Access fish_history from $XDG_DATA_HOME

Add new functions path_get_data and path_create_data which parallel existing
functions path_get_config and path_create_data.  The new functions refer to
XDG_DATA_HOME, if it is defined, or ./local/share if not.

Modify history_filename to use the new function path_get_data.

As a consequence, fish_history will now be located in XDG_DATA_HOME,
not XDG_CONFIG_HOME.

Note that these changes mirror what is already used in
fish-shell/share/tools/create_manpage_completions.py, which stores the
completions in XDG_DATA_HOME

This change matches recommendations in the xdg basedir spec at
http://standards.freedesktop.org/basedir-spec/basedir-spec-0.7.html
($XDG_DATA_HOME defines the base directory relative to which user specific data
files should be stored. If $XDG_DATA_HOME is either not set or empty, a default
equal to $HOME/.local/share should be used.)

It addresses suggestions from the following issues:

1. Don't put history in $XDG_CONFIG_HOME (closes #744)
   https://github.com/fish-shell/fish-shell/issues/744

2. Fish is placing non-config files in $XDG_CONFIG_HOME #1257
   https://github.com/fish-shell/fish-shell/issues/1257

3. Move non-config data out of $XDG_CONFIG_HOME #1669
   https://github.com/fish-shell/fish-shell/issues/1669

src/history.cpp

@@ -1275,7 +1275,7 @@ static void unescape_yaml(std::string *str)
 static wcstring history_filename(const wcstring &name, const wcstring &suffix)
 {
     wcstring path;
-    if (! path_get_config(path))
+    if (! path_get_data(path))
         return L"";
 
     wcstring result = path;

src/path.cpp

@@ -276,6 +276,42 @@ static wcstring path_create_config()
     return res;
 }
 
+static wcstring path_create_data()
+{
+    bool done = false;
+    wcstring res;
+
+    const env_var_t xdg_dir  = env_get_string(L"XDG_DATA_HOME");
+    if (! xdg_dir.missing())
+    {
+        res = xdg_dir + L"/fish";
+        if (!create_directory(res))
+        {
+            done = true;
+        }
+    }
+    else
+    {
+        const env_var_t home = env_get_string(L"HOME");
+        if (! home.missing())
+        {
+            res = home + L"/.local/share/fish";
+            if (!create_directory(res))
+            {
+                done = true;
+            }
+        }
+    }
+
+    if (! done)
+    {
+        res.clear();
+
+        debug(0, _(L"Unable to create a data directory for fish. Your history will not be saved. Please set the $XDG_DATA_HOME variable to a directory where the current user has write access."));
+    }
+    return res;
+}
+
 /* Cache the config path */
 bool path_get_config(wcstring &path)
 {
@@ -284,6 +320,14 @@ bool path_get_config(wcstring &path)
     return ! result.empty();
 }
 
+/* Cache the data path */
+bool path_get_data(wcstring &path)
+{
+    static const wcstring result = path_create_data();
+    path = result;
+    return ! result.empty();
+}
+
 __attribute__((unused))
 static void replace_all(wcstring &str, const wchar_t *needle, const wchar_t *replacement)
 {

src/path.h

@@ -20,13 +20,26 @@
 
 /**
    Returns the user configuration directory for fish. If the directory
-   or one of it's parents doesn't exist, they are first created.
+   or one of its parents doesn't exist, they are first created.
 
    \param path The directory as an out param
    \return whether the directory was returned successfully
 */
 bool path_get_config(wcstring &path);
 
+/**
+   Returns the user data directory for fish. If the directory
+   or one of its parents doesn't exist, they are first created.
+
+   Volatile files presumed to be local to the machine,
+   such as the fish_history and all the generated_completions,
+   will be stored in this directory.
+
+   \param path The directory as an out param
+   \return whether the directory was returned successfully
+*/
+bool path_get_data(wcstring &path);
+
 /**
    Finds the full path of an executable. Returns YES if successful.