sbt in --client mode loses stdout/stderr of the forked processes in continuous (watch) task mode after the task ir reevaluated

[arturaz]

• sbt: 1.9.9
• JVM: 21

It seems that sbt --client has a problem losing stdout/stderr when a task is re-evaluated in the watch mode.

This is somewhat obscure, as the task has to launch a process in the background using the Fork API. It was originally noticed in sbt-revolver plugin (spray/sbt-revolver#99). After quick investigation it seems that sbt-revolver does not do anything fancy, except launching the forked process (https://github.com/spray/sbt-revolver/blob/4d28f091d3f2ce2f7f28b0165bac6705effded9a/src/main/scala/spray/revolver/Actions.scala#L139).

Reproduction case: https://github.com/arturaz/sbt-watch-loses-stdout

Expected behaviour

If we launch sbt in non-client mode everything works as expected:

PS E:\projects\sbt-watch-loses-stdout> sbt
[info] welcome to sbt 1.9.9 (Oracle Corporation Java 21.0.1)
[info] loading global plugins from C:\Users\x11\.sbt\1.0\plugins
[info] loading settings for project sbt-watch-loses-stdout-build-build-build from metals.sbt ...
[info] loading project definition from E:\projects\sbt-watch-loses-stdout\project\project\project
[info] loading settings for project sbt-watch-loses-stdout-build-build from metals.sbt ...
[info] loading project definition from E:\projects\sbt-watch-loses-stdout\project\project
[success] Generated .bloop\sbt-watch-loses-stdout-build-build.json
[success] Total time: 2 s, completed 2024-04-07 14:44:54
[info] loading settings for project sbt-watch-loses-stdout-build from metals.sbt,plugins.sbt ...
[info] loading project definition from E:\projects\sbt-watch-loses-stdout\project
[success] Generated .bloop\sbt-watch-loses-stdout-build.json
[success] Total time: 1 s, completed 2024-04-07 14:44:56
[info] loading settings for project sbt-watch-loses-stdout from build.sbt ...
[info] set current project to sbt-watch-loses-stdout (in build file:/E:/projects/sbt-watch-loses-stdout/)
[info] sbt server started at local:sbt-server-88c85ad2cae54cd28c14
[info] started sbt server
sbt:sbt-watch-loses-stdout> ~reStart
[info] compiling 1 Scala source to E:\projects\sbt-watch-loses-stdout\target\scala-2.12\classes ...
[warn] one deprecation (since 2.11.0); re-run with -deprecation for details
[warn] one warning found
[info] Application sbt-watch-loses-stdout not yet started
[info] Starting application sbt-watch-loses-stdout in the background ...
sbt-watch-loses-stdout Starting app.Main.main()
[success] Total time: 3 s, completed 2024-04-07 14:45:54
[info] 1. Monitoring source files for sbt-watch-loses-stdout/reStart...
[info]    Press <enter> to interrupt or '?' for more options.
sbt-watch-loses-stdout Hello, World!
sbt-watch-loses-stdout ... finished with exit code 0
[info] Build triggered by E:\projects\sbt-watch-loses-stdout\src\main\scala\app\Main.scala. Running 'reStart'.
[info] compiling 1 Scala source to E:\projects\sbt-watch-loses-stdout\target\scala-2.12\classes ...
[warn] one deprecation (since 2.11.0); re-run with -deprecation for details
[warn] one warning found
[info] Application sbt-watch-loses-stdout not yet started
[info] Starting application sbt-watch-loses-stdout in the background ...
sbt-watch-loses-stdout Starting app.Main.main()
[success] Total time: 1 s, completed 2024-04-07 14:46:04
[info] 2. Monitoring source files for sbt-watch-loses-stdout/reStart...
[info]    Press <enter> to interrupt or '?' for more options.
sbt-watch-loses-stdout Hello, World!
sbt-watch-loses-stdout ... finished with exit code 0

Notice that "Hello, World!" is printed every time sbt-revolver restarts the background process.

Problem

However, if we launch it with sbt --client, this behaviour breaks down.

PS E:\projects\sbt-watch-loses-stdout> sbt --client
[info] entering *experimental* thin client - BEEP WHIRR
[info] server was not detected. starting an instance
[info] welcome to sbt 1.9.9 (Oracle Corporation Java 21.0.1)
[info] loading global plugins from C:\Users\x11\.sbt\1.0\plugins
[info] loading settings for project sbt-watch-loses-stdout-build-build-build from metals.sbt ...
[info] loading project definition from E:\projects\sbt-watch-loses-stdout\project\project\project
[info] loading settings for project sbt-watch-loses-stdout-build-build from metals.sbt ...
[info] loading project definition from E:\projects\sbt-watch-loses-stdout\project\project
[success] Generated .bloop\sbt-watch-loses-stdout-build-build.json
[success] Total time: 2 s, completed 2024-04-07 14:51:43
[info] loading settings for project sbt-watch-loses-stdout-build from metals.sbt,plugins.sbt ...
[info] loading project definition from E:\projects\sbt-watch-loses-stdout\project
[success] Generated .bloop\sbt-watch-loses-stdout-build.json
[success] Total time: 1 s, completed 2024-04-07 14:51:44
[info] loading settings for project sbt-watch-loses-stdout from build.sbt ...
[info] set current project to sbt-watch-loses-stdout (in build file:/E:/projects/sbt-watch-loses-stdout/)
[info] sbt server started at local:sbt-server-88c85ad2cae54cd28c14
[info] started sbt server
[info] terminate the server with `shutdown`
[info] disconnect from the server with `exit`
sbt:sbt-watch-loses-stdout> ~reStart
[info] Application sbt-watch-loses-stdout not yet started
[info] Starting application sbt-watch-loses-stdout in the background ...
sbt-watch-loses-stdout Starting app.Main.main()
[success] Total time: 0 s, completed 2024-04-07 14:51:48
[info] 1. Monitoring source files for sbt-watch-loses-stdout/reStart...
[info]    Press <enter> to interrupt or '?' for more options.
sbt-watch-loses-stdout Hello, World!
sbt-watch-loses-stdout ... finished with exit code 0
[info] Build triggered by E:\projects\sbt-watch-loses-stdout\src\main\scala\app\Main.scala. Running 'reStart'.
[info] compiling 1 Scala source to E:\projects\sbt-watch-loses-stdout\target\scala-2.12\classes ...
[warn] one deprecation (since 2.11.0); re-run with -deprecation for details
[warn] one warning found
[info] Application sbt-watch-loses-stdout not yet started
[info] Starting application sbt-watch-loses-stdout in the background ...
sbt-watch-loses-stdout Starting app.Main.main()
[success] Total time: 3 s, completed 2024-04-07 14:51:55
[info] 2. Monitoring source files for sbt-watch-loses-stdout/reStart...
[info]    Press <enter> to interrupt or '?' for more options.

Only the first run prints "Hello, World!", while the subsequent runs do not have any stdout/stderr output anymore.

Expectation

sbt and sbt --client should act identically.

Notes

eed3si9n — Yesterday at 8:29 PM
~ used to be defined mostly in one file https://github.com/sbt/sbt/blob/1.10.x/main/src/main/scala/sbt/internal/Continuous.scala. I am not sure if that's the case lately. in any case, feel free to open an issue on sbt/sbt so other maintainers may be able to take a look

I would be interested in fixing this issue if more guidance would be provided.

This might also be related: #6866