Trac #30157: Undo sqlite3's munging of DLL search paths

Includes a Cython function specifically targeted to fixing the sqlite3
issue, and adds a monkey-patching at sage import time which wraps
sqlite3.connect (the first place where the issue that causes this
is invoked) to call the workaround function.

src/module_list.py

@@ -655,6 +655,9 @@
     ################################
 
     Extension('*', ['sage/misc/*.pyx']),
+    Extension('sage.misc.sage_ostools',
+              sources = ['sage/misc/sage_ostools.pyx'],
+              libraries = ['sqlite3']),
 
     ################################
     ##
@@ -1120,10 +1123,10 @@
 
     Extension('sage.rings.polynomial.ore_polynomial_element',
               sources = ['sage/rings/polynomial/ore_polynomial_element.pyx']),
-    
+
     Extension('sage.rings.polynomial.skew_polynomial_element',
               sources = ['sage/rings/polynomial/skew_polynomial_element.pyx']),
-    
+
     Extension('sage.rings.polynomial.skew_polynomial_finite_order',
               sources = ['sage/rings/polynomial/skew_polynomial_finite_order.pyx']),
 

src/sage/__init__.py

@@ -4,6 +4,7 @@
 # to many other Python packages.
 from sage.version import version as __version__
 
+import sys
 # Make sure that the correct zlib library is loaded. This is needed
 # to prevent the system zlib to be loaded instead of the Sage one.
 # See https://trac.sagemath.org/ticket/23122
@@ -66,3 +67,40 @@ def isfunction(obj):
     pass  # Python 2
 else:
     del ExtensionFileLoader.get_source
+
+
+# Work around a Cygwin-specific bug caused by sqlite3; see
+# https://trac.sagemath.org/ticket/30157 and the docstring for
+# fix_for_ticket_30157
+# Here we monkey-patch the sqlite3 module to ensure the fix is
+# applied the very first time a connection is made to a sqlite3
+# database
+if sys.platform == 'cygwin':
+    def patch_sqlite3():
+        try:
+            from sage.misc.sage_ostools import fix_for_ticket_30157
+        except ImportError:
+            # The module might not have been re-built yet; don't worry about it
+            # then
+            return
+
+        import sqlite3, functools
+        orig_sqlite3_connect = sqlite3.connect
+
+        @functools.wraps(orig_sqlite3_connect)
+        def connect(*args, **kwargs):
+            if fix_for_ticket_30157():
+                raise RuntimeError(
+                    'patch for Trac ticket #30157 failed; please report this '
+                    'bug to https://trac.sagemath.org')
+
+            # Undo the monkey-patch
+            try:
+                return orig_sqlite3_connect(*args, **kwargs)
+            finally:
+                sqlite3.connect = orig_sqlite3_connect
+
+        sqlite3.connect = connect
+
+    patch_sqlite3()
+    del patch_sqlite3

src/sage/misc/sage_ostools.pyx

@@ -8,6 +8,7 @@ from cpython.exc cimport PyErr_SetFromErrno
 import os
 import contextlib
 
+
 def have_program(program, path=None):
     """
     Return ``True`` if a ``program`` executable is found in the path
@@ -302,3 +303,43 @@ cdef class redirection:
             if self.close_dest:
                 self.dest_file.close()
                 self.dest_fd = -1
+
+
+IF PY_PLATFORM == 'cygwin':
+    from libc.stddef cimport wchar_t
+
+    cdef extern from "Windows.h":
+        int SetDllDirectoryW(wchar_t* lpPathName)
+
+    cdef extern from "sqlite3.h":
+        int sqlite3_initialize()
+
+    def fix_for_ticket_30157():
+        """
+        Cygwin-only workaround for an issue caused by the sqlite3 library.  See
+        trac:`30157`.
+
+        The issue here is that when the sqlite3 library is first initialized
+        it modifies Windows' default DLL search path order, which can possibly
+        break the correct search path for subsequent DLL loads.
+
+        This workaround ensures that the sqlite3 library is initialized very
+        early on (this does not add any significant overhead) and then
+        immediately undoes its deleterious effects.  In particular, calling
+        SetDllDirectoryW(NULL) restores the default DLL search path.
+
+        To be clear, there's no reason sqlite3 needs this to function
+        correctly; it's just a poorly-considered hack that attempted to work
+        around a problem that doesn't affect us.
+
+        Returns 0 if it succeeeds or a non-zero value if not.
+        """
+
+        ret = sqlite3_initialize()
+
+        if ret != 0:
+            # Library initialization failed for some reason
+            return ret
+
+        # SetDllDirectory returns 1 if it succeeds.
+        return not SetDllDirectoryW(NULL)

src/sage_setup/command/sage_build_cython.py

@@ -163,6 +163,7 @@ def finalize_options(self):
         self.compile_time_env = dict(
             PY_VERSION_HEX=sys.hexversion,
             PY_MAJOR_VERSION=sys.version_info[0],
+            PY_PLATFORM=sys.platform
         )
 
         # We check the Cython version and some relevant configuration