when handling a returned error, if the expression/block does not try/return error, reset error return trace index to 0

[Sahnvour]

When trying to run the first example code from #1813 with an executable that can't be found, the error trace is weird. It looks like the return address array is filled with addresses from windowsCreateProcess.
I believe this is due to the loop in spawnWindows that tries every directory from PATH and returns a lot of errors because the file doesn't exist.

As for why it is sometimes PATH_NOT_FOUND and others FILE_NOT_FOUND, I assume my PATH contains some old directories that don't exist anymore.

Example output:

$ zig run childprocess.zig
error: Unexpected
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:693:45: 0x7ff663ac4a97 in windowsCreateProcess (run.obj)
            windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
                                            ^
D:\DEV\Install\zig\lib\zig\std\os\index.zig:2253:5: 0x7ff663aa3442 in unexpectedErrorWindows (run.obj)
    return error.Unexpected;
    ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:696:21: 0x7ff663ac4a5d in windowsCreateProcess (run.obj)
            else => return os.unexpectedErrorWindows(err),
                    ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:611:21: 0x7ff663ac34d6 in ChildProcess::ChildProcess_spawnWindows (run.obj)
                    return err;
                    ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:127:13: 0x7ff663ac09ce in ChildProcess::ChildProcess_spawn (run.obj)
            return self.spawnWindows();
            ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:209:9: 0x7ff663ac01bb in ChildProcess::ChildProcess_exec (run.obj)
        try child.spawn();
        ^
D:\DEV\Zig\error-traces\childprocess.zig:5:18: 0x7ff663abff2a in main (run.obj)
  const result = try std.os.ChildProcess.exec(
                 ^
D:\DEV\Install\zig\lib\zig\std\os\child_process.zig:692:45: 0x7ff663ac4a7a in windowsCreateProcess (run.obj)
            windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,

[andrewrk]

I think there are 2 components to this issue:

• bug: the stack trace printer is printing the error return trace in backwards (or otherwise incorrect) order
• enhancement: "caught" errors still show up in errror return traces. This is a little tricky because we can definitely change the behavior but it has performance implications to consider.

[Sahnvour]

catch and if else don't decrement stack_trace.index at the moment ?

[andrewrk]

That's correct - and intentional. The example in the docs here shows how this is useful: https://ziglang.org/documentation/master/#Error-Return-Traces

If we wanted to avoid giving this up, what we could do is "reset" the error return trace when a function returns a non-error, by setting the index to 0. This would solve the problem, but require an extra memory store for every return in functions which return error unions. Maybe that is worth it.

[Sahnvour]

I think that would be insufficient, in my example as the executable is never found, it would still keep all of windowsCreateProcess's errors since it always returned an error, but the first line we want to see in the error trace is

zig/std/os/child_process.zig

Lines 613 to 616 in 7843c96

	} else {
	// Every other error would have been returned earlier.
	return error.FileNotFound;
	}

How about this : in a catch or else (handling a returned error), if the block does not try or return error.xyz, set the index to 0 ?

[andrewrk]

How about this : in a catch or else (handling a returned error), if the block does not try or return error.xyz, set the index to 0 ?

I like this idea a lot! And I think it will be straightforward to implement too. 💯 from me

[andrewrk]

BTW I believe this is a duplicate of #1810

[andrewrk]

Closing as a duplicate of #1810 but with the note that @Sahnvour's proposed fix in #1923 (comment) is planned.

[andrewrk]

I don't know why I thought this was a duplicate of #1810. The issues are entirely unrelated.