Provide a way to specify nimcache dir via env variable

[jakubgs]

Nim Version

Any in the last 8 years.

Description

The use of a shared nimcache folder can be a source of very bizarre errors, especially when used on a CI host with jobs running in parallel. One such issue was encountered in:

• fix(ci): set nimcache later to avoid null value status-im/status-app#19958 (comment)

Where either via some sort of corruption or other weird race condition the nimcache directory became broken and had to be remove to stop the following error from happening:

x86_64-w64-mingw32/bin/ld.exe:
  C:\Users\jenkins\nimcache\koch_d\@mlib@sstd@sprivate@swin_setenv.nim.c.o:@mlib@sstd@sprivate@swin_setenv.nim.c:
  (.rdata$.refptr.__imp__environ[.refptr.__imp__environ]+0x0): undefined reference to `__imp__environ'
collect2.exe: error: ld returned 1 exit status

On Linux this can be partially mitigated by pointing XDG_CACHE_HOME at something like WORKSPACE_TMP or similar, but in case of Windows it just uses os.getHomeDir():

Nim/compiler/options.nim

Lines 806 to 810 in ab00c56

	proc getOsCacheDir(): string =
	when defined(posix):
	result = getEnv("XDG_CACHE_HOME", getHomeDir() / ".cache") / "nim"
	else:
	result = getHomeDir() / genSubDir.string

The issue with that is that overriding USERPROFILE on Windows can have way more unexpected results than just overriding XDG_CACHE_HOME on Linux. This is also not the first time I've encountered issues with shared nimcache in CI.

For this reason I'd like to propose either:

• A NIMCACHE environment variable to specify the directory explicitly instead of indirectly.
• A modification to getOsCacheDir that would prevent weird errors like above from happening.

Current Output

Expected Output

Known Workarounds

Remove nimcache after every build... except that can kill another parallel job running that uses it.

Additional Information

No response

[Araq]

Set --nimcache in your config, we don't do env-vars, they are machine specific and suck.