fs.mitm: add and use memlet module to temporarily steal applet memory while building romfs images.

Starting in 20.0.0, the browser needs more applet memory to function, so we can't steal as much any more.
Thus, we now steal 14 MB on 20.0.0+ instead of 40MB.

However, since this reduces memory available for custom system modules, we are adjusting to compensate.
ams.mitm's heap size has been reduced from 32MB to 12MB (recovering 20MB).
In addition, fs.mitm now uses a new mechanism for stealing memory from the applet pool while romfs is being built.

On net, we are compromising:
    * Custom sysmodules lose memory available to them.
        On 19.0.0/AMS 1.8.0, there was 30 MB available for custom sysmodules.
        Stealing 14 MB instead of 40 MB, we lose 26 MB of that. Reducing ams.mitm's usage will gain us back 20.
        Nintendo also appears to...use 4 extra MB, in 20.0.0, from my test homebrew.
        So on 20.0.0/AMS 1.9.0, there should be 20 MB available for custom sysmodules.
        On the bright side, on <20.0.0/AMS 1.9.0, I guess there will be 50 MB available for custom sysmodules now?
    * totk mods will lose the ability to...put every file in the romfs on sd card. There will be some unknown maximum filecount for totk mods.
        On the bright side, implementing the transient memory stealing should improve compatibility for some mods which strictly add files?

stratosphere/ams_mitm/source/fs_mitm/memlet/memlet.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#define NX_SERVICE_ASSUME_NON_DOMAIN
+#include "service_guard.h"
+#include "memlet.h"
+
+static Service g_memletSrv;
+
+NX_GENERATE_SERVICE_GUARD(memlet);
+
+Result _memletInitialize(void) {
+    return smGetService(&g_memletSrv, "memlet");
+}
+
+void _memletCleanup(void) {
+    serviceClose(&g_memletSrv);
+}
+
+Service* memletGetServiceSession(void) {
+    return &g_memletSrv;
+}
+
+Result memletCreateAppletSharedMemory(Handle *out_shmem_h, u64 *out_size, u64 desired_size) {
+    return serviceDispatchInOut(&g_memletSrv, 65000, desired_size, *out_size,
+        .out_handle_attrs = { SfOutHandleAttr_HipcMove },
+        .out_handles = out_shmem_h,
+    );
+}

stratosphere/ams_mitm/source/fs_mitm/memlet/memlet.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+#include <switch.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+Result memletInitialize(void);
+void memletExit(void);
+Service* memletGetServiceSession(void);
+
+Result memletCreateAppletSharedMemory(Handle *out_shmem_h, u64 *out_size, u64 desired_size);
+
+#ifdef __cplusplus
+}
+#endif

stratosphere/ams_mitm/source/fs_mitm/memlet/service_guard.h

@@ -0,0 +1,65 @@
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <switch/types.h>
+#include <switch/result.h>
+#include <switch/kernel/mutex.h>
+#include <switch/sf/service.h>
+#include <switch/services/sm.h>
+
+typedef struct ServiceGuard {
+    Mutex mutex;
+    u32 refCount;
+} ServiceGuard;
+
+NX_INLINE bool serviceGuardBeginInit(ServiceGuard* g)
+{
+    mutexLock(&g->mutex);
+    return (g->refCount++) == 0;
+}
+
+NX_INLINE Result serviceGuardEndInit(ServiceGuard* g, Result rc, void (*cleanupFunc)(void))
+{
+    if (R_FAILED(rc)) {
+        cleanupFunc();
+        --g->refCount;
+    }
+    mutexUnlock(&g->mutex);
+    return rc;
+}
+
+NX_INLINE void serviceGuardExit(ServiceGuard* g, void (*cleanupFunc)(void))
+{
+    mutexLock(&g->mutex);
+    if (g->refCount && (--g->refCount) == 0)
+        cleanupFunc();
+    mutexUnlock(&g->mutex);
+}
+
+#define NX_GENERATE_SERVICE_GUARD_PARAMS(name, _paramdecl, _parampass) \
+\
+static ServiceGuard g_##name##Guard; \
+NX_INLINE Result _##name##Initialize _paramdecl; \
+static void _##name##Cleanup(void); \
+\
+Result name##Initialize _paramdecl \
+{ \
+    Result rc = 0; \
+    if (serviceGuardBeginInit(&g_##name##Guard)) \
+        rc = _##name##Initialize _parampass; \
+    return serviceGuardEndInit(&g_##name##Guard, rc, _##name##Cleanup); \
+} \
+\
+void name##Exit(void) \
+{ \
+    serviceGuardExit(&g_##name##Guard, _##name##Cleanup); \
+}
+
+#define NX_GENERATE_SERVICE_GUARD(name) NX_GENERATE_SERVICE_GUARD_PARAMS(name, (void), ())
+
+#ifdef __cplusplus
+}
+#endif