next, exitcode);
+
+	return exitcode;
+}
+
 int dagger_evolve_or_abandon(struct dagger *d)
 {
 	int exitcode, err;
 
 	exitcode = dagger_evolve(d);
+	dagger_prune(d);
 	if (!exitcode)
 		return 0;