dune build --watch + dune exec + DUNE_BUILD_DIR does not work

[yakobowski]

After #12473, running dune build --watch and dune exec in parallel works great. That's fantastic, thanks a lot. However, I observe an issue when DUNE_BUILD_DIR is also set.

Expected Behavior

The binary under $DUNE_BUILD_DIR is rebuilt if needed, and executed.

Actual Behavior

Error message about not being able to build the binary

Error: Don't know how to build

Reproduction

• PR with a reproducing test:

1. Repro code

dune:

(executable
 (name test_watch))

dune-project:

(lang dune 2.8)

test_watch.ml:

let () = print_endline "Hello, World!"

dune build -w &
Success, waiting for filesystem changes...
dune exec ./test_watch.exe 
Hello, World!

Works! (make sure to kill the watcher)

export DUNE_BUILD_DIR=$HOME/tmp/dune
mkdir -p $DUNE_BUILD_DIR
dune build -w &
Success, waiting for filesystem changes...
dune exec ./test_watch.exe 
Error: Don't know how to build /home/byakobow/tmp/dune/default/test_watch.exe
Had 1 error, waiting for filesystem changes...Warning:
Your build request is being forwarded to a running Dune instance (pid:
4113118). Note that certain command line arguments may be ignored.
Error: Don't know how to build /home/byakobow/tmp/dune/default/test_watch.exe
Error: Build failed with 1 error.
Had 1 error, waiting for filesystem changes...

Specifications

• Version of dune (output of dune --version): 3.20.2-1064-ge29a36c (trunk as of right now)
• Version of ocaml (output of ocamlc --version): 4.14.2
• Operating system (distribution and version): Debian oldstable

[Alizter]

The issue here is because Path.of_string incorrectly classifies /a/b as External even when the build prefix is set as /a. The same doesn't appear to happen when the build prefix is inside the workspace.

[Alizter]

Another complication is due to RPC clients sending all paths as absolute so the server always has to re-interpret them. We could clarify this situation by preserving the paths as the user wrote them, but also send a current-working-directory alongside the request. That way the server can better interpret build targets without having to guess.

[m-peron]

I am wondering if, in the given reproduction example, we could expect dune exec test_watch (so only using executable's name) to work?

[Alizter]

You would have to give it a public_name but it's worth trying. I'm unsure myself.