Use shared pointers in the memory source accessor

This allows aliasing, like hard links.

src/libutil-tests/git.cc

@@ -233,30 +233,30 @@ TEST_F(GitTest, both_roundrip)
         .contents{
             {
                 "foo",
-                File::Regular{
+                make_ref<File>(File::Regular{
                     .contents = "hello\n\0\n\tworld!",
-                },
+                }),
             },
             {
                 "bar",
-                File::Directory{
+                make_ref<File>(File::Directory{
                     .contents =
                         {
                             {
                                 "baz",
-                                File::Regular{
+                                make_ref<File>(File::Regular{
                                     .executable = true,
                                     .contents = "good day,\n\0\n\tworld!",
-                                },
+                                }),
                             },
                             {
                                 "quux",
-                                File::Symlink{
+                                make_ref<File>(File::Symlink{
                                     .target = "/over/there",
-                                },
+                                }),
                             },
                         },
-                },
+                }),
             },
         },
     };

src/libutil/include/nix/util/memory-source-accessor.hh

@@ -35,7 +35,7 @@ struct MemorySourceAccessor : virtual SourceAccessor
         {
             using Name = std::string;
 
-            std::map<Name, File, std::less<>> contents;
+            std::map<Name, ref<File>, std::less<>> contents;
 
             bool operator==(const Directory &) const noexcept;
             // TODO libc++ 16 (used by darwin) missing `std::map::operator <=>`, can't do yet.
@@ -89,13 +89,21 @@ struct MemorySourceAccessor : virtual SourceAccessor
     SourcePath addFile(CanonPath path, std::string && contents);
 };
 
-inline bool MemorySourceAccessor::File::Directory::operator==(
+inline bool
-    const MemorySourceAccessor::File::Directory &) const noexcept = default;
+MemorySourceAccessor::File::Directory::operator==(const MemorySourceAccessor::File::Directory & other) const noexcept
+{
+    return std::ranges::equal(contents, other.contents, [](const auto & lhs, const auto & rhs) -> bool {
+        return lhs.first == rhs.first && *lhs.second == *rhs.second;
+    });
+};
 
 inline bool
 MemorySourceAccessor::File::Directory::operator<(const MemorySourceAccessor::File::Directory & other) const noexcept
 {
-    return contents < other.contents;
+    return std::ranges::lexicographical_compare(
+        contents, other.contents, [](const auto & lhs, const auto & rhs) -> bool {
+            return lhs.first < rhs.first && *lhs.second < *rhs.second;
+        });
 }
 
 inline bool MemorySourceAccessor::File::operator==(const MemorySourceAccessor::File &) const noexcept = default;

src/libutil/memory-source-accessor.cc

@@ -24,11 +24,11 @@ MemorySourceAccessor::File * MemorySourceAccessor::open(const CanonPath & path,
                     i,
                     {
                         std::string{name},
-                        File::Directory{},
+                        make_ref<File>(File::Directory{}),
                     });
             }
         }
-        cur = &i->second;
+        cur = &*i->second;
     }
 
     if (newF && create)
@@ -92,7 +92,7 @@ MemorySourceAccessor::DirEntries MemorySourceAccessor::readDirectory(const Canon
     if (auto * d = std::get_if<File::Directory>(&f->raw)) {
         DirEntries res;
         for (auto & [name, file] : d->contents)
-            res.insert_or_assign(name, file.lstat().type);
+            res.insert_or_assign(name, file->lstat().type);
         return res;
     } else
         throw Error("file '%s' is not a directory", path);