loader: build a loose RomFS once and only when it is read

Every loader of an extracted title built the whole RomFS from its romfs/
directory in its constructor, though most are made only for metadata
(the game list, title lookups), and each build keeps an object per file:
title 1's 206k files left four copies alive, ~0.6 GB of the heap. The
RomFS is now built on first read, and loaders of the same directory share
one build while any of them holds it.

(cherry picked from commit be47212cac130e6a895423644751c0525dc6f169)

2 files changed, 34 insertions, 5 deletions

diff --git a/src/core/loader/deconstructed_rom_directory.cpp b/src/core/loader/deconstructed_rom_directory.cpp
index cda1645548..04b05d188f 100644
--- a/src/core/loader/deconstructed_rom_directory.cpp
+++ b/src/core/loader/deconstructed_rom_directory.cpp
@@ -7,6 +7,9 @@
 #include <algorithm>
 #include <cctype>
 #include <cstring>
+#include <map>
+#include <memory>
+#include <mutex>
 #include <string_view>
 #include "common/logging.h"
 #include "common/random.h"
@@ -90,8 +93,19 @@ FileSys::VirtualFile FindLooseRomFS(const FileSys::VirtualDir& exefs_dir) {
     }
     if (const auto parent = exefs_dir->GetParentDirectory()) {
         if (const auto romfs_dir = parent->GetSubdirectory("romfs")) {
+            // Building it walks every file and keeps an object for each (title 1: 206k files, ~150
+            // MiB), so loaders of the same directory share one build while any of them holds it.
+            static std::mutex cache_mutex;
+            static std::map<std::string, std::weak_ptr<FileSys::VfsFile>> cache;
+            std::scoped_lock lk{cache_mutex};
+            auto& cached = cache[romfs_dir->GetFullPath()];
+            if (auto built = cached.lock()) {
+                return built;
+            }
             LOG_INFO(Loader, "Using RomFS directory next to the ExeFS");
-            return FileSys::CreateRomFS(romfs_dir);
+            auto built = FileSys::CreateRomFS(romfs_dir);
+            cached = built;
+            return built;
         }
     }
     return nullptr;
@@ -233,7 +247,7 @@ AppLoader_DeconstructedRomDirectory::AppLoader_DeconstructedRomDirectory(FileSys
         name = control->GetApplicationName();
     }
 
-    romfs = FindLooseRomFS(file_dir);
+    romfs_exefs_dir = file_dir;
     if (const auto title_dir = file_dir->GetParentDirectory()) {
         romfs_version = RomFSProductVersion(title_dir->GetSubdirectory("romfs"));
     }
@@ -245,7 +259,17 @@ AppLoader_DeconstructedRomDirectory::AppLoader_DeconstructedRomDirectory(
     , dir(std::move(directory))
     , override_update(override_update_)
 {
-    romfs = FindLooseRomFS(dir);
+    romfs_exefs_dir = dir;
+}
+
+// Loaders made only for metadata (the game list, title lookups) never need the RomFS, and several
+// of them were alive with a whole copy each.
+const FileSys::VirtualFile& AppLoader_DeconstructedRomDirectory::LooseRomFS() {
+    if (!romfs_resolved) {
+        romfs_resolved = true;
+        romfs = FindLooseRomFS(romfs_exefs_dir);
+    }
+    return romfs;
 }
 
 FileType AppLoader_DeconstructedRomDirectory::IdentifyType(const FileSys::VirtualFile& dir_file) {
@@ -304,7 +328,7 @@ AppLoader_DeconstructedRomDirectory::LoadResult AppLoader_DeconstructedRomDirect
 
     // Loose-ExeFS loads bypass the NCA/NSP loaders, which are the ones that normally apply
     // LayeredFS to the RomFS; apply it here so mods in user/load/<titleid>/ work too.
-    if (romfs != nullptr) {
+    if (LooseRomFS() != nullptr) {
         const FileSys::PatchManager patch_manager(metadata.GetTitleID(),
                                                   system.GetFileSystemController(),
                                                   system.GetContentProvider());
@@ -414,7 +438,7 @@ AppLoader_DeconstructedRomDirectory::LoadResult AppLoader_DeconstructedRomDirect
 }
 
 ResultStatus AppLoader_DeconstructedRomDirectory::ReadRomFS(FileSys::VirtualFile& out_dir) {
-    if (romfs == nullptr) {
+    if (LooseRomFS() == nullptr) {
         return ResultStatus::ErrorNoRomFS;
     }
 
diff --git a/src/core/loader/deconstructed_rom_directory.h b/src/core/loader/deconstructed_rom_directory.h
index b976b71ecb..db0120f4ad 100644
--- a/src/core/loader/deconstructed_rom_directory.h
+++ b/src/core/loader/deconstructed_rom_directory.h
@@ -58,8 +58,13 @@ public:
     ResultStatus ReadNSOModules(Modules& out_modules) override;
 
 private:
+    /// The loose RomFS next to the ExeFS, found (and built) on first use.
+    const FileSys::VirtualFile& LooseRomFS();
+
     FileSys::ProgramMetadata metadata;
     FileSys::VirtualFile romfs;
+    FileSys::VirtualDir romfs_exefs_dir; ///< Where LooseRomFS looks
+    bool romfs_resolved{};
     FileSys::VirtualDir dir;
 
     std::vector<u8> icon_data;