populate: add prune

Populate never removes what it delivered for sources that aren't
configured anymore.  For flake and flake-input sources, that keeps their
garbage collector roots, so what they delivered never gets collected.

With prune, populate removes everything from the target directory that
isn't a source, except .populate, after syncing the sources.  It also
removes the roots in .populate/flake-inputs of names that aren't flake or
flake-input sources, and uploads left behind by interrupted runs.
Pruning is opt-in, because the target directory may contain other things
worth keeping, or be shared by deployments with different sources.

writeDeploy, writeTest, and writeCommand pass prune on to populate.

A feature scenario deploys sources, deploys fewer without pruning, then
with pruning, and checks that the removed sources' entries and roots are
gone, while the remaining sources and .populate stay.

1 files changed, 51 insertions, 2 deletions

diff --git a/tests/scenarios.nix b/tests/scenarios.nix
index 22431a2..c9774bf 100644
--- a/tests/scenarios.nix
+++ b/tests/scenarios.nix
@@ -64,10 +64,10 @@ let
     path = "/var/krops/${name}";
     target = targets.${targetKind} path;
     vars = "T=${path}\n";
-    phase = { sources, expect ? "success", check ? "", mutate ? null }: let
+    phase = { sources, prune ? false, expect ? "success", check ? "", mutate ? null }: let
       command = populate {
         source = krops.evalSource [ sources ];
-        inherit target;
+        inherit prune target;
         force = true;
       };
       evaluated = builtins.tryEval (builtins.seq (toString command) true);
@@ -480,6 +480,55 @@ in {
         }
       ];
     })
+    # Pruning removes everything sources don't deliver anymore, including
+    # garbage collector roots, and keeps everything else.
+    (s "prune" (let
+      sources = {
+        a.file = "/fixtures/alt";
+        b.flake = { flake = "/fixtures/top"; inherit fetchOnTarget; };
+        c.flake-input = { flake = "/fixtures/top"; name = "dep"; inherit fetchOnTarget; };
+        d.symlink = "a";
+      };
+    in {
+      prepare = ''
+        mkdir -p "$T"
+        echo other > "$T"/other
+      '';
+      phases = [
+        {
+          inherit sources;
+          check = ''
+            test -e "$T"/.populate/flake-inputs/b || fail "roots of b missing"
+            test -e "$T"/.populate/flake-inputs/c || fail "roots of c missing"
+          '';
+        }
+        {
+          # Without pruning, nothing gets removed.
+          sources = { inherit (sources) a; };
+          check = ''
+            for name in b c d other .populate/flake-inputs/b .populate/flake-inputs/c; do
+              test -e "$T"/$name || fail "$name removed"
+            done
+          '';
+        }
+        {
+          # b changes its type, so its roots go, while b stays.
+          sources = { inherit (sources) a; b.symlink = "a"; };
+          prune = true;
+          check = ''
+            test -e "$T"/a/flake.nix || fail "a removed"
+            expect_eq b "$(readlink "$T"/b)" a
+            test -d "$T"/.populate || fail ".populate removed"
+            for name in c d other .populate/flake-inputs; do
+              test ! -e "$T"/$name || fail "$name not pruned"
+            done
+            if nix-store --gc --print-roots 2>/dev/null | grep -F "$T/"; then
+              fail "garbage collector roots remain"
+            fi
+          '';
+        }
+      ];
+    }))
     (s "cycle" {
       phases = [{
         sources = {