[bug#65486] [PATCH] syscalls: Add support for musl libc

["Sören Tempel" <soeren@soeren-tempel.net>]

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Hi Ludovic,

Ludovic Courtès <ludo@gnu.org> wrote:
> You could call it ‘musl?’ instead, to (hopefully) convey we’re
> interested in the C library specifically.

I used musl-libc? instead to make it more clear that we are interested
in the C library for this case-distinction. This is implemented in the
attached git-format-patch(1). Would that be suitable for inclusion in
Guix?

> No no, I meant something like:
> 
>   (or (false-if-exception (dynamic-func "readdir64" (dynamic-link)))
>       (dynamic-func "readdir" (dynamic-link)))
> 
> Of course, it’s not as simple as this because we’d rather have it
> integrated with ‘syscall->procedure’ (maybe by adding an
> #:alternative-name argument for the Musl name?), but you get the idea.

Also this check doesn't ensure struct layout compatibility, e.g. if
readdir uses 32-bit types so not sure if this is necessarily better
than the musl libc check I proposed above.

Let me know what you think.

Greetings
Sören


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From b1d478defc7f3e794974be2b9665cd4a58030569 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=B6ren=20Tempel?= <soeren+git@soeren-tempel.net>
Date: Thu, 14 Sep 2023 12:35:38 +0000
Subject: [PATCH] syscalls: Add support for musl libc

This commit allows using Guix on a foreign distro which uses musl libc,
for example, Alpine Linux. Usage of musl libc is detected via a new
musl-libc? variable using the Guile %host-type.

Using the new musl-libc? variable, we can now implement musl-specific
quirks. The two compatibility problems I encountered in this regard are
that musl dose not export a readdir64 and statfs64 symbol. On musl,
these two functions are implemented as CPP macros that expand to
readdir/statfs. To workaround that, a case-distinction was added.

The existing linux? variable has been modified to return true if the
%host-system contains "linux-" in order to ensure it is true for both
linux-gnu as well as linux-musl host systems.

The patch has been tested on Alpine Linux and is already used for the
downstream Guix package shipped in Alpine Linux's package repository.

* guix/build/syscalls.scm (musl-libc?): New variable.
* guix/build/syscalls.scm (linux?): Truth value on any linux system.
* guix/build/syscalls.scm (readdir-procedure): Support musl libc.
* guix/build/syscalls.scm (statfs): Support musl libc.
---
 guix/build/syscalls.scm | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/guix/build/syscalls.scm b/guix/build/syscalls.scm
index c9c0bf594d..b845b8aab9 100644
--- a/guix/build/syscalls.scm
+++ b/guix/build/syscalls.scm
@@ -836,7 +836,8 @@ (define-record-type <file-system>
 (define-syntax fsword                             ;fsword_t
   (identifier-syntax long))
 
-(define linux? (string-contains %host-type "linux-gnu"))
+(define musl-libc? (string-contains %host-type "linux-musl"))
+(define linux? (string-contains %host-type "linux-"))
 
 (define-syntax define-statfs-flags
   (syntax-rules (linux hurd)
@@ -905,7 +906,7 @@ (define-c-struct %statfs                          ;<bits/statfs.h>
   (spare            (array fsword 4)))
 
 (define statfs
-  (let ((proc (syscall->procedure int "statfs64" '(* *))))
+  (let ((proc (syscall->procedure int (if musl-libc? "statfs" "statfs64") '(* *))))
     (lambda (file)
       "Return a <file-system> data structure describing the file system
 mounted at FILE."
@@ -1232,7 +1233,7 @@ (define closedir*
 
 (define (readdir-procedure name-field-offset sizeof-dirent-header
                            read-dirent-header)
-  (let ((proc (syscall->procedure '* "readdir64" '(*))))
+  (let ((proc (syscall->procedure '* (if musl-libc? "readdir" "readdir64") '(*))))
     (lambda* (directory #:optional (pointer->string pointer->string/utf-8))
       (let ((ptr (proc directory)))
         (and (not (null-pointer? ptr))