gn build: Add gn.py wrapper script that adds appropriate --dotfile= and --root= paramers

[thakis]

Summary

Since people weren't enthused about moving the .gn file to the toplevel in D56419, here's a script to make gn at least somewhat more pleasant to invoke (useful for gn clean, gn args --list, gn desc, etc).

[phosek]

llvm/utils/gn/gn.py

25

Shouldn't we check if help is the first argument (i.e. argv[1])? Otherwise this would catch even cases like gn gen help (which is admittedly a strange name for output directory but nevertheless valid).

[thakis]

address comment

llvm/utils/gn/gn.py

25

I think either check will have false positives (e.g. gn --nocolor help gen doesn't work when checking argv[1]). It's not _that_ much code to get the first non-flag arg, so I did that.

[serge-sans-paille]

LGTM, but only from the point of view of python 2/3 compatibility :-)

[phosek]

LGTM, although I'll probably try to fix the gn help behavior to avoid similar workarounds.

llvm/utils/gn/.gn

-# FIXME: Once it's possible to add files to the root directory of the
-# monorepo, move this file to there.  Until then, you need to pass
-# `--dotfile=llvm/utils/gn/.gn --root=.` to the `gn gen` command.
+# Since this can't be at the toplevel, you either need to pass
+# `--dotfile=llvm/utils/gn/.gn --root=.` to the `gn gen` command
+# or use llvm/utils/gn/build/gn.py which calls gn with these two flags added.
 
 buildconfig = "//llvm/utils/gn/build/BUILDCONFIG.gn"
 
 # Disallow all calls to exec_script. We should be very conservative about
 # whitelisting things here.
 exec_script_whitelist = []
 
 # The normal GN setup is to have BUILD.gn files in-tree and a root BUILD.gn
 # file.  Since LLVM's GN build is unofficial, set secondary_source to tell GN
 # that e.g. llvm/lib/Demangle/BUILD.gn is found at
 # llvm/utils/gn/secondary/llvm/lib/Demangle/BUILD.gn and that the root BUILD.gn
 # file is at llvm/utils/gn/secondary/BUILD.gn
 secondary_source = "//llvm/utils/gn/secondary/"

llvm/utils/gn/README.rst

 
 Quick start
 ===========
 
 GN only works in the monorepo layout.
 
 #. Obtain a `gn binary <https://gn.googlesource.com/gn/#getting-started>`_.
 
-#. In the root of the monorepo, run
-   `gn gen --dotfile=$PWD/llvm/utils/gn/.gn --root=. out/gn` (`out/gn` is the
-   build directory, it can have any name, and you can have as many as you want,
-   each with different build settings).
+#. In the root of the monorepo, run `llvm/utils/gn/build/gn.py gen out/gn`.
+   `out/gn` is the build directory, it can have any name, and you can have as
+   many as you want, each with different build settings.  (The `gn.py` script
+   adds `--dotfile=llvm/utils/gn/.gn --root=.` and just runs regular `gn`;
+   you can manually pass these parameters and not use the wrapper if you
+   prefer.)
 
 #. Run e.g. `ninja -C out/gn check-lld` to build all prerequisites for and
    run the LLD tests.
 
 By default, you get a release build with assertions enabled that targets
 the host arch. You can set various build options by editing `out/gn/args.gn`,
 for example putting `is_debug = true` in there gives you a debug build. Run
-`gn args --list out/gn` to see a list of all possible options. After touching
-`out/gn/args.gn`, just run ninja, it will re-invoke gn before starting the
-build.
+`llvm/utils/gn/build/gn.py args --list out/gn` to see a list of all possible
+options. After touching `out/gn/args.gn`, just run ninja, it will re-invoke gn
+before starting the build.
 
 GN has extensive built-in help; try e.g. `gn help gen` to see the help
 for the `gen` command. The full GN reference is also `available online
 <https://gn.googlesource.com/gn/+/master/docs/reference.md>`_.
 
 GN has an autoformatter: `git ls-files '*.gn' '*.gni' | xargs -n 1 gn format`
 after making GN build changes is your friend.
 

llvm/utils/gn/gn.py

+#!/usr/bin/env python
+"""Calls `gn` with the right --dotfile= and --root= arguments for LLVM."""
+
+# GN normally expects a file called '.gn' at the root of the repository.
+# Since LLVM's GN build isn't supported, putting that file at the root
+# is deemed inappropriate, which requires passing --dotfile= and -root= to GN.
+# Since that gets old fast, this script automatically passes these arguments.
+
+import os
+import subprocess
+import sys
+
+
+THIS_DIR = os.path.dirname(__file__)
+ROOT_DIR = os.path.join(THIS_DIR, '..', '..', '..')
+
+
+def main():
+    # Find real gn executable. For now, just assume it's on PATH.
+    # FIXME: Probably need to append '.exe' on Windows.
+    gn = 'gn'
+
+    # Compute --dotfile= and --root= args to add.
+    extra_args = []
+    if 'help' not in sys.argv:  # `gn help` gets confused by the switches.
+        cwd = os.getcwd()
+        dotfile = os.path.relpath(os.path.join(THIS_DIR, '.gn'), cwd)
+        root = os.path.relpath(ROOT_DIR, cwd)
+        extra_args = [ '--dotfile=' + dotfile, '--root=' + root ]
+
+    # Run GN command with --dotfile= and --root= added.
+    cmd = [gn] + extra_args + sys.argv[1:]
+    sys.exit(subprocess.call(cmd))
+
+
+if __name__ == '__main__':
+    main()