Misinstallation of Python in Cygwin

[embray]

I've encountered yet another problem in my quest to get sage building on Cygwin. I'm surprised this hasn't come up before though which makes me wonder if I'm doing something wrong.

The DLL loader library, libpython2.7.dll.a, gets installed to $(SAGE_LOCAL)/lib/python2.7/config/ which is not normally on the ld search path (similar on Python 3 I think).

This means that when building extension modules -lpython2.7 is actually linking against my system Python (in this case the Python that came with cygwin--also Python 2.7.10). This is of course wrong, but isn't a problem for the majority of modules, including sage itself (at least it wasn't a problem for compiling). It was actually PIL(low) where this fell apart because it used some PyUnicode_ functions that were not found in my system libpython because it was compiled with --enable-unicode=ucs2, whereas Sage builds Python with --enable-unicode=ucs4.

When installing Python, Sage should be making a symlink to $(SAGE_LOCAL)/lib/python2.7/config/libpython2.7.dll.a in $(SAGE_LOCAL)/lib.

I'm leaning toward calling this an upstream bug--I don't understand why this isn't done automatically by Python's Makefile. The relevant portion begins ​here. It seems that if a DLL is built it does not install a symlink to the loader library in lib/. I've brought this up here: ​https://mail.python.org/pipermail/python-dev/2016-April/144315.html

For reference, Cygwin's own system package for Python installs this symlink manually... It seems odd not to do it by default.

[embray]

After applying the following patch, I rebuilt python2, and was subsequently able to build pillow:

build/pkgs/python2/spkg-install

@@ -129 +129 @@
 if [ "$UNAME" = "Darwin" ] && \
     [ `uname -r | cut '-d.' -f1` -gt 9 ]; then
     rm -f "$SAGE_LOCAL/lib/python2.7/config/libpython2.7.a"
+elif [ "$UNAME" = "CYGWIN" ]; then
+    # See http://trac.sagemath.org/ticket/20437
+    ln -sf "python2.7/config/libpython2.7.dll.a" "$SAGE_LOCAL/lib/libpython2.7.dll.a"
 fi
 
 # Make sure extension modules were built correctly.

Should probably do the same for Python 3.

[embray]

Added a patch for python2 and python2. Basically the same as the above diff, but slightly enhanced to not use a hard-coded pythonX.Y version.

---

New commits:

​e45b216

Create symlinks in $SAGE_LOCAL/lib to $SAGE_LOCAL/lib/pythonX.Y/config/libpythonX.Y.dll.a. Also get Python version from python instead of hard-coding. Note: This is required on Cygwin in order for extension modules to link to the correct DLL import library for the python in sage. Cygwin itself does this in its system python package.

[vdelecroix]

Hi Eric,

only sided notes (as I am not a Cygwin guy):

• in the ticket, you should always fill the "Authors" field (with your full name). The reason is that patchbots completely ignore tickets where it is not set.

• in a commit it is standard to have one short line description, a linebreak and then an optional description. That way the history is much more readable (e.g. with git log --oneline).

[embray]

Sounds reasonable.

[embray]

Incidentally, is this documented somewhere? I don't see anything about this use of the "Authors" field under ​http://doc.sagemath.org/html/en/developer/trac.html#section-trac-fields or here: ​https://wiki.sagemath.org/buildbot. It would be nice if there were a single, easy to point to checklist for tickets--sort of a choose your own adventure story.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​66ceeb2

On Cygwin: Create symlinks in $SAGE_LOCAL/lib to $SAGE_LOCAL/lib/pythonX.Y/config/libpythonX.Y.dll.a.

[embray]

Build passed--any comments?

[jdemeyer]

One detail: you should quote $SAGE_LOCAL/bin/python as "$SAGE_LOCAL/bin/python" in case of funny characters in $SAGE_LOCAL.

Did you report this upstream? The "report upstream" field says "Not yet reported upstream; Will do shortly."

[embray]

Replying to jdemeyer:

One detail: you should quote $SAGE_LOCAL/bin/python as "$SAGE_LOCAL/bin/python" in case of funny characters in $SAGE_LOCAL.

Sure.

Did you report this upstream? The "report upstream" field says "Not yet reported upstream; Will do shortly."

I'll do that now.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​71062f0

Adding quotes around python call in case of funny business in SAGE_LOCAL.

[gouezel]

Works for me. You can set if to positive review in a few days, if there is no answer from upstream.

[embray]

No response from python-dev yet. I only asked on the mailing list though--might get more response if I opened a bug report instead. I was just being polite and not assuming it was definitely a bug :)

It's only Monday though, and early yet in the Americas so I'll wait a little longer.

[embray]

I opened a bug report about this in Python, and included a patch: ​http://bugs.python.org/issue27374 as this is more likely to at least be acknowledged than my original mailing list post.

I wouldn't be surprised if this isn't acted on, or even if it is it won't be of any immediately help. So it would be nice to go ahead and get this merged, given a positive review.