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

1 files changed, 29 insertions, 0 deletions

diff --git a/doc/manual/rl-next/fix-gc-dry-run.md b/doc/manual/rl-next/fix-gc-dry-run.md
new file mode 100644
index 000000000..6095d7727
--- /dev/null
+++ b/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
+$
+```