Then Hashdist launcher

This is a very small program that facilitates launching applications from relocateable bin directories, typically as the result of creating Hashdist profiles. It has the following usecases:

Relative shebangs:

While Unix allows $!some/relative/path, it is completely useless, as it uses the CWD(!). Using launcher allows for proper relative shebangs. If a script is launched, then the launcher will act directly as the process launcher and inspect the shebang. Shebangs are interpreted as by the OS, except that the variable $PROFILE_BIN_DIR and $ORIGIN is expanded, and one can try multiple interpreters, so that one can do:

#!${PROFILE_BIN_DIR}/python:${ORIGIN}/../../../python/xkl4/bin/python

Process redirection (useful for relocateable sys.prefix for Python):

Since the program is an executable and not a script, if launcher is used to execute Python then it will first look in a location relative to the launcher executable for its libraries (../lib/), and if found use that as its library path ("the virtualenv technique")

Usage

When launcher is run we go through the following steps:

1): argv[0] is used to find the last symlink pointing to launcher; e.g., for:

foo -> launcher
bar -> foo

then the last symlink will be foo if either foo or bar is executed. It will be referred to as $launchlink below.

2): The launcher looks for the file $launchlink.link. That file is read and the contents followed as if it was a symlink, i.e. relative paths are relative to the location of the link file (the reason for not using a symlink is to avoid cluttering up PATH-based auto-completion).

If such a file is not found, it is assumed that the program/script to execute is named $launchlink.real.

3): Once the program/script to be launched is decided on, we look for the shebang #!. If found, the launcher does the shebang launching like the OS but with some extra features:

Variable expansion: ${ORIGIN} is expanded to the real path (using realpath()) of the directory containing the script. ${PROFILE_BIN_DIR} is expanded to the first directory in the chain of links to contain a file named is-profile-bin.

Secondly, it is allowed to have multiple interpreters that will be tried in turn, separated by :.

Example:

#!${PROFILE_BIN_DIR}/python:${ORIGIN}/../../../python/xkl4/bin/python
import sys
...

If a shebang is not found, a simple execv is done without changing argv. The consequence of that is that if the program is Python, it will think its binary is launcher, and search for ../lib/pythonX.Y relative to the launcher used.

Other

Errors in the launcher causes return with code 127.

Testing

Set HDIST_LAUNCHER_DEBUG=1 for debug printing. Unit tests in nosetests test_launcher.py