fix: improve GC error when manifest is out of sync with server.conf (#4115)

At the weekend I ran GC on lhcb.cern.ch which normally has
`CVMFS_GARBAGE_COLLECTION=false`. While I figured it out, I think the
error messages could be clearer about what is wrong so I'm making this
PR to suggest an improvement. Feel free to close if you'd leave things
as-is or handle it a different way.

See also: cvmfs/doc-cvmfs#258

Author: chrisburr

cvmfs/server/cvmfs_server_gc.sh

@@ -164,6 +164,12 @@ cvmfs_server_gc() {
     if [ $reconstruct_reflog -eq 1 ]; then reflog_reconstruct_msg="yes"; fi
 
     echo "Affected Repositories:         $names"
+    for _gc_name in $names; do
+      load_repo_config $_gc_name
+      if [ x"$CVMFS_GARBAGE_COLLECTION" != x"true" ]; then
+        echo "  WARNING: CVMFS_GARBAGE_COLLECTION is not enabled for $_gc_name"
+      fi
+    done
     echo "Dry Run (no actual deletion):  $dry_run_msg"
     echo "Needs Reflog reconstruction:   $reflog_reconstruct_msg"
     if [ $preserve_revisions -ge 0 ]; then
@@ -318,6 +324,11 @@ __do_gc_cmd()
       return 0
     fi
     is_garbage_collectable $name || die "Garbage Collection is not enabled for $name"
+    if is_stratum0 $name; then
+      if [ x"$(get_repo_info_from_url $CVMFS_STRATUM0 -g)" != x"yes" ]; then
+        die "Garbage collection is enabled in server.conf but not yet in the repository manifest. Run 'cvmfs_server transaction $name && cvmfs_server publish $name' to update the manifest, then retry."
+      fi
+    fi
     is_owner_or_root       $name || die "Permission denied: Repository $name is owned by $user"
 
     # figure out the URL of the repository

cvmfs/swissknife_gc.cc

@@ -121,7 +121,10 @@ int CommandGc::Main(const ArgumentList &args) {
 
   if (!manifest->garbage_collectable()) {
     LogCvmfs(kLogCvmfs, kLogStderr,
-             "repository does not allow garbage collection");
+             "repository does not allow garbage collection "
+             "(manifest does not have the garbage collection flag set; "
+             "ensure CVMFS_GARBAGE_COLLECTION=true in server.conf and "
+             "run a transaction + publish to update the manifest)");
     return 1;
   }