Add wreaddir_for_dirs function

Eliminates some stat calls, speeding up wildcard expansion

src/wildcard.cpp

@@ -814,7 +814,7 @@ void wildcard_expander_t::expand_trailing_slash(const wcstring &base_dir)
 void wildcard_expander_t::expand_intermediate_segment(const wcstring &base_dir, DIR *base_dir_fp, const wcstring &wc_segment, const wchar_t *wc_remainder)
 {
     wcstring name_str;
-    while (!interrupted() && wreaddir(base_dir_fp, name_str))
+    while (!interrupted() && wreaddir_for_dirs(base_dir_fp, &name_str))
     {
         /* Note that it's critical we ignore leading dots here, else we may descend into . and .. */
         if (! wildcard_match(name_str, wc_segment, true))
@@ -849,7 +849,7 @@ void wildcard_expander_t::expand_literal_intermediate_segment_with_fuzz(const wc
     // This only works with tab completions
     // Ordinary wildcard expansion should never go fuzzy
     wcstring name_str;
-    while (!interrupted() && wreaddir(base_dir_fp, name_str))
+    while (!interrupted() && wreaddir_for_dirs(base_dir_fp, &name_str))
     {
         /* Don't bother with . and .. */
         if (contains(name_str, L".", L".."))

src/wutil.cpp

@@ -115,6 +115,40 @@ bool wreaddir(DIR *dir, std::wstring &out_name)
     return true;
 }
 
+bool wreaddir_for_dirs(DIR *dir, wcstring *out_name)
+{
+    struct dirent *result = NULL;
+    while (result == NULL)
+    {
+        struct dirent *d = readdir(dir);
+        if (!d) break;
+        
+#if HAVE_STRUCT_DIRENT_D_TYPE
+        switch (d->d_type)
+        {
+            // These may be directories
+            case DT_DIR:
+            case DT_LNK:
+            case DT_UNKNOWN:
+                result = d;
+                break;
+                
+            // Nothing else can
+            default:
+                break;
+        }
+#else
+        /* We can't determine if it's a directory or not, so just return it */
+        result = d;
+#endif
+    }
+    if (result && out_name)
+    {
+        *out_name = str2wcstring(result->d_name);
+    }
+    return result != NULL;
+}
+
 
 wchar_t *wgetcwd(wchar_t *buff, size_t sz)
 {

src/wutil.h

@@ -94,6 +94,14 @@ wchar_t *wrealpath(const wcstring &pathname, wchar_t *resolved_path);
 bool wreaddir(DIR *dir, std::wstring &out_name);
 bool wreaddir_resolving(DIR *dir, const std::wstring &dir_path, std::wstring &out_name, bool *out_is_dir);
 
+/**
+ Like wreaddir, but skip items that are known to not be directories.
+ If this requires a stat (i.e. the file is a symlink), then return it.
+ Note that this does not guarantee that everything returned is a directory,
+ it's just an optimization for cases where we would check for directories anyways.
+*/
+bool wreaddir_for_dirs(DIR *dir, wcstring *out_name);
+
 /**
    Wide character version of dirname()
 */