Mill 0.10.9 BSP unable to initialize with nightly Metals

[ckipp01]

So I think some combination of these prs are causing things to break or behave in weird ways when using Mill as a build server with the nightly version of Metals. Mainly, the BSP server doesn't respond at all.

• chore(deps): bump millw to 0.4.4 scalameta/metals#4609
• refactor: change where we stick millw scalameta/metals#4586
• Set the current cli as mill.main.cli sysprop lefou/millw#41
• Add actual Mill cli as sysprop and use it in BSP config #2080

To illustrate I'll start with a fresh project.

g8 com-lihaoyi/mill-scala-hello.g8

Everything working

So I'll start with the following:

• Metals version: 0.11.9 (brings in 0.4.2 of millw and sticks it in a temp dir)
• Mill 0.10.9

Steps taken

1. Open the project
2. Answer "Not now" to build import
3. Trigger a "Switch Build Server" and select mill-bsp
4. Everything works as expected

.bsp/mill-bsp.json

{
  "name": "mill-bsp",
  "argv": [
    "/Users/ckipp/.cache/mill/download/0.10.9",
    "--bsp",
    "--disable-ticker",
    "--color",
    "false",
    "--jobs",
    "1"
  ],
  "millVersion": "0.10.9",
  "bspVersion": "2.0.0",
  "languages": [
    "scala",
    "java"
  ]
}

jps output is what I'd expect

❯ jps
9601 MillServerMain
9658 Jps
9628 MillClientMain
9565 metals

Stuff not working

Now I'll bump the Metals version to the latest nightly (0.11.9+131-601042ab-SNAPSHOT) which brings in the new placement and version of millw, and I'll fully clean the workspace.

Steps taken

1. Open the project
2. Answer "Not now" to build import
3. Trigger a "Switch Build Server" and select mill-bsp
4. Wait and see that Metals times out waiting for the BSP connection

.bsp/mill-bsp.json

{
  "name": "mill-bsp",
  "argv": [
    "/Users/ckipp/Documents/scala-workspace/mill-bsp-test/.metals/millw",
    "--bsp",
    "--disable-ticker",
    "--color",
    "false",
    "--jobs",
    "1"
  ],
  "millVersion": "0.10.9",
  "bspVersion": "2.0.0",
  "languages": [
    "scala",
    "java"
  ]
}

There is a mill-bsp.trace file created, but with nothing inside of it and no other files created by Mill.

When I look at the BSP trace file from Metals all I see is the following:

[Trace - 08:59:46 am] Sending request 'build/initialize - (1)'
Params: {
  "rootUri": "file:///Users/ckipp/Documents/scala-workspace/mill-bsp-test/",
  "displayName": "Metals",
  "version": "0.11.9+131-601042ab-SNAPSHOT",
  "bspVersion": "2.1.0-M3",
  "capabilities": {
    "languageIds": [
      "scala",
      "java"
    ]
  },
  "data": {
    "javaSemanticdbVersion": "0.7.4",
    "semanticdbVersion": "4.6.0",
    "supportedScalaVersions": [
      "2.13.10",
      "2.12.17",
      "2.12.16",
      "2.12.15",
      "2.12.14",
      "2.12.13",
      "2.12.12",
      "2.13.5",
      "2.13.6",
      "2.13.7",
      "2.13.8",
      "2.13.9",
      "2.11.12",
      "2.12.9",
      "2.12.10",
      "2.12.11",
      "2.13.1",
      "2.13.2",
      "2.13.3",
      "2.13.4"
    ]
  }
}

So the request is sent, but never responds. Looking at the output of jps I do see that Mill has started:

❯ jps
10118 MillClientMain
10109 metals
10126 MillServerMain
10127 Jps

Other research

I also tried reverting the place that Metals stores the millw back to a tmp dir and that still doesn't work. For example the BSP entry in this case looks like:

{
  "name": "mill-bsp",
  "argv": [
    "/var/folders/fq/nx_jsnyd6550xp03czx898d40000gn/T/metals17064985890367699422/millw",
    "--bsp",
    "--disable-ticker",
    "--color",
    "false",
    "--jobs",
    "1"
  ],
  "millVersion": "0.10.9",
  "bspVersion": "2.0.0",
  "languages": [
    "scala",
    "java"
  ]
}

And the same behavior of an initialize being sent, but not responded to.

Conclusions

From what I can tell it just seems problematic that the BSP entry is pointing to the millw file. I'm not 100% sure how to dig in further to see why Mill isn't responding here or why pointing towards millw is an issue. Any ideas or anything else I can provide?

[lefou]

Thanks for the report!

It took me a while to see what the issue is, but I think, as we now pass -D as the first option, Mill is no longer correctly switching to the BSP mode. For this, the --bsp options needs to come as a first option. I just checked with other options, which need to be given as the first one like --repl or --interactice, and indeed, they no longer work.

This is actually an issue with the bundled mill script in Mill, as well as with millw 0.4.4.