[debuginfo-tests] Always use the system python to invoke llgdb.py.

[ab]

Summary

llgdb.py almost always ends up using the system LLDB python and framework. LLDB.framework is linked against the system python. If we have a different python installed, and first in PATH, we're going to try using that, which invariably fails because of internal data structure incompatibilities.

Don't pretend we can use any arbitrary python: just use the system one.

[MatzeB]

usr/bin/env is the recommended way to invoke python... Or is lldb somehow hardcoded to use /usr/bin/python and you need to match this here?

[ab]

Precisely: /usr/bin/env is recommended as a cross-platform way to find python. But:

• we're only using lldb on darwin, where we know python (or at least, the xcrun-style shortcut) is in /usr/bin/
• the python interpreter in LLDB comes from /S/L/F, for instance:

        $ otool -L /Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework/LLDB | grep Python
	    /System/Library/Frameworks/Python.framework/Versions/2.7/Python (compatibility version 2.7.0, current version 2.7.10)

so when we use the lldb python module, it calls into the swig/python support in the lldb framework, and if there's a mismatch between the interpreter and the linked python, weird things occur.

In theory, this should be done by:

• looking for the LLDB framework (llgdb.py does some of that)
• finding the binary inside the framework
• looking for the Python it was linked against (otool -L)
• finding the interpreter executable inside the Python.framework

But in practice, that's only different if we use a custom LLDB framework/pythonpath when running these tests, and AFAIK nobody does that right now, so the code would be dead anyway.

[aprantl]

From a pragmatic point of view this change is okay for all the reasons you outlined above.

[aprantl]

Please do watch the LLDB linux bots on lab.llvm.org:8011 after landing this.

test_debuginfo.pl

 close(INPUT);
 close(OUTPUT);
 
 # setup debugger and debugger options to run a script.
 my $my_debugger = $ENV{'DEBUGGER'};
 if (!$my_debugger) {
     if ($use_lldb) {
         my $path = dirname(Cwd::abs_path($0));
-        $my_debugger = "/usr/bin/env python $path/llgdb.py";
+        $my_debugger = "/usr/bin/python $path/llgdb.py";
     } else {
         $my_debugger = "gdb";
     }
 }
 
 # quiet / exit after cmdline / no init file / execute script
 my $debugger_options = "-q -batch -n -x";