Refactor/extend BKE API to get special user directories

The previous `BKE_appdir_folder_default()` was confusing, it would return the
home directory on Linux and macOS, but the Documents directory on Windows.
Plus, for the Asset Browser, we want to use the Documents directory for the
default asset library on all platforms.
This attempts to clean up the API to avoid confusion, while adding the newly
needed functionality.

* `BKE_appdir_folder_default()` should behave as before, but the implementation
  changed:
** Removes apparently incorrect usage of `XDG_DOCUMENTS_DIR` on Unix systems -
   this seems to be a config file variable, not an environment variable. Always
   use `$HOME` instead, which this ended up using anyway.
** On Windows it doesn't attempt to use `%HOME%` anymore and gets the Documents
   directory directly.
* Add `BKE_appdir_folder_home()` to gives the top-level user directory on all
  platforms.
* Add `BKE_appdir_folder_documents()` to always get the user documents
  directory on all platforms.

There should be no user noticable behavior change.

Differential Revision: https://developer.blender.org/D9800

Reviewed by: Brecht Van Lommel

source/blender/blenkernel/BKE_appdir.h

@@ -31,6 +31,8 @@ void BKE_appdir_exit(void);
 /* note on naming: typical _get() suffix is omitted here,
  * since its the main purpose of the API. */
 const char *BKE_appdir_folder_default(void);
+const char *BKE_appdir_folder_home(void);
+bool BKE_appdir_folder_documents(char *dir);
 bool BKE_appdir_folder_id_ex(const int folder_id,
                              const char *subfolder,
                              char *path,

source/blender/blenkernel/intern/appdir.c

@@ -36,6 +36,8 @@
 #include "BKE_appdir.h" /* own include */
 #include "BKE_blender_version.h"
 
+#include "BLT_translation.h"
+
 #include "GHOST_Path-api.h"
 
 #include "MEM_guardedalloc.h"
@@ -146,49 +148,76 @@ static char *blender_version_decimal(const int version)
  * \{ */
 
 /**
- * This is now only used to really get the user's default document folder.
+ * Get the folder that's the "natural" starting point for browsing files on an OS. On Unix that is
+ * $HOME, on Windows it is %userprofile%/Documents.
  *
- * \note On Windows `Users/{MyUserName}/Documents` is used
- * as it's the default location to save documents.
+ * \note On Windows `Users/{MyUserName}/Documents` is used as it's the default location to save
+ *       documents.
  */
 const char *BKE_appdir_folder_default(void)
 {
 #ifndef WIN32
-  const char *const xdg_documents_dir = BLI_getenv("XDG_DOCUMENTS_DIR");
-
-  if (xdg_documents_dir) {
-    return xdg_documents_dir;
-  }
-
   return BLI_getenv("HOME");
 #else  /* Windows */
   static char documentfolder[MAXPATHLEN];
-  HRESULT hResult;
 
-  /* Check for `%HOME%` environment variable. */
-  if (uput_getenv("HOME", documentfolder, MAXPATHLEN)) {
-    if (BLI_is_dir(documentfolder)) {
-      return documentfolder;
-    }
-  }
-
-  /* Add user profile support for WIN 2K / NT.
-   * This is `%APPDATA%`, which translates to either:
-   * - `%USERPROFILE%\Application Data` or...
-   * - `%USERPROFILE%\AppData\Roaming` (since Vista).
-   */
-  hResult = SHGetFolderPath(NULL, CSIDL_PERSONAL, NULL, SHGFP_TYPE_CURRENT, documentfolder);
-
-  if (hResult == S_OK) {
-    if (BLI_is_dir(documentfolder)) {
-      return documentfolder;
-    }
+  if (BKE_appdir_folder_documents(documentfolder)) {
+    return documentfolder;
   }
 
   return NULL;
 #endif /* WIN32 */
 }
 
+/**
+ * Get the user's home directory, i.e. $HOME on UNIX, %userprofile% on Windows.
+ */
+const char *BKE_appdir_folder_home(void)
+{
+#ifndef WIN32
+  return BLI_getenv("HOME");
+#else /* Windows */
+  return BLI_getenv("userprofile");
+#endif
+}
+
+/**
+ * Get the user's document directory, i.e. $HOME/Documents on Linux, %userprofile%/Documents on
+ * Windows. If this can't be found using OS queries (via Ghost), try manually finding it.
+ *
+ * \returns True if the path is valid and points to an existing directory.
+ */
+bool BKE_appdir_folder_documents(char *dir)
+{
+  dir[0] = '\0';
+
+  const char *documents_path = (const char *)GHOST_getUserSpecialDir(
+      GHOST_kUserSpecialDirDocuments);
+
+  /* Usual case: Ghost gave us the documents path. We're done here. */
+  if (documents_path && BLI_is_dir(documents_path)) {
+    BLI_strncpy(dir, documents_path, FILE_MAXDIR);
+    return true;
+  }
+
+  /* Ghost couldn't give us a documents path, let's try if we can find it ourselves.*/
+
+  const char *home_path = BKE_appdir_folder_home();
+  if (!home_path || !BLI_is_dir(home_path)) {
+    return false;
+  }
+
+  char try_documents_path[FILE_MAXDIR];
+  /* Own attempt at getting a valid Documents path. */
+  BLI_path_join(try_documents_path, sizeof(try_documents_path), home_path, N_("Documents"), NULL);
+  if (!BLI_is_dir(try_documents_path)) {
+    return false;
+  }
+
+  BLI_strncpy(dir, try_documents_path, FILE_MAXDIR);
+  return true;
+}
+
 /**
  * Gets a good default directory for fonts.
  */