Make lfs.dir error out when the directory can't be opened

Both platform implementations of DirectoryIterator swallow open
failures and produce an empty iteration, but require 'lfs' in
automation scripts transparently resolves to aegisub.lfs, and vanilla
lfs.dir raises "cannot open <path>: <reason>" immediately. Scripts
using pcall lfs.dir to detect missing or unreadable directories took
the wrong branch under Aegisub.

Validate the path in dir_new rather than changing DirectoryIterator,
since the rest of the app plausibly relies on silently scanning
possibly-missing directories. This also makes the error path in
lfs.moon's dir reachable, which until now was dead code on every
platform.

Author: CoffeeFlux

automation/tests/modules/lfs.moon

@@ -85,3 +85,16 @@ describe 'lfs', ->
       os.remove name .. '/a'
       os.remove name .. '/b'
       lfs.rmdir name
+
+    it 'should error for a nonexistent path like vanilla lfs', ->
+      ok, err = pcall -> lfs.dir temp_name!
+      assert.is.false ok
+      assert.is.truthy err\find 'cannot open', 1, true
+
+    it 'should error when given a file rather than a directory', ->
+      name = temp_name!
+      lfs.touch name
+      ok, err = pcall -> lfs.dir name
+      assert.is.false ok
+      assert.is.truthy err\find 'cannot open', 1, true
+      os.remove name

libaegisub/lua/modules/lfs.cpp

@@ -18,6 +18,8 @@
 #include "libaegisub/lua/ffi.h"
 
 #include <chrono>
+#include <stdexcept>
+#include <system_error>
 
 using namespace agi::fs;
 using namespace agi::lua;
@@ -94,6 +96,12 @@ void dir_free(DirectoryIterator *it, char **) {
 
 DirectoryIterator *dir_new(const char *path, char **err) {
 	return wrap(err, [=]{
+		// DirectoryIterator gives a silent empty iteration when the directory
+		// can't be opened, but lfs.dir is documented to error out instead
+		std::error_code ec;
+		sfs::directory_iterator(agi::fs::path(path), ec);
+		if (ec)
+			throw std::runtime_error("cannot open " + std::string(path) + ": " + ec.message());
 		return new DirectoryIterator(path, "");
 	});
 }