std.Io.Threaded: debug file handle leaks #30788

As promised, one of the neat features of std.Io is that it makes it nice to debug resource leaks such as file descriptors. Here is a proof-of-concept of this working:

Demo

conststd=@import("std");constIo=std.Io;pubfnmain(init:std.process.Init)!void{constio=init.io;constargs=tryinit.minimal.args.toSlice(init.arena.allocator());for(args[1..])|arg|{constfile=tryIo.Dir.cwd().openFile(io,arg,.{});_=file;}}

$ zig run test.zig -- hello.zig windows_h.zig 
error: file handle 3 leaked: 
/home/andy/src/zig/lib/std/Io/Dir.zig:508:33: 0x10cd890 in openFile (std.zig)
 return io.vtable.dirOpenFile(io.userdata, dir, sub_path, flags);
 ^
/home/andy/src/zig/build-release/test.zig:9:47: 0x11ae0f5 in main (test.zig)
 const file = try Io.Dir.cwd().openFile(io, arg, .{});
 ^
/home/andy/src/zig/lib/std/start.zig:714:30: 0x11aebe6 in callMain (std.zig)
 return wrapMain(root.main(.{
 ^
/home/andy/src/zig/lib/std/start.zig:190:5: 0x11ade51 in _start (std.zig)
 asm volatile (switch (native_arch) {
 ^
???:?:?: 0x0 in ??? (???)
???:?:?: 0x0 in ??? (???)
error: file handle 4 leaked: 
/home/andy/src/zig/lib/std/Io/Dir.zig:508:33: 0x10cd890 in openFile (std.zig)
 return io.vtable.dirOpenFile(io.userdata, dir, sub_path, flags);
 ^
/home/andy/src/zig/build-release/test.zig:9:47: 0x11ae0f5 in main (test.zig)
 const file = try Io.Dir.cwd().openFile(io, arg, .{});
 ^
/home/andy/src/zig/lib/std/start.zig:714:30: 0x11aebe6 in callMain (std.zig)
 return wrapMain(root.main(.{
 ^
/home/andy/src/zig/lib/std/start.zig:190:5: 0x11ade51 in _start (std.zig)
 asm volatile (switch (native_arch) {
 ^
???:?:?: 0x0 in ??? (???)
???:?:?: 0x0 in ??? (???)

I've pushed a counter-proposal to the branch debug-file-leaks-differently (link to commit). There are a few small tweaks in the implementation, but that's not really important: the difference I want to discuss is that instead of hardcoding this logic into Io.Threaded, I've instead written it as a separate implementation, Io.Debug, which wraps another Io (like how std.heap.ArenaAllocator wraps another Allocator). I think this might be a better idea for a few reasons:

• The implementation is more readable. Most obviously, this PR makes the big Io.Threaded even bigger. But another downside of the approach you've gone with is that you've had to put calls to trackOpenFile separately in the code paths for every target (dirOpenFilePosix, dirOpenFileWtf16, etc)---there's no need for that mistake (missing the call on some target[s]) to even be possible!
• It allows you to trivially plug in different implementations to this debugging framework. I think this is a good idea for two reasons:
  • Evented may have significantly better performance characteristics for some applications, so even in debug mode with Threaded available, some users may prefer to use Evented.
  • On some targets, Threaded may not be an available implementation. For instance, WASM in the browser might have no threads available to it, and choose to use an Io implementation based on stackless coroutines. You still want nice debugging capabilities in that case! Of course, there's also freestanding development / BYOS, where you need to write your own Io implementation.

EDIT: oh, and just to be clear, the implementation behaves just the same as yours on your example:

$ zig run repro.zig -- README.md CMakeLists.txt
error: file handle '3' leaked:
/home/mlugg/zig/master/lib/std/Io/Dir.zig:508:33: 0x10ebfb0 in openFile (std.zig)
 return io.vtable.dirOpenFile(io.userdata, dir, sub_path, flags);
 ^
/home/mlugg/zig/master/repro.zig:6:47: 0x11c7a1e in main (repro.zig)
 const file = try Io.Dir.cwd().openFile(io, arg, .{});
 ^
/home/mlugg/zig/master/lib/std/start.zig:728:30: 0x11c85f7 in callMain (std.zig)
 return wrapMain(root.main(.{
 ^
/home/mlugg/zig/master/lib/std/start.zig:190:5: 0x11c7751 in _start (std.zig)
 asm volatile (switch (native_arch) {
 ^
error: file handle '4' leaked:
/home/mlugg/zig/master/lib/std/Io/Dir.zig:508:33: 0x10ebfb0 in openFile (std.zig)
 return io.vtable.dirOpenFile(io.userdata, dir, sub_path, flags);
 ^
/home/mlugg/zig/master/repro.zig:6:47: 0x11c7a1e in main (repro.zig)
 const file = try Io.Dir.cwd().openFile(io, arg, .{});
 ^
/home/mlugg/zig/master/lib/std/start.zig:728:30: 0x11c85f7 in callMain (std.zig)
 return wrapMain(root.main(.{
 ^
/home/mlugg/zig/master/lib/std/start.zig:190:5: 0x11c7751 in _start (std.zig)
 asm volatile (switch (native_arch) {
 ^