Document macOS usage

[phil-blain]

Issue: It's unclear how this package is supposed to work on macOS. I haven't been able to use the conda-forge LLDB at all.

Just doing lldb <executable>, where lldb is installed from this package, does not work (at least on my macOS 10.11.6 system). For a simple "hello-world"-type program, I get::

$ conda create -n lldb lldb
$ conda activate lldb
$ lldb hello
(lldb) target create "hello"
Current executable set to '/path/to/hello' (x86_64).
(lldb) r
error: process launch failed: failed to get reply to handshake packet

The conda-forge lldb recipe uses LLDB_USE_SYSTEM_DEBUGSERVER=ON on macOS:

lldb-feedstock/recipe/build.sh

Lines 8 to 12 in 131e70d

	if [[ "$target_platform" == osx* ]]; then
	LLDB_USE_SYSTEM_DEBUGSERVER=ON
	else
	LLDB_USE_SYSTEM_DEBUGSERVER=OFF
	fi

which I guess was done to avoid having to codesign the executable.

The error "process launch failed: failed to get reply to handshake packet" seems to be related to inability for the lldb executable to connect with the system debugserver.

Note: I have several copies of debugserver on my system, none of them are in the PATH. The system LLDB works fine this way...

/Applications/CLion.app/Contents/bin/lldb/mac/LLDB.framework/Resources/debugserver
/Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework/Versions/A/Resources/debugserver 
/Library/Developer/CommandLineTools/Library/PrivateFrameworks/LLDB.framework/Versions/A/Resources/debugserver
/Users/Philippe/.vscode/extensions/ms-vscode.cpptools-0.28.1/debugAdapters/lldb/bin/debugserver

First reported here: conda-forge/gdb-feedstock#23 (comment)

---

I have been able (a few times, but not consistently) to debug an executable using these steps:

1. Adding a call to sleep() in my program
2. Launching my program: ./sleep
3. In a separate shell, launch debugserver:

/Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework/Versions/A/Resources/debugserver localhost:16000 --attach=sleep

4. In a third shell, launch lldb and attach to debugserver:

$ lldb
(lldb)  process connect connect://localhost:16000

Maybe it would be easier to actually codesign the executable, as was done in conda-forge/gdb-feedstock#23 ?