Add a 'target' field for symlinkattributes.

It returns the resolved path of the symlink.

Original version by Hisham <hisham@gobolinux.org>, modified to
use a different strategy for sizing the readlink() buffer.

doc/us/manual.html

@@ -242,6 +242,8 @@ LuaFileSystem offers the following functions:
     <dt><a name="symlinkattributes"></a><strong><code>lfs.symlinkattributes (filepath [, aname])</code></strong></dt>
     <dd>Identical to <a href="#attributes">lfs.attributes</a> except that
     it obtains information about the link itself (not the file it refers to).
+    It also adds a <strong><code>target</code></strong> field, containing
+    the file name that the symlink points to.
     On Windows this function does not yet support links, and is identical to
     <code>lfs.attributes</code>.
     </dd>

src/lfs.c

@@ -849,11 +849,58 @@ static int file_info (lua_State *L) {
 }
 
 
+/*
+** Push the symlink target to the top of the stack.
+** Assumes the file name is at position 1 of the stack.
+** Returns 1 if successful (with the target on top of the stack),
+** 0 on failure (with stack unchanged, and errno set).
+*/
+static int push_link_target(lua_State *L) {
+#ifdef _WIN32
+        errno = ENOSYS;
+        return 0;
+#else
+        const char *file = luaL_checkstring(L, 1);
+        char *target = NULL;
+        int tsize, size = 256; /* size = initial buffer capacity */
+        while (1) {
+            target = realloc(target, size);
+            if (!target) /* failed to allocate */
+                return 0;
+            tsize = readlink(file, target, size);
+            if (tsize < 0) { /* a readlink() error occurred */
+                free(target);
+                return 0;
+            }
+            if (tsize < size)
+                break;
+            /* possibly truncated readlink() result, double size and retry */
+            size *= 2;
+        }
+        target[tsize] = '\0';
+        lua_pushlstring(L, target, tsize);
+        free(target);
+        return 1;
+#endif
+}
+
 /*
 ** Get symbolic link information using lstat.
 */
 static int link_info (lua_State *L) {
-        return _file_info_ (L, LSTAT_FUNC);
+        int ret;
+        if (lua_isstring (L, 2) && (strcmp(lua_tostring(L, 2), "target") == 0)) {
+                int ok = push_link_target(L);
+                return ok ? 1 : pusherror(L, "could not obtain link target");
+        }
+        ret = _file_info_ (L, LSTAT_FUNC);
+        if (ret == 1 && lua_type(L, -1) == LUA_TTABLE) {
+                int ok = push_link_target(L);
+                if (ok) {
+                        lua_setfield(L, -2, "target");
+                }
+        }
+        return ret;
 }
 
 

tests/test.lua

@@ -91,6 +91,8 @@ io.flush()
 if lfs.link (tmpfile, "_a_link_for_test_", true) then
   assert (lfs.attributes"_a_link_for_test_".mode == "file")
   assert (lfs.symlinkattributes"_a_link_for_test_".mode == "link")
+  assert (lfs.symlinkattributes"_a_link_for_test_".target == tmpfile)
+  assert (lfs.symlinkattributes("_a_link_for_test_", "target") == tmpfile)
   assert (lfs.link (tmpfile, "_a_hard_link_for_test_"))
   assert (lfs.attributes (tmpfile, "nlink") == 2)
   assert (os.remove"_a_link_for_test_")