Fix dry-run flag for nix-collect-garbage

`nix-collect-garbage --dry-run` previously elided the entire garbage
collection check, meaning that it would just exit the script without
printing anything.

This change makes the dry run flag instead set the GC action to
`gcReturnDead` rather than `gcDeleteDead`, and then continue with the
script. So if you set `--dry-run`, it will print the paths it *would*
have garbage collected, but not actually delete them.

I filed a bug for this: https://git.lix.systems/lix-project/lix/issues/432 but then realised I could give fixing it a go myself.

Change-Id: I062dbf1a80bbab192b5fd0b3a453a0b555ad16f2

doc/manual/change-authors.yml

@@ -96,6 +96,10 @@ puck:
   forgejo: puck
   github: puckipedia
 
+quantumjump:
+  display_name: Quantum Jump
+  github: QuantumBJump
+
 r-vdp:
   github: r-vdp
 

doc/manual/rl-next/fix-gc-dry-run.md

@@ -0,0 +1,29 @@
+---
+synopsis: Fix nix-collect-garbage --dry-run
+issues: [fj#432]
+cls: [1566]
+category: Fixes
+credits: [quantumjump]
+---
+
+`nix-collect-garbage --dry-run` did not previously give any output - it simply
+exited without even checking to see what paths would be deleted.
+
+```
+$ nix-collect-garbage --dry-run
+$
+```
+
+We updated the behaviour of the flag such that instead it prints out how many
+paths it *would* delete, but doesn't actually delete them.
+
+```
+$ nix-collect-garbage --dry-run
+finding garbage collector roots...
+determining live/dead paths...
+...
+<nix store paths>
+...
+2670 store paths deleted, 0.00MiB freed
+$
+```

src/nix-collect-garbage/nix-collect-garbage.cc

@@ -89,12 +89,21 @@ static int main_nix_collect_garbage(int argc, char * * argv)
 
         // Run the actual garbage collector.
         if (!dryRun) {
-            auto store = openStore();
-            auto & gcStore = require<GcStore>(*store);
             options.action = GCOptions::gcDeleteDead;
-            GCResults results;
-            PrintFreed freed(true, results);
-            gcStore.collectGarbage(options, results);
+        } else {
+            options.action = GCOptions::gcReturnDead;
+        }
+        auto store = openStore();
+        auto & gcStore = require<GcStore>(*store);
+        GCResults results;
+        PrintFreed freed(true, results);
+        gcStore.collectGarbage(options, results);
+
+        if (dryRun) {
+            // Only print results for dry run; when !dryRun, paths will be printed as they're deleted.
+            for (auto & i : results.paths) {
+                printInfo("%s", i);
+            }
         }
 
         return 0;

tests/functional/meson.build

@@ -73,6 +73,7 @@ functional_tests_scripts = [
   'flakes/flake-in-submodule.sh',
   'gc.sh',
   'nix-collect-garbage-d.sh',
+  'nix-collect-garbage-dry-run.sh',
   'remote-store.sh',
   'legacy-ssh-store.sh',
   'lang.sh',

tests/functional/nix-collect-garbage-dry-run.sh

@@ -0,0 +1,40 @@
+source common.sh
+
+clearStore
+
+## Test `nix-collect-garbage --dry-run`
+
+
+testCollectGarbageDryRun () {
+    clearProfiles
+    # Install then uninstall a package
+    # This should leave packages ready to be garbage collected.
+    nix-env -f ./user-envs.nix -i foo-1.0
+    nix-env -f ./user-envs.nix -e foo-1.0
+
+
+    nix-env --delete-generations old
+    [[ $(nix-store --gc --print-dead | wc -l) -eq 7 ]]
+
+    nix-collect-garbage --dry-run
+    [[ $(nix-store --gc --print-dead | wc -l) -eq 7 ]]
+
+}
+
+testCollectGarbageDryRun
+
+# Run the same test, but forcing the profiles an arbitrary location.
+rm ~/.nix-profile
+ln -s $TEST_ROOT/blah ~/.nix-profile
+testCollectGarbageDryRun
+
+# Run the same test, but forcing the profiles at their legacy location under
+# /nix/var/nix.
+#
+# Note that we *don't* use the default profile; `nix-collect-garbage` will
+# need to check the legacy conditional unconditionally not just follow
+# `~/.nix-profile` to pass this test.
+#
+# Regression test for #8294
+rm ~/.nix-profile
+testCollectGarbageDryRun --profile "$NIX_STATE_DIR/profiles/per-user/me"