fix(install): prefer the musl asset on NixOS and other non-FHS hosts

NixOS keeps glibc in the store, so a generic `-gnu` release binary cannot
find its dynamic loader. The old detection missed this twice over: `ldd
--version` reports plain GNU libc there, and `/lib64/ld-linux-x86-64.so.2`
does exist — as a stub that only prints an error and exits 127. The
installer downloaded the gnu asset, `executable()` returned 1, and the
binary could never start.

Detect non-FHS distributions (`/etc/NIXOS`, `ID=nixos`/`ID=guix` in
os-release) and pick the statically linked musl asset there. Turn the
single target triple into an ordered candidate list so glibc hosts fall
back to musl too, and verify the staged binary actually execs — exit 126
or 127 means the kernel or loader refused it — before it lands in `bin/`.

Applied to both installers (Lua and the Vim build hook, which the shell
test harnesses source). Adds `g:nuwiki_ls_target` to force a triple.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012qPCSWXxATFvjEpVxQMhG9

scripts/download_bin.vim

@@ -20,24 +20,71 @@ let s:rs_repo    = fnamemodify(s:plugin_dir, ':h:h') . '/nuwiki-rs'
 
 call mkdir(s:bin_dir, 'p')
 
-function! s:target() abort
-  if has('win32')
-    return 'x86_64-pc-windows-msvc'
-  elseif has('mac')
-    if has('arm64')
-      return 'aarch64-apple-darwin'
-    endif
-    return 'x86_64-apple-darwin'
-  elseif has('linux')
-    let l:ldd = system('ldd --version 2>&1')
-    let l:musl = stridx(l:ldd, 'musl') >= 0
-    let l:uname_m = trim(system('uname -m'))
-    if l:uname_m ==# 'aarch64' || l:uname_m ==# 'arm64'
-      return l:musl ? 'aarch64-unknown-linux-musl' : 'aarch64-unknown-linux-gnu'
-    endif
-    return l:musl ? 'x86_64-unknown-linux-musl' : 'x86_64-unknown-linux-gnu'
+function! s:is_musl(arch) abort
+  if filereadable('/lib/ld-musl-' . a:arch . '.so.1')
+    return 1
   endif
-  return ''
+  if !executable('ldd')
+    return 0
+  endif
+  " musl's ldd writes its banner to stderr.
+  return stridx(system('ldd --version 2>&1'), 'musl') >= 0
+endfunction
+
+" NixOS and Guix System are not FHS distributions: glibc lives in the store,
+" so the /lib64/ld-linux-*.so.2 a generic -gnu binary is linked against is
+" either absent or — on NixOS — a stub that only prints an error and exits
+" 127. `ldd --version` still reports GNU libc there, so libc detection alone
+" picks the wrong asset. The musl builds are statically linked and need no
+" loader at all, so they run on these systems unchanged.
+function! s:is_non_fhs_linux() abort
+  if filereadable('/etc/NIXOS')
+    return 1
+  endif
+  if !filereadable('/etc/os-release')
+    return 0
+  endif
+  for l:line in readfile('/etc/os-release')
+    let l:id = matchstr(l:line, '^ID=\s*"\?\zs[[:alnum:]._-]\+')
+    if l:id ==# 'nixos' || l:id ==# 'guix'
+      return 1
+    endif
+  endfor
+  return 0
+endfunction
+
+" Release triples to try, best first. More than one entry means the later ones
+" are fallbacks for a host that turns out not to be able to run the first.
+function! s:targets() abort
+  if get(g:, 'nuwiki_ls_target', '') !=# ''
+    return [g:nuwiki_ls_target]
+  endif
+  if has('win32')
+    return ['x86_64-pc-windows-msvc']
+  elseif has('mac')
+    return [has('arm64') ? 'aarch64-apple-darwin' : 'x86_64-apple-darwin']
+  elseif has('linux')
+    let l:uname_m = trim(system('uname -m'))
+    let l:arch = (l:uname_m ==# 'aarch64' || l:uname_m ==# 'arm64')
+          \ ? 'aarch64' : 'x86_64'
+    let l:musl = l:arch . '-unknown-linux-musl'
+    if s:is_musl(l:arch) || s:is_non_fhs_linux()
+      return [l:musl]
+    endif
+    " Static musl runs anywhere, so keep it as a last resort on glibc hosts
+    " too (old glibc, exotic loader layouts).
+    return [l:arch . '-unknown-linux-gnu', l:musl]
+  endif
+  return []
+endfunction
+
+" Can this host actually start the binary? Exit codes 126 and 127 mean the
+" kernel or the dynamic loader refused to exec it (missing/stub loader, wrong
+" architecture); any other code means it ran and exited on its own terms.
+" system() gives the child an empty stdin, so the LSP loop sees EOF and stops.
+function! s:can_exec(path) abort
+  call system(shellescape(a:path) . ' --version')
+  return v:shell_error != 126 && v:shell_error != 127
 endfunction
 
 function! s:build_from_source() abort
@@ -65,16 +112,12 @@ function! s:build_from_source() abort
   return executable(s:bin)
 endfunction
 
-function! s:download_release() abort
-  let l:target = s:target()
-  if empty(l:target)
-    return 0
-  endif
+function! s:download_release(target) abort
   if !executable('curl') || !executable('tar')
     return 0
   endif
   let l:url = 'https://code.gfran.co/gffranco/nuwiki-rs/releases/download/latest/nuwiki-ls-'
-        \ . l:target . '.tar.gz'
+        \ . a:target . '.tar.gz'
   let l:archive = tempname() . '.tar.gz'
   call system('curl -fsSL -o ' . shellescape(l:archive) . ' ' . shellescape(l:url))
   if v:shell_error != 0
@@ -86,14 +129,28 @@ function! s:download_release() abort
   if v:shell_error != 0
     return 0
   endif
-  call system('cp ' . shellescape(l:extract . '/nuwiki-ls' . s:suffix) . ' ' . shellescape(s:bin))
+  " Vet the staged copy before it becomes the installed one, so a binary this
+  " host cannot exec never lands at s:bin for the LSP client to pick up.
+  let l:staged = l:extract . '/nuwiki-ls' . s:suffix
+  call system('chmod +x ' . shellescape(l:staged))
+  if !executable(l:staged) || !s:can_exec(l:staged)
+    return 0
+  endif
+  call system('cp ' . shellescape(l:staged) . ' ' . shellescape(s:bin))
   call system('chmod +x ' . shellescape(s:bin))
   return executable(s:bin)
 endfunction
 
-if s:download_release()
-  echom 'nuwiki: installed ' . s:bin
-else
+let s:installed = 0
+for s:t in s:targets()
+  if s:download_release(s:t)
+    echom 'nuwiki: installed ' . s:bin . ' (' . s:t . ')'
+    let s:installed = 1
+    break
+  endif
+endfor
+
+if !s:installed
   echohl WarningMsg | echom 'nuwiki: download failed, falling back to source build' | echohl None
   if !s:build_from_source()
     echohl ErrorMsg | echom 'nuwiki: install failed' | echohl None