dune build --passive-watch-mode: `Error: open(_build/.sync\0): No such file or directory`

[rusv-simcorp]

Expected Behavior

1. I open Terminal A and run the command dune build --passive-watch-mode
2. I open Terminal B and run the command dune build --watch
3. Terminal B builds the project in the current directory and notifies me of any errors

Actual Behavior

1. I open Terminal A and run the command dune build --passive-watch-mode
2. I open Terminal B and run the command dune build --watch
3. Terminal B prints the following and hangs (no more output)

   • Warning: Your build request is being forwarded to a running Dune instance.
     Note that certain command line arguments may be ignored.
     

4. Terminal A prints:

   • Error: open(_build/.sync\0): No such file or directory
     RPC accepted the last client. No more clients will be accepted.
     

Reproduction

I am running dune within Cygwin on Windows, so I don't know exactly how to reproduce this, as I think this may be a factor.

Specifications

• Version of dune (output of dune --version):
  • 3.20.2
• Version of ocaml (output of ocamlc --version):
  • 4.14.2+LEXIFI
• Operating system (distribution and version):
  • Cygwin on Windows 11 Enterprise 25H2

Additional information

Terminal A output of dune build --verbose --passive-watch-mode

Shared cache: enabled
Shared cache location: E:\repos\tmp\dune-3.20.2-cache
Workspace root: "E:\\Repos\\myproject"
Auto-detected concurrency: 8
Error: open(_build/.sync\0): No such file or directory
RPC accepted the last client. No more clients will be accepted.

Terminal B output of dune build --verbose --watch:

Shared cache: enabled
Shared cache location: E:\repos\tmp\dune-3.20.2-cache
Workspace root: "E:\\Repos\\myproject"
Warning: Your build request is being forwarded to a running Dune instance.
Note that certain command line arguments may be ignored.
Auto-detected concurrency: 8
Error: { payload = Some [ [ "id"; [ "auto"; "0" ] ] ]
; message =
    "connection terminated. this request will never receive a response"
; kind = Connection_dead
}
No cache trimming necessary (cache location: E:\repos\tmp\dune-3.20.2-cache; cache size: 15GB)

[Alizter]

We don't support running two watch mode RPC servers on the same repository at the same time. --passive-watch-mode is just a less eager-to-build version of --watch we use for testing purposes. If you mean to connect a client to an existing watch mode, i.e. dune build -w together with dune build then the second should detect the first and connect to it.

Warning: Your build request is being forwarded to a running Dune instance.
Note that certain command line arguments may be ignored.

Means that the --watch version isn't even considering that argument and pretending to be a regular dune build client.

Does it not work with dune build -w and dune build? If not then there might be something we need to check here.

[rusv-simcorp]

Does it not work with dune build -w and dune build? If not then there might be something we need to check here.

When I instead run dune build -w in Terminal A and dune build in Terminal B, all Terminal B ever shows is:

Warning: Your build request is being forwarded to a running Dune instance.
Note that certain command line arguments may be ignored.

Regardless of whether I make a change to the source code to trigger a build in Terminal A.

However, I don't see the open(_build/.sync\0): No such file or directory error in Terminal A.

[Alizter]

@rusv-simcorp Does the second dune build hang or return 0? You should be able to give specific build requests like dune build @fmt from the second one that weren't being done as part of the first. There is no progress update from the client dune at the moment, but errors do get transmitted.

[rusv-simcorp]

Does the second dune build hang or return 0?

It hangs. Displaying only the above text (Warning: Your build request is being forwarded to a running Dune instance. etc.).

There is no progress update from the client dune at the moment, but errors do get transmitted.

How are errors transmitted? I assume I should be able to see them in the terminal? The only thing I see in Terminal B (the one running dune build as a client) is the above (Warning: Your build request is being forwarded to a running Dune instance. etc.). Nothing changes even when I introduce an error and Terminal A (dune build -w) shows the error.

[Alizter]

Errors are transmitted over the Dune RPC protocol. When the watch mode is started, it starts and RPC server which creates a Unix socket in a known location that allows clients to connect and interface. I don't know how well Cygwin emulates Unix sockets on Windows so that could be causing problems.

The other thing I noticed is that your Dune version is quite old. 3.23.1 which was recently released has since had a bunch of improvements to RPC and watch mode. It may solve your hang if you upgrade to a newer version.

Watch mode on Cygwin is completely untested, but my experience with Cygwin is that it does a pretty good job of acting like a Unix system. Running Dune directly on Windows however does currently have issues with the watch server, but we haven't had the time or resources to look into it yet.

[nojb]

I don't know how well Cygwin emulates Unix sockets on Windows so that could be causing problems.

Watch mode on Cygwin is completely untested, but my experience with Cygwin is that it does a pretty good job of acting like a Unix system.

I would be surprised if the OP was running the Cygwin version of Dune; instead I am guessing they are using Cygwin only as a shell, but the Dune version used is most probably the native Windows one.