DevinLeamy opened a new pull request, #497:
URL: https://github.com/apache/mesos/pull/497
Recently ran into a issue where `AM_PYTHON_CHECK_VERSION` was failing with
code 127 "command not found" despite that the `$PYTHON` value set by the user
when `./configure` was invoked was valid.
This was resulting in the following error message: """
configure: error: Mesos requires Python >= 2.6
------------------------------------------------------------------- The
detected Python version is 2.7.
If you already have Python 2.6+ installed (and it's the default python on
the path), you might want to check if you have the PYTHON environment variable
set to an older version of Python.
------------------------------------------------------------------- """
Which is rather confusing:
> "Requires Python >= 2.6 but finds 2.7 and throws an error?!"
What was happening is that:
1. $PYTHON=<path to python>
2. $PYTHON was being used to set $PYTHON_VERSION
3. $PYTHON_VERSION was then being used to update $PYTHON
```
if test -n "$PYTHON_VERSION"; then
AC_SUBST([PYTHON], [python$PYTHON_VERSION])
fi
```
4. This made $PYTHON="python2.7" which was not in the path.
5. AM_PYTHON_CHECK_VERSION was called which was failing to find python2.7
6. We received an error.
To fix this, we move the logic that sets $PYTHON using $PYTHON_VERSION to
run **before** the logic that sets $PYTHON_VERSION. This will preserve the
value of $PYTHON, if it was provided. In other words,
$PYTHON=/usr/bin/python2.7 will not become $PYTHON=python2.7 (the problem), but
will remain $PYTHON=/usr/bin/python2.7.
The make similar issues more obvious in the future, we additionally log the
current values of $PYTHON_VERSION **and** $PYTHON in the error message.
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