Better error messages when file does not exist

[anka-213]

Currently, we get cryptic error messages like:

createProcess: runInteractiveProcess: exec: does not exist (No such file or directory)

Which tells us nothing about what it is that we were trying to run that does not exist. It would be very helpful for debugging if the path that does not exist was included in the error message.

[snoyberg]

I don't see a problem with such a change, would you be interested in submitting a PR?

[juhp]

+:100: for this

This has bothered me for a long time actually...

$ cat > myprog.hs
import System.Process
main = rawSystem "a-tool" []
$ ghc myprog
$ ./myprog
myprog: a-tool: rawSystem: runInteractiveProcess: exec: does not exist (No such file or directory)

The error message should be something like:

myprog: rawSystem: a-tool: command not found

I tried a bit to track down where/how the error message is built, but didn't succeed yet: any pointers would be helpful.

[Fuuzetsu]

I'm making an uneducated guess and going to say here:

process/cbits/posix/runProcess.c

Line 292 in 17b8a15

execvp(args[0], args);

Edit: notably, childFailed function. As the message comes from the OS, I think probably you want to examine the error code somehow and do some special behaviour there that adds the process name?

[juhp]

Thanks @Fuuzetsu for the helpful comment: it is also confusing that the C function is also called runInteractiveProcess.
Indeed the error substring "runInteractiveProcess: exec" comes from the runInteractiveProcess in runProcess.c
along with a list of other potential failure errors (pipe, chdir, setuid, initgroups, etc).

I am still trying to work out how to move the executable name to later in the error message.

[juhp]

I am wondering if this is could even be a ghc bug: maybe something like throwIO# in ghc-prim possibly? since the process command filepath is put into the IOException:

So far I tracked down from System.Process: createProcess_ (process) -> createProcess_Internal.
Then perhaps withFilePathException fpath -> ioError (ioeSetFileName ex fpath) (base) -> ioException -> throwIO -> raiseIO# (ghc-prim).

I think I need a ghc build tree to look at the generated ghc-prim files, to check what it does, though the actual final error message might still be assembled higher up in the stack.

[Fuuzetsu]

I don't really follow. I haven't read the code closely, but isn't it just the case of adding args[0] to this line?

process/cbits/posix/runProcess.c

Line 339 in 17b8a15

*failed_doing = "runInteractiveProcess: exec";

[juhp]

Probably and removing it from the start of the error string...

[anka-213]

I'm trying to figure out how the cmd name is added to the beginning. It might be this line?

process/System/Process/Posix.hs

Line 123 in 9410f77

withFilePathException cmd $

And then the rest is composed here

process/System/Process/Posix.hs

Line 163 in 9410f77

throwErrno (fun ++ ": " ++ failedDoing)

And finally, the actual error message is extracted from the C errno by the throwErrNo function and combined with the context from the failedDoing variable.

https://hackage.haskell.org/package/base-4.16.0.0/docs/src/Foreign.C.Error.html#throwErrno