docs: generalize manpage generation script as json-to-tree.py

This should be capable of replacing every invocation of
nix eval --write-to.

Change-Id: I60387bc9b0fc54a91244eddb639beaa64d705878

3 files changed, 85 insertions, 80 deletions

diff --git a/doc/manual/generate-manpage.py b/doc/manual/generate-manpage.py
deleted file mode 100755
index f082caffa..000000000
--- a/doc/manual/generate-manpage.py
+++ /dev/null
@@ -1,63 +0,0 @@
-#!/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())
diff --git a/doc/manual/json-to-tree.py b/doc/manual/json-to-tree.py
new file mode 100755
index 000000000..27c8e417c
--- /dev/null
+++ b/doc/manual/json-to-tree.py
@@ -0,0 +1,61 @@
+#!/usr/bin/env python3
+
+"""
+This script is a helper for this project's Meson buildsystem, to replace its
+usage of `nix eval --write-to`. Writing a JSON object as a nested directory
+tree is more generic, easier to maintain, and far, far less cursed. Nix
+has 'good' support for JSON output. Let's just use it.
+"""
+
+import argparse
+from pathlib import Path
+import json
+import sys
+
+name = 'json-to-tree.py'
+
+def log(*args, **kwargs):
+    kwargs['file'] = sys.stderr
+    return print(f'{name}:', *args, **kwargs)
+
+def write_dict_to_directory(current_directory: Path, data: dict, files_written=0):
+    current_directory.mkdir(parents=True, exist_ok=True)
+    for key, value in data.items():
+        nested_path = current_directory / key
+        match value:
+            case dict(nested_data):
+                files_written += write_dict_to_directory(nested_path, nested_data)
+
+            case str(content):
+                nested_path.write_text(content)
+                files_written += 1
+
+            case rest:
+                assert False, \
+                    f'should have been called on a dict or string, not {type(rest)=}\n\t{rest=}'
+
+    return files_written
+
+def main():
+    parser = argparse.ArgumentParser(name)
+    parser.add_argument('-i', '--input', type=argparse.FileType('r'), default='-',
+        help='The JSON input to operate on and output as a directory tree',
+    )
+    parser.add_argument('-o', '--output', type=Path, required=True,
+        help='The place to put the directory tree',
+    )
+    args = parser.parse_args()
+
+    json_string = args.input.read()
+
+    try:
+        data = json.loads(json_string)
+    except json.JSONDecodeError:
+        log(f'could not decode JSON from input: {json_string}')
+        raise
+
+
+    files_written = write_dict_to_directory(args.output, data)
+    log(f'wrote {files_written} files')
+
+sys.exit(main())
diff --git a/doc/manual/meson.build b/doc/manual/meson.build
index dafec9a4c..55373d15c 100644
--- a/doc/manual/meson.build
+++ b/doc/manual/meson.build
@@ -7,13 +7,13 @@ nix_env_for_docs = {
 }
 
 nix_for_docs = [ nix, '--experimental-features', 'nix-command' ]
-nix_eval_for_docs = nix_for_docs + [
+nix_eval_for_docs_common = nix_for_docs + [
   'eval',
   '-I', 'nix/corepkgs=corepkgs',
   '--store', 'dummy://',
   '--impure',
-  '--raw',
 ]
+nix_eval_for_docs = nix_eval_for_docs_common + '--raw'
 
 nix_conf_file_json = custom_target(
   command : nix_for_docs + [ 'show-config', '--json' ],
@@ -132,27 +132,34 @@ 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.
+
+# Intermediate step for manpage generation.
+# This splorks the output of generate-manpage.nix as JSON,
+# which gets written as a directory tree below.
+nix3_cli_files_json = custom_target(
+  command : nix_eval_for_docs_common + [
+    '--json',
+    '--expr',
+    'import @INPUT0@ true (builtins.readFile @INPUT1@)',
+  ],
+  input : [
+    'generate-manpage.nix',
+    nix3_cli_json,
+  ],
+  capture : true,
+  output : 'new-cli.json',
+  env : nix_env_for_docs,
+)
 nix3_cli_files = custom_target(
   command : [
     python,
     '@INPUT0@',
-    '--nix=@INPUT1@',
-    '--generator=@INPUT2@',
-    '--cli-json=@INPUT3@',
-    '--output=@OUTPUT@',
+    '-i', '@INPUT1@',
+    '-o', '@OUTPUT@',
   ],
   input : [
-    'generate-manpage.py', # INPUT0
-    nix, # INPUT1
-    'generate-manpage.nix', # INPUT2
-    nix3_cli_json, # INPUT3
-    'utils.nix',
+    'json-to-tree.py',
+    nix3_cli_files_json,
   ],
   output : 'new-cli',
 )