Use --keep-going to make the garbage collector continue when an error occurs

Currently, when an error occurs during garbage collection, the process
is aborted and the error is reported. This is not ideal, as it means
that the garbage collection is not completed and garbage will continue
to be accumulated.

Example:

```
error: cannot delete path '/nix/store/4haq90hy2jz3i9n359zpw3bfan9jm0ss-glib-networking-2.78.0' because it is in use by '/nix/store/94nz5lnlsi4ss53izwg1ikifk4bxrs6g-seahorse-43.0', '/nix/store/mfw6k5a1712zqd6q2h4h3ipmx3q0lb1n-geoclue-2.7.0'
```

In this commit, we reuse the existing option `--keep-going` to allow the
garbage collection to continue when an error occurs.

Co-authored-by: Ramses <ramses@well-founded.dev>

doc/manual/src/command-ref/nix-collect-garbage.md

@@ -4,7 +4,7 @@
 
 # Synopsis
 
-`nix-collect-garbage` [`--delete-old`] [`-d`] [`--delete-older-than` *period*] [`--max-freed` *bytes*] [`--dry-run`]
+`nix-collect-garbage` [`--delete-old`] [`-d`] [`--delete-older-than` *period*] [`--max-freed` *bytes*] [`--dry-run`] [`--keep-going`]
 
 # Description
 

doc/manual/src/command-ref/nix-store/gc.md

@@ -4,7 +4,7 @@
 
 # Synopsis
 
-`nix-store` `--gc` [`--print-roots` | `--print-live` | `--print-dead`] [`--max-freed` *bytes*]
+`nix-store` `--gc` [`--print-roots` | `--print-live` | `--print-dead`] [`--max-freed` *bytes*] [`--keep-going`]
 
 # Description
 

doc/manual/src/command-ref/opt-common.md

@@ -118,6 +118,7 @@ Most Nix commands accept the following command-line options:
   Keep going in case of failed builds, to the greatest extent possible.
   That is, if building an input of some derivation fails, Nix will still build the other inputs, but not the derivation itself.
   Without this option, Nix stops if any build fails (except for builds of substitutes), possibly killing builds in progress (in case of parallel or distributed builds).
+  In the context of garbage collection, this option indicates that the garbage collector should continue deleting garbage even if an error occurs while deleting a path.
 
 - <span id="opt-keep-failed">[`--keep-failed`](#opt-keep-failed)</span> / `-K`
 

src/libstore/gc.cc

@@ -1,6 +1,7 @@
 #include "derivations.hh"
 #include "globals.hh"
 #include "local-store.hh"
+#include "posix-fs-canonicalise.hh"
 #include "finally.hh"
 #include "unix-domain-socket.hh"
 #include "signals.hh"
@@ -781,9 +782,19 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         for (auto & path : topoSortPaths(visited)) {
             if (!dead.insert(path).second) continue;
             if (shouldDelete) {
-                invalidatePathChecked(path);
-                deleteFromStore(path.to_string());
-                referrersCache.erase(path);
+                try {
+                    invalidatePathChecked(path);
+                    deleteFromStore(path.to_string());
+                    referrersCache.erase(path);
+                } catch (PathInUse & e) {
+                    if (settings.keepGoing) {
+                        printError("warning: %s"
+                                   "Ignoring as `--keep-going` is used.",
+                                   e.msg());
+                    } else {
+                        throw;
+                    }
+                }
             }
         }
     };

src/nix/store-gc.md

@@ -14,6 +14,12 @@ R""(
   # nix store gc --max 1G
   ```
 
+* Keep deleting garbage even if an error occurs while deleting a path:
+
+  ```console
+  # nix store gc --keep-going
+  ```
+
 # Description
 
 This command deletes unreachable paths in the Nix store.