draft implementation of prefix directory for intermediate compilation results

[corpix]

Some systems or package managers (like Nix and Guix) use read-only filesystem to store dependencies. Gambit writes intermediate compilation objects into the directory of the input file, just changing the extension.

This pull request adds GAMBIT_OUTPUT_PREFIX environment variable which change intermediate compilation objects location. Motivated by the error I've got while using Gerbil to write an application and packaging it's dependencies using Nix package manager:

$ gxc -static -exe -o app app.ss
/nix/store/i2fr4hsl5bs704bp8kihxdxbnvcsm3h3-gerbil-cli/gerbil/lib/static/cli__cli.scm:
*** ERROR IN c#targ-start-dump -- Read-only file system
(open-output-file "/nix/store/i2fr4hsl5bs704bp8kihxdxbnvcsm3h3-gerbil-cli/gerbil/lib/static/cli__cli.c")
#f*** ERROR IN gxc#compile-executable-module/separate --
*** ERROR IN ?
--- Syntax Error at (compile-exe app.ss): Compilation error; process exit with nonzero status
... form:   ("/nix/store/58g7klswv88zp6d3kmdzyk67rf3x5035-gerbil-gerbil-unstable-2024-05-11/gerbil/v0.18.1/bin/gsc"
             "-link"
             "/nix/store/58g7klswv88zp6d3kmdzyk67rf3x5035-gerbil-gerbil-unstable-2024-05-11/gerbil/v0.18.1/lib/static/gerbil__runtime__gambit.c"
             ...

Problem is Gambit try to write intermediate compilation objects onto a read-only filesystem (/nix/store/...).

Here is a simple demonstration of what effect this patch has. Let's say we have some Scheme file, want to generate C code from it:

 ~/tmp  λ  cat test.scm
(display "hello")

Before:

 ~/tmp  λ  gsc -c test.scm
 ~/tmp  λ  ls -la
.rw-r--r-- 3.4k user 18 Jul 18:34 test.c
.rw-r--r--   18 user 18 Jul 18:30 test.scm

After:

 ~/tmp  λ  GAMBIT_OUTPUT_PREFIX=prefix gsc -c test.scm
 ~/tmp  λ  tree prefix/
prefix/
└── home
    └── user
        └── tmp
            └── test.c

4 directories, 1 file
 ~/tmp  λ  ls -la
drwxr-xr-x  - user 18 Jul 18:33 prefix
.rw-r--r-- 18 user 18 Jul 18:30 test.scm

Key questions:

• what do community think about this feature?
• what may be improved? (probably we may want a CLI flag for this)

If you are interested:

• running GAMBIT_OUTPUT_PREFIX=prefix gsc -obj test.scm read-only-media/some-other-file.scm is still not working, not sure why, probably I need a help with this

[maconcg]

I think it's cool, it just addresses a pretty specific scenario that most people don't run into. So adding this to Gambit would be kind of a heavyweight solution for this. Here's my attempt to explain:

Environment variables are inherently global, which means they don't really mesh with my understanding of functional programming. At a glance over the documentation, I see mentions of only two Gambit-specific environment variables: GAMBOPT and GAMBUILD_VERBOSE. If you solve one problem by adding an environment variable, that might not seem so bad, but if you solve 20 problems by adding 20 environment variables...

I think there's also general reluctance to modify parts closer to the "core" of Gambit unless it's to fix something that can't be adequately addressed some other way -- it's better to avoid the complexity there. Looking at this file's commit history, I see that file has only been patched a few times a year (if that), and almost all those commits come from Prof. Feeley.

If I were you, I would definitely add this or something like it to ~/.gambini. I just don't use a package manager that needs it.

[corpix]

Thank you for your answer. Sure, environment variable exists only as a proof of concept, it probably should be a command line option or something.
I understand parts of core Gambit should not be modified without a serious reason to do that, this pull request was made to start discussion and highlight a difference Gambit has from other programming languages (like Python which use separate directory to create intermediate compilation objects like bytecode).
Not sure what to do with this changes because I gave up trying to use Gambit & Gerbil for my projects (reasons are out of scope), so I am not sure I will find time to follow on this pull request, but this may be useful for other people who want make packaging of Gambit&Gerbil on platforms like Nixos/Guix more easy.

[feeley]

As you know the -o command line option to gsc indicates the directory where output files are stored. Perhaps the temporary files should default to go there? Or maybe the current directory (like gcc)?

Environment variables that affect the execution of gsi and gsc should be kept to a minimum, as this causes endless troubles.