nix store gc: account for auto-optimised store

Before the change on a system with `auto-optimise-store = true`:

    $ nix store gc --verbose --max 1

deleted all the paths instead of one path (we requested 1 byte limit).

It happens because every file in `auto-optimise-store = true` has at
least 2 links: file itself and a link in /nix/store/.links/ directory.

The change conservatively assumes that any file that has one (as before)
or two links (assume auto-potimise mode) will free space.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

src/libstore/gc.cc

@@ -841,7 +841,8 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
             if (unlink(path.c_str()) == -1)
                 throw SysError("deleting '%1%'", path);
 
-            results.bytesFreed += st.st_size;
+            /* Do not accound for deleted file here. Rely on deletePath()
+               accounting.  */
         }
 
         struct stat st;

src/libutil/util.cc

@@ -406,8 +406,29 @@ static void _deletePath(int parentfd, const Path & path, uint64_t & bytesFreed)
         throw SysError("getting status of '%1%'", path);
     }
 
-    if (!S_ISDIR(st.st_mode) && st.st_nlink == 1)
-        bytesFreed += st.st_size;
+    if (!S_ISDIR(st.st_mode)) {
+        /* We are about to delete a file. Will it likely free space? */
+
+        switch (st.st_nlink) {
+            /* Yes: last link. */
+            case 1:
+                bytesFreed += st.st_size;
+                break;
+            /* Maybe: yes, if 'auto-optimise-store' or manual optimisation
+               was performed. Instead of checking for real let's assume
+               it's an optimised file and space will be freed.
+
+               In worst case we will double count on freed space for files
+               with exactly two hardlinks for unoptimised packages.
+             */
+            case 2:
+                bytesFreed += st.st_size;
+                break;
+            /* No: 3+ links. */
+            default:
+                break;
+        }
+    }
 
     if (S_ISDIR(st.st_mode)) {
         /* Make the directory accessible. */