Platform.resolvedExecutable gives invalid path for network UNC

[timsneath]

See fluttercommunity/plus_plugins#1483 (comment) for more information.

In Platform::ResolveExecutablePath, if Windows returns a UNC such as \\?\UNC\server\share\folder\file.exe from GetModuleFileNameW, this will become stripped to UNC\server\share\folder\file.exe, which is not a valid Windows file path.

I believe this is due to some incorrect URL editing here:

sdk/runtime/bin/file_win.cc

Lines 1060 to 1065 in 6e18b08

	// Remove leading \\?\ if possible, unless input used it.
	int offset = 0;
	if ((result_size > 4) && (wcsncmp(path.get(), L"\\\\?\\", 4) == 0) &&
	(strncmp(pathname, "\\\\?\\", 4) != 0)) {
	offset = 4;
	}

Documentation on valid file path formats on Windows can be found here:
https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats

/cc @bkonyi @aam

[timsneath]

It's not clear why we remove \\?\ at all? Is it out of a fear that it's not a valid file syntax?

To this untrained PM eye, it seems dangerous to be editing paths like this by hand if at all possible.

[aam]

I've tried reproducing this in dart on Windows, but things seem to work as expected:

PS C:\src\d1\sdk> type r.dart
import 'dart:io';

main() {
  print(Platform.executable);
}
PS C:\src\d1\sdk> \\aam0\c$\src\d1\sdk\out\ReleaseX64\dart.exe r.dart
\\aam0\c$\src\d1\sdk\out\ReleaseX64\dart.exe

[timsneath]

OK, I'll trade you for a broken one :)

import 'dart:io';

main() {
  print(Platform.executable);
  print(Platform.resolvedExecutable);
}

[C:\src\scratch] \\.\UNC\MICA\Flutter\bin\cache\dart-sdk\bin\dart.exe io.dart
\\.\UNC\MICA\Flutter\bin\cache\dart-sdk\bin\dart.exe
UNC\MICA\flutter\bin\cache\dart-sdk\bin\dart.exe

the former is a valid file path; the latter is not.

(This is a machine named MICA where I've shared C:\src\flutter as \\MICA\Flutter.)

[aam]

Ah, I see, thanks. Platform.resolvedExecutable gets malformed when we attempt to canonicalize Windows path and remove \\?\, which was thought to be used specifically to prefix long-paths, letting Windows to bypass MAX_PATH limitation.
UNC paths were not considered, but somehow they should be.