Trac #20736: Upgrade patchbot to 2.6.1 as a system package

The patchbot is currently installing in a very not kosher way, as
explained in

sagemath/sage-patchbot#79

This ticket will change completely the installation mode. The patchbot
will become a python package, to be installed using pip in
the system python as explained below.

This ticket also updates the patchbot, in particular to cope with the
new https trac.

New tarball for patchbot: http://www-irma.u-strasbg.fr/~chapoton/sage-
patchbot-2.6.1.tar.gz

installation (two equivalent methods at your disposal):
1. Download the tarball and then
{{{
pip install --user sage-patchbot-2.6.1.tar.gz
}}}

2. (recommended)
{{{
pip install --user git+https://github.com/sagemath/sage-
patchbot.git@2.6.1
}}}

There are now two possible ways to launch the patchbot:
1.
{{{
python -m sage_patchbot.patchbot --sage-root=XXXX
}}}
or
2. Checkout the branch here and run
{{{
make && ./sage --patchbot
}}}

URL: https://trac.sagemath.org/20736
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): Jeroen Demeyer

src/bin/sage

@@ -276,8 +276,26 @@ fi
 # an unclobbered environment before testing unsafe tickets.
 if [ "$1" = '-patchbot' -o "$1" = "--patchbot" ]; then
     shift
-    cd "$SAGE_ROOT"
-    exec local/bin/patchbot/patchbot.py "$@"
+    # We ask the Python from Sage where the patchbot is installed.
+    # We set PYTHONPATH to that directory such that the system Python
+    # should also find the sage_patchbot package.
+    cmd='import sage_patchbot as p; import os; print(os.path.dirname(p.__path__[0]))'
+    export PYTHONPATH=`"$SAGE_ROOT/sage" --python -c "$cmd"`
+    if [ -z "$PYTHONPATH" ]; then
+        # Something went wrong, assume that the patchbot is not installed
+        echo >&2 "Error: cannot find installation path for sage_patchbot"
+        echo >&2 "See https://wiki.sagemath.org/buildbot for instructions"
+        exit 1
+    fi
+
+    # Try "python2.7", then "python2", then "python"
+    shopt -s execfail  # Do not exit if "exec" fails
+    exec python2.7 -m sage_patchbot.patchbot "$@"
+    exec python2 -m sage_patchbot.patchbot "$@"
+    exec python -m sage_patchbot.patchbot "$@"
+    echo >&2 "Error: cannot find a suitable Python program."
+    echo >&2 "The SageMath patchbot requires a system Python installation."
+    exit 127
 fi
 
 # Check for '--upgrade' before sourcing sage-env: the top-level "make"