Use argv[0] to internally relaunch meson.py

When installing Meson, distutils may choose to put shim scripts in the
`PATH` that only set up the egg requirements before launching the real
`meson.py` contained in the egg.

This means that `__file__` points to the real `meson.py` file, but
launching it directly is doomed to fail as it's missing the metadata
contained in the shim to set up the path egg, resulting in errors when
trying to import the `mesonbuild` module.

A similar issue affects Meson when installed as a zipapp, with the
current code going great lengths to figure out how to relaunch itself.

Using `argv[0]` avoids these issues as it gives us the way the current
executable has been launched, so we are pretty much guaranteed that
using it will create another instance of the same executable. We only
need to resolve relative paths as the current working directory may
get changed before re-launching the script, and using `realpath()` for
that saves us the trouble of manually resolving links and getting caught
in endless loops.

This also mean that `meson_script_file` no longer necessarily point to a
absolute file, so rename it to `_launcher` which hopefully would be less
prone to inducing false assumptions.

4 files changed, 14 insertions, 23 deletions

diff --git a/authors.txt b/authors.txt
index f0c2006..229a2ff 100644
--- a/authors.txt
+++ b/authors.txt
@@ -46,3 +46,4 @@ Patrick Griffis
 Iain Lane
 Daniel Brendle
 Franz Zapata
+Emanuele Aina
diff --git a/meson.py b/meson.py
index 7e21c78..492f70d 100755
--- a/meson.py
+++ b/meson.py
@@ -15,13 +15,11 @@
 # limitations under the License.
 
 from mesonbuild import mesonmain
-import sys, os
+import sys, os, os.path
 
-thisfile = __file__
-if not os.path.isabs(thisfile):
-    thisfile = os.path.normpath(os.path.join(os.getcwd(), thisfile))
+launcher = sys.argv[0]
+# resolve the command path if not launched from $PATH
+if os.path.split(launcher)[0]:
+    launcher = os.path.realpath(launcher)
 
-# The first argument *must* be an absolute path because
-# the user may have launched the program from a dir
-# that is not in path.
-sys.exit(mesonmain.run(thisfile, sys.argv[1:]))
+sys.exit(mesonmain.run(launcher, sys.argv[1:]))
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index e411687..86c23ae 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -179,12 +179,10 @@ class Environment():
     coredata_file = os.path.join(private_dir, 'coredata.dat')
     version_regex = '\d+(\.\d+)+(-[a-zA-Z0-9]+)?'
 
-    def __init__(self, source_dir, build_dir, main_script_file, options, original_cmd_line_args):
-        assert(os.path.isabs(main_script_file))
-        assert(not os.path.islink(main_script_file))
+    def __init__(self, source_dir, build_dir, main_script_launcher, options, original_cmd_line_args):
         self.source_dir = source_dir
         self.build_dir = build_dir
-        self.meson_script_file = main_script_file
+        self.meson_script_launcher = main_script_launcher
         self.scratch_dir = os.path.join(build_dir, Environment.private_dir)
         self.log_dir = os.path.join(build_dir, Environment.log_dir)
         os.makedirs(self.scratch_dir, exist_ok=True)
@@ -198,7 +196,7 @@ class Environment():
             # re-initialized with project options by the interpreter during
             # build file parsing.
             self.coredata = coredata.CoreData(options)
-            self.coredata.meson_script_file = self.meson_script_file
+            self.coredata.meson_script_launcher = self.meson_script_launcher
             self.first_invocation = True
         if self.coredata.cross_file:
             self.cross_info = CrossBuildInfo(self.coredata.cross_file)
@@ -253,7 +251,7 @@ class Environment():
         return self.coredata
 
     def get_build_command(self):
-        return self.meson_script_file
+        return self.meson_script_launcher
 
     def is_header(self, fname):
         return is_header(fname)
diff --git a/mesonbuild/mesonmain.py b/mesonbuild/mesonmain.py
index f7da1e0..7574c27 100644
--- a/mesonbuild/mesonmain.py
+++ b/mesonbuild/mesonmain.py
@@ -65,7 +65,7 @@ parser.add_argument('directories', nargs='*')
 
 class MesonApp():
 
-    def __init__(self, dir1, dir2, script_file, handshake, options, original_cmd_line_args):
+    def __init__(self, dir1, dir2, script_launcher, handshake, options, original_cmd_line_args):
         (self.source_dir, self.build_dir) = self.validate_dirs(dir1, dir2, handshake)
         if not os.path.isabs(options.prefix):
             raise RuntimeError('--prefix value \'{0}\' must be an absolute path: '.format(options.prefix))
@@ -76,7 +76,7 @@ class MesonApp():
                 pass
             else:
                 options.prefix = options.prefix[:-1]
-        self.meson_script_file = script_file
+        self.meson_script_launcher = script_launcher
         self.options = options
         self.original_cmd_line_args = original_cmd_line_args
 
@@ -124,7 +124,7 @@ itself as required.'''
             env.coredata.pkgconf_envvar = curvar
 
     def generate(self):
-        env = environment.Environment(self.source_dir, self.build_dir, self.meson_script_file, self.options, self.original_cmd_line_args)
+        env = environment.Environment(self.source_dir, self.build_dir, self.meson_script_launcher, self.options, self.original_cmd_line_args)
         mlog.initialize(env.get_log_dir())
         mlog.debug('Build started at', datetime.datetime.now().isoformat())
         mlog.debug('Python binary:', sys.executable)
@@ -262,12 +262,6 @@ def run(mainfile, args):
             dir2 = args[1]
         else:
             dir2 = '.'
-    while os.path.islink(mainfile):
-        resolved = os.readlink(mainfile)
-        if resolved[0] != '/':
-            mainfile = os.path.join(os.path.dirname(mainfile), resolved)
-        else:
-            mainfile = resolved
     try:
         app = MesonApp(dir1, dir2, mainfile, handshake, options, sys.argv)
     except Exception as e: