Fix #115056: man-page fails to build with non-portable install

This effectively reverts [0] which required the installed Blender
along with the `bpy` module to load for the install target to generate
the man-page. It turns out running a full Blender at install time can be
quite involved.

Now the man-page generator runs `blender --help` & `blender --version`,
converting the output into a man-page from a Python script.

Some minor changes have also been made:

- Use PYTHON_EXECUTABLE so the systems Python
  (which may not be compatible) isn't used.
- Remove leading blender version from the description
  which was unintentionally being included.

[0]: 61d99d450e

doc/manpage/blender.1.py

@@ -7,7 +7,7 @@
 This script generates the blender.1 man page, embedding the help text
 from the Blender executable itself. Invoke it as follows:
 
-    ./blender.bin -b --python doc/manpage/blender.1.py -- --output <output-filename>
+    blender.1.py --blender <path-to-blender> --output <output-filename>
 
 where <path-to-blender> is the path to the Blender executable,
 and <output-filename> is where to write the generated man page.
@@ -15,8 +15,8 @@ and <output-filename> is where to write the generated man page.
 
 import argparse
 import os
+import subprocess
 import time
-import sys
 
 from typing import (
     Dict,
@@ -30,28 +30,77 @@ def man_format(data: str) -> str:
     return data
 
 
-def blender_extract_info() -> Dict[str, str]:
-    # Only use of `bpy` in this file.
-    import bpy  # type: ignore
-    blender_help_text = bpy.app.help_text()
-    blender_version_text = bpy.app.version_string
-    blender_build_date_text = bpy.app.build_date
+def blender_extract_info(blender_bin: str) -> Dict[str, str]:
+    blender_env = {
+        "ASAN_OPTIONS": (
+            os.environ.get("ASAN_OPTIONS", "") +
+            ":exitcode=0:check_initialization_order=0:strict_init_order=0"
+        ).lstrip(":"),
+    }
 
-    if blender_build_date_text == b'Unknown':
+    # NOTE: in some ways it's more elegant to use `bpy.app.help_text()` which was done but had to be reverted.
+    # however - this requires Blender to run with a full environment (initializing it's Python environment).
+    # See #115056 & !115320 for details.
+
+    blender_help = subprocess.run(
+        [blender_bin, "--help"],
+        env=blender_env,
+        check=True,
+        stdout=subprocess.PIPE,
+    ).stdout.decode(encoding="utf-8")
+
+    blender_version_output = subprocess.run(
+        [blender_bin, "--version"],
+        env=blender_env,
+        check=True,
+        stdout=subprocess.PIPE,
+    ).stdout.decode(encoding="utf-8")
+
+    # Extract information from the version string.
+    # Note that some internal modules may print errors (e.g. color management),
+    # check for each lines prefix to ensure these aren't included.
+    blender_version = ""
+    blender_date = ""
+
+    # The full text (use to manipulate `blender_version_text`).
+    blender_version_text = ""
+
+    for l in blender_version_output.split("\n"):
+        if l.startswith("Blender "):
+            if blender_version_text == "":
+                blender_version_text = l
+            # Remove `Blender` prefix.
+            blender_version = l.split(" ", 1)[1].strip()
+        elif l.lstrip().startswith("build date:"):
+            # Remove `build date:` prefix.
+            blender_date = l.split(":", 1)[1].strip()
+        if blender_version and blender_date:
+            break
+
+    # The `--help` text also contains the version, skip it so as not to include it twice.
+    if blender_version_text:
+        i = blender_help.find(blender_version_text)
+        if i != -1:
+            blender_help = blender_help[i + len(blender_version_text) + 1:]
+        del i
+
+    if not blender_date:
         # Happens when built without WITH_BUILD_INFO e.g.
-        blender_date = time.strftime("%B %d, %Y", time.gmtime(int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))))
+        date_string = time.strftime("%B %d, %Y", time.gmtime(int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))))
     else:
-        blender_date = time.strftime("%B %d, %Y", time.strptime(blender_build_date_text.decode(), "%Y-%m-%d"))
+        date_string = time.strftime("%B %d, %Y", time.strptime(blender_date, "%Y-%m-%d"))
 
     return {
-        "help": blender_help_text,
-        "version": blender_version_text,
-        "date": blender_date,
+        "help": blender_help,
+        "version": blender_version,
+        "date": date_string,
     }
 
 
-def man_page_from_blender_help(fh: TextIO, verbose: bool) -> None:
-    blender_info = blender_extract_info()
+def man_page_from_blender_help(fh: TextIO, blender_bin: str, verbose: bool) -> None:
+    if verbose:
+        print("Extracting help text:", blender_bin)
+    blender_info = blender_extract_info(blender_bin)
 
     # Header Content.
     fh.write(
@@ -144,6 +193,11 @@ def create_argparse() -> argparse.ArgumentParser:
         required=True,
         help="The man page to write to."
     )
+    parser.add_argument(
+        "--blender",
+        required=True,
+        help="Path to the blender binary."
+    )
     parser.add_argument(
         "--verbose",
         default=False,
@@ -156,15 +210,15 @@ def create_argparse() -> argparse.ArgumentParser:
 
 
 def main() -> None:
-    argv = sys.argv[sys.argv.index("--") + 1:]
     parser = create_argparse()
-    args = parser.parse_args(argv)
+    args = parser.parse_args()
 
     output_filename = args.output
+    blender_bin = args.blender
     verbose = args.verbose
 
     with open(output_filename, "w", encoding="utf-8") as fh:
-        man_page_from_blender_help(fh, verbose)
+        man_page_from_blender_help(fh, blender_bin, verbose)
         if verbose:
             print("Written:", output_filename)