nix-env: Minor change to '--delete-generations Nd' semantics

The option '--delete-generations Nd' deletes all generations older than N
days. However, most likely the user does not want to delete the
generation that was active N days ago.

For example, say that you have these 3 generations:

1: <30 days ago>
2: <15 days ago>
3: <1 hour ago>

If you do --delete-generations 7d (say, as part of a cron job), most
likely you still want to keep generation 2, i.e. the generation that was
active 7 days ago (and for most of the past 7 days, in fact).

This patch fixes this issue. Note that this also affects
'nix-collect-garbage --delete-older-than Nd'.

Thanks to @roconnor for noticing the issue!

1 files changed, 10 insertions, 4 deletions

diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 8a80526347..3db84ff5c7 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -1315,11 +1315,17 @@ static void opDeleteGenerations(Globals & globals,
 
             oldTime = curTime - days * 24 * 3600;
 
-            for (Generations::iterator j = gens.begin(); j != gens.end(); ++j) {
-                if (j->number == curGen) continue;
-
-                if (j->creationTime < oldTime)
+            bool canDelete = false;
+            for (Generations::reverse_iterator j = gens.rbegin(); j != gens.rend(); ++j) {
+                if (canDelete) {
+                    assert(j->creationTime < oldTime);
                     deleteGeneration2(globals, j->number);
+                } else if (j->creationTime < oldTime) {
+                    /* We may now start deleting generations, but we don't delete
+                       this generation yet, because this generation was still the
+                       one that was active at the requested point in time. */
+                    canDelete = true;
+                }
             }
         } else {
             int n;