replace /usr/local as a source of packages in the python relocatable interpreter

I was playing with the python3 interpreter trying to get pip to work,
just to see how far we can go. We don't really need pip, but I figured
it would be a good stress test to make sure that the process is working
and robust.

And it didn't really work, because although pip will correctly install
things into $relocatable_root/local/lib, sys.path will still refer to a
hardcoded /usr/local. While this should not affect Scylla, since we
expect to have all our modules in out path anyway -- and that path is
searched before /usr/local, it is still dangerous to make an absolute
reference like this.

Unfortunately, /usr/local/ it is included unconditionally by site.py,
which is executed when the interpreter is started and there is no
environment variable I found to change that (the help string refers to
PYTHONNOUSERSITE, but I found no mention of that in site.py whatsoever)

There is a way to tell site.py not to bother to add user sites, by
passing the -s flag, which this patch does.

Aside from doing that, we also enhance PYTHONPATH to include a reference
to ./local/{lib,lib64}/python<version>/site-packages.

After applying this patch, I was able to build an interpreter containing
only python3-pip and python3-setuptools, and build the relocatable
environment from there.

Signed-off-by: Glauber Costa <glauber@scylladb.com>
Message-Id: <20190206052104.25927-1-glauber@scylladb.com>

scripts/create-relocatable-python.py

@@ -102,16 +102,21 @@ def fix_dynload(ar, binpath, targetpath):
     ar.add(patched_binary, arcname=targetpath, recursive=False)
 
 def gen_python_thunk(ar, pybin):
-    thunk=b'''\
+    base_thunk='''\
 #!/bin/bash
 x="$(readlink -f "$0")"
 b="$(basename "$x")"
 d="$(dirname "$x")/.."
 ldso="$d/libexec/ld.so"
 realexe="$d/libexec/$b.bin"
-exec -a "$0" "$ldso" "$realexe" "$@"
+PYTHONPATH="$d/{sitepackages}:$d/{sitepackages64}:$PYTHONPATH" exec -a "$0" "$ldso" "$realexe" -s "$@"
 '''
 
+    sitepackages = os.path.join("local/lib/", pybin, "site-packages")
+    sitepackages64 = os.path.join("local/lib64/", pybin, "site-packages")
+
+    thunk = base_thunk.format(sitepackages=sitepackages, sitepackages64=sitepackages64).encode()
+
     ti = tarfile.TarInfo(name=os.path.join("bin", pybin))
     ti.size = len(thunk)
     ti.mode = 0o755