Improve diagnostics for kernel supervisor startup failures

[jmcphers]

This change improves diagnostics and logging for (hopefully rare) cases in which the kernel supervisor itself cannot start. This is distinct from the cases where R or Python can't start. So far we've seen just one of these in the wild, as a result of running on an unsupported OS.

The approach is to borrow the wrapper script technique from the Jupyter Adapter (formerly used to invoke the kernels themselves). The wrapper script acts as a sort of supervisor for the supervisor; it eats the output of the supervisor process and writes it to a file. If the supervisor exits unexpectedly at startup, the output file is written to the log channel, and the user is directed there to view errors.

As an additional benefit, this runs the supervisor under a bash process on Unix-alikes, so any environment variables or configuration set up in .bashrc (etc) will now be available to the supervisor.

Addresses #5611 .

May help us figure out #5337.

QA Notes

An easy way to test this is to replace your kcserver binary with a shell script that emits some nonsense and then exits immediately with a nonzero status code. If you're feeling ambitious, you could also test this on the OS named in #5611.

Also, did you know that the longest worm in the world can reach up to 55 meters? Crazy.

[petetronic]

Confirmed on macOS that replacing kcserver binary with:

echo "Foo"
exit 197

Showed in the output logs, as well as the new information message notification directing you to open those logs.

2024-12-12 14:23:04.701 [info] Starting Kallichore server /Users/username/dev/git/posit/positron/extensions/positron-supervisor/resources/kallichore/kcserver on port 59681
Waiting for Kallichore server to start (attempt 1, 120ms)
Supervisor terminal closed with exit code 197; output:
/Users/username/dev/git/posit/positron/extensions/positron-supervisor/resources/kallichore/kcserver --port 59681 --token /var/folders/fz/p04t93s91vdd27ljtdr6rrg40000gp/T/kallichore-d071ecc4.token --log-level debug --log-file /var/folders/fz/p04t93s91vdd27ljtdr6rrg40000gp/T/kallichore-d071ecc4.log
Foo

Failed to start Kallichore server: Error: The supervisor process exited before the server was ready.

[sharon-wang]

LGTM on Windows 👍

I replaced the kcserver binary with a kcserver.bat:

@echo off
echo "worm"
exit /b 55

2024-12-12 15:17:22.454 [info] Positron Kernel Supervisor activated
2024-12-12 15:17:22.454 [info] Kallichore server PID 19200 is not running
2024-12-12 15:17:22.454 [info] Could not reconnect to Kallichore server at http://localhost:42305. Starting a new server
2024-12-12 15:17:22.454 [info] Starting Kallichore server c:\Users\sharon\positron\extensions\positron-supervisor\resources\kallichore\kcserver.bat on port 44281
Waiting for Kallichore server to start (attempt 1, 633ms)
Waiting for Kallichore server to start (attempt 6, 1660ms)
Waiting for Kallichore server to start (attempt 11, 1975ms)
Supervisor terminal closed with exit code 55; output:
c:\Users\sharon\positron\extensions\positron-supervisor\resources\kallichore\kcserver.bat --port 44281 --token C:\Users\sharon\AppData\Local\Temp\kallichore-9b27dbe3.token --log-level debug --log-file C:\Users\sharon\AppData\Local\Temp\kallichore-9b27dbe3.log 
"worm"

Failed to start Kallichore server: Error: The supervisor process exited before the server was ready.