Trac #30149: Cygwin: problem with DLL search order when using system …

…Python

I noticed a problem when trying to run the tests for the latest Sage,
that the wrong version of `libR.dll` was being loaded, because I have R
installed via Sage, but I also have the R package for Cygwin installed.

My path starts with:

{{{
/home/embray/src/sagemath/sage/local/lib/R/lib:/home/embray/src/sagemath
/sage/local/lib:/home/embray/src/sagemath/sage/build/bin:/home/embray/sr
c/sagemath/sage/src/bin:/home/embray/src/sagemath/sage/local/bin:/usr/lo
cal/bin:/usr/bin
}}}

so when importing `rpy2` it should link with the `libR.dll` that's in
`$SAGE_LOCAL/lib/R/lib`.  Normally this has been the case.

But when using the system Python this is not so.  This is because
according to the standard [https://docs.microsoft.com/en-us/previous-
versions/7d83bc18(v=vs.140)?redirectedfrom=MSDN DLL search path] the
first place it looks is:

> The directory where the executable module for the current process is
located.

Well, when running the system Python that's `/usr/bin` where
`python3.7m.exe` lives.  However, that's also where the system's
`libR.dll` lives.

This is just an example, but it's a broader problem: For any DLL linked
to by a compiled Python module, it will always privilege the one in
`/usr/bin` over a copy provided by Sage.

What we might have to do is, at least on Cygwin, when creating the venv
it should actually copy the Python executable instead of just symlinking
to it.  I believe venv has an option for this.

URL: https://trac.sagemath.org/30149
Reported by: embray
Ticket author(s): Erik Bray
Reviewer(s): Matthias Koeppe

build/bin/sage-venv

@@ -4,7 +4,7 @@
 # Adapted from examples in https://docs.python.org/3/library/venv.html
 
 import venv
-import os
+import sys
 import argparse
 
 parser = argparse.ArgumentParser(prog=__name__,
@@ -37,8 +37,8 @@ options = parser.parse_args()
 if options.upgrade and options.clear:
  raise ValueError('you cannot supply --upgrade and --clear together.')
 
-if os.name == 'nt':
- # default for Windows
+if sys.platform == 'cygwin':
+ # default for Cygwin; see https://trac.sagemath.org/ticket/30149
  use_symlinks = False
 else:
  # default for posix