getUserDirectory: return libretro Save Directory instead of $USER env

 - Fixes a mem corruption bug: adding a trailing slash was corrupting the env table of the process
 - converted implementation to use `lual_dostring` thus performing non-trivial string operations within the safe GC'd confines of Lua

getUserDirectory returns nil if the libretro frontend doesn't support it. This is done rather than using a substitution because there is no suitable substitution that is guaranteed to be writable. It's better to fail on returning a valid path than to fail in unexpected ways later on when trying to write data to the directory. The love/lutro game app will need to check for hand handle such cases appropriately if it wants to be well-behaved running under libretro.

It occurs to me that it could be swell if there was an IDE and/or C preprocessor that allowed embedding lua into C code. Something similar perhaps to how one can embed PHP code inside HTML, as an example.

filesystem.c

@@ -13,6 +13,18 @@
 #include <stdlib.h>
 #include <string.h>
 
+int _raw_get_user_writable_dir(lua_State *L)
+{
+   char* savedir;
+   if ((*settings.environ_cb)(RETRO_ENVIRONMENT_GET_SAVE_DIRECTORY, &savedir)) {
+      lua_pushstring(L, savedir);
+      return 1;
+   }
+
+   // in the case that no savedir is available: return nil rather than seeking a fallback.
+   return 0;
+}
+
 int lutro_filesystem_preload(lua_State *L)
 {
    static const luaL_Reg fs_funcs[] =  {
@@ -23,17 +35,76 @@ int lutro_filesystem_preload(lua_State *L)
       { "getRequirePath", fs_getRequirePath },
       { "load",        fs_load },
       { "setIdentity", fs_setIdentity },
-      { "getUserDirectory", fs_getUserDirectory },
       { "getAppdataDirectory", fs_getAppdataDirectory },
       { "isDirectory", fs_isDirectory },
       { "isFile",      fs_isFile },
       { "createDirectory", fs_createDirectory },
       { "getDirectoryItems", fs_getDirectoryItems },
+
+      // internal helpers for lua-authored functions.
+      { "_raw_get_user_writable_dir", _raw_get_user_writable_dir },
       {NULL, NULL}
    };
 
    lutro_newlib(L, fs_funcs, "filesystem");
 
+   // lutro.filesystem._appendTrailingSlash - appends a trailing slash to match behavior of LOVE.
+   // Because slash or backslash is platform dependent, this implementation takes the laziest
+   // approach: if the incoming path has a trailing backslash then it checks if the OS environment
+   // is Windows (WINDIR) and if yes then it doesn't append a trailing slash. It does not attempt
+   // to append trialing backslashes. Windows tolerates forward slash and mixed-slash paths well.
+   if (1) {
+      int ret = luaL_dostring(L, "\
+      lutro.filesystem._appendTrailingSlash = function(path)\n\
+         if path:sub(-1) == '\\\\' then\n\
+            local winDir = os.getenv('WINDIR')\n\
+            if winDir ~= nil and winDir ~= '' then\n\
+               path = path .. '/'\n\
+            end\n\
+         end\n\
+         if path:sub(-1) ~= '/' then\n\
+            path = path .. '/'\n\
+         end\n\
+         return path\n\
+      end");
+
+      if (ret) {
+         // don't hard-fail - no need to block apps that might not use the function.
+         lutro_errorf("failed to assign lutro.filesystem.getUserDirectory: %s\n", lua_tostring(L, -1));
+         lua_pop(L, 1); // pop error message
+      }
+   }
+
+   // lutro.filesystem.getUserDirectory
+   // 
+   // Returns the writable save directory provided by libretro frontend. The directory
+   // may or may not be sandboxed according to a current user, depending on the design
+   // and capabilities of the platform OS. Funtion may return nil if the libretro frontend
+   // does not provide a writable save directory.
+   //
+   // implementation notes:
+   //  - UserDirectory should always have a trailing slash (as confirmed on Love2D)
+   //  - Cache the UserDir to avoid costly re-calculation of env var lookups. UserDir and all env vars
+   //    are static for the lifetime of the process.
+   //  - https://love2d.org/wiki/love.filesystem.getUserDirectory
+
+   if (1) {
+      int ret = luaL_dostring(L, "\
+      lutro.filesystem.getUserDirectory = function()\n\
+         local savedir = lutro.filesystem._raw_get_user_writable_dir()\n\
+         if savedir == nil or savedir == '' then\n\
+            return nil\n\
+         end\n\
+         return lutro.filesystem._appendTrailingSlash(savedir)\n\
+      end");
+
+      if (ret) {
+         // don't hard-fail - no need to block apps that might not use the function.
+         lutro_errorf("failed to assign lutro.filesystem.getUserDirectory: %s\n", lua_tostring(L, -1));
+         lua_pop(L, 1); // pop error message
+      }
+   }
+
    return 1;
 }
 
@@ -201,35 +272,6 @@ int fs_setIdentity(lua_State *L)
    return 0;
 }
 
-/**
- * lutro.filesystem.getUserDirectory
- *
- * https://love2d.org/wiki/love.filesystem.getUserDirectory
- */
-int fs_getUserDirectory(lua_State *L)
-{
-   // Retrieve the user's home directory from environment variables.
-   char *homedir = getenv("HOME");
-   if (homedir == NULL) {
-      homedir = getenv("HOMEDRIVE");
-      if (homedir == NULL) {
-         // TODO: Figure out what to do when HOME isn't available.
-         homedir = ".";
-      }
-   }
-
-   // If needed, append a / at the end of the homedir.
-   size_t len = strlen(homedir);
-   if (homedir[len] != '/') {
-      homedir[len++] = '/';
-      homedir[len] = '\0';
-   }
-
-   lua_pushstring(L, homedir);
-
-   return 1;
-}
-
 /**
  * lutro.filesystem.getAppdataDirectory
  *

filesystem.h

@@ -21,8 +21,8 @@ int fs_setIdentity(lua_State *L);
 int fs_isFile(lua_State *L);
 int fs_isDirectory(lua_State *L);
 int fs_createDirectory(lua_State *L);
-int fs_getUserDirectory(lua_State *L);
 int fs_getAppdataDirectory(lua_State *L);
+int fs_getWorkingDirectory(lua_State *L);
 int fs_getDirectoryItems(lua_State *L);
 
 #endif // FILESYSTEM_H

test/unit/modules/filesystem.lua

@@ -20,10 +20,15 @@ function lutro.filesystem.getAppdataDirectoryTest()
 end
 
 function lutro.filesystem.getUserDirectoryTest()
-    local homeDir = lutro.filesystem.getUserDirectory()
-    -- @todo Find out how to make os.getenv('HOME') work on Windows?
-    local luaHomeDir = os.getenv("HOME")
-    unit.assertEquals(homeDir, luaHomeDir)
+    -- UserDirectory should always have a trailing slash. This is easy to verify.
+    -- Other aspects of UserDirectory are platform specific and non-trivial to calculate and there
+    -- isn't much value to trying to replicate it here.
+    local userDir = lutro.filesystem.getUserDirectory()
+    local userDirFixed = getUserDir
+    if userDirFixed:sub(-1) ~= '/' then
+        userDirFixed = userDirFixed .. '/'
+    end
+    unit.assertEquals(userDir, userDirFixed)
 end
 
 function lutro.filesystem.getDirectoryItemsTest()