meson: fix warm nix3 CLI manual generation

`nix eval --write-to` refuses to write to a directory that exists at
all, so now we generate in a temporary directory, and copy the generated
tree to the build directory. This is equivalent to what the Make
buildsystem did, actually, but hopefully more robust.

Future work: documenting the doc generation architecture in the
top-level meson.build outline comment.

Change-Id: Ic3eb6d26e3cc249a1c042fd3ced22d637ac66a69

doc/manual/generate-manpage.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+
+"""
+This script is a helper for this project's Meson buildsystem, to generate
+manpages as mdbook markdown for nix3 CLI commands. It is an analogue to an
+inline sequence of bash commands in the autoconf+Make buildsystem, which works
+around a limitation in `nix eval --write-to`, in that it refuses to write to any
+directory that already exists.
+
+Basically, this script is a glorified but hopefully-more-robust version of:
+$ rm -rf $output
+$ nix eval --write-to $output.tmp --expr 'import doc/manual/generate-manpage.nix true
+    (builtins.readFile ./generate-manpage.nix)'
+$ mv $output.tmp $output
+"""
+
+import argparse
+import os.path
+import shlex
+import shutil
+import subprocess
+import sys
+import tempfile
+
+name = 'generate-manpage.py'
+
+def log(*args, **kwargs):
+    kwargs['file'] = sys.stderr
+    return print(f'{name}:', *args, **kwargs)
+
+def main():
+    parser = argparse.ArgumentParser(name)
+    parser.add_argument('--nix', required=True, help='Full path to the nix binary to use')
+    parser.add_argument('-o', '--output', required=True, help='Output directory')
+    parser.add_argument('--generator', required=True, help='Path to generate-manpage.nix')
+    parser.add_argument('--cli-json', required=True, help='Path to the nix.json output from Nix')
+    args = parser.parse_args()
+
+    with tempfile.TemporaryDirectory() as tempdir:
+
+        temp_out = os.path.join(tempdir, 'new-cli')
+
+        nix_args = [
+            args.nix,
+            '--experimental-features',
+            'nix-command',
+            'eval',
+            '-I', 'nix/corepkgs=corepkgs',
+            '--store', 'dummy://',
+            '--impure',
+            '--raw',
+            '--write-to', temp_out,
+            '--expr',
+            f'import {args.generator} true (builtins.readFile {args.cli_json})',
+        ]
+
+        log('generating nix3 man pages with', shlex.join(nix_args))
+
+        subprocess.check_call(nix_args)
+
+        shutil.copytree(temp_out, args.output, dirs_exist_ok=True)
+
+sys.exit(main())

doc/manual/meson.build

@@ -132,15 +132,26 @@ nix3_cli_json = custom_target(
   capture : true,
   output : 'nix.json',
 )
+# `nix eval --write-to` is rather picky, and refuses to write to a directory that exists at all.
+# This is fine for clean builds because it creates the directory and then is never run again,
+# but during development if nix.json changes, then using `nix eval --write-to` will error,
+# since that part of the build directory already exists.
+# Thus, another Python script. It runs the relevant `nix eval --write-to` command in a temporary
+# directory, and then copies that tree to our build output directory.
 nix3_cli_files = custom_target(
-  command : nix_eval_for_docs + [
-    '--write-to', '@OUTPUT@',
-    '--expr',
-    'import @INPUT1@ true (builtins.readFile @INPUT0@)',
+  command : [
+    python,
+    '@INPUT0@',
+    '--nix=@INPUT1@',
+    '--generator=@INPUT2@',
+    '--cli-json=@INPUT3@',
+    '--output=@OUTPUT@',
   ],
   input : [
-    nix3_cli_json,
-    'generate-manpage.nix',
+    'generate-manpage.py', # INPUT0
+    nix, # INPUT1
+    'generate-manpage.nix', # INPUT2
+    nix3_cli_json, # INPUT3
     'utils.nix',
   ],
   output : 'new-cli',