python: have %%dve and %%pml honor SPOT_TMPDIR and TMPDIR

* python/spot/aux.py (tmpdir): New context manager.
* python/spot/ltsmin.i: Use it for the two magics.
* NEWS: Mention this.

NEWS

@@ -127,6 +127,10 @@ New in spot 2.0.3a (not yet released)
     with spins, and dynamically load them.   This is
     akin to the %%dve magic that was already supported.
 
+  * The %%dve and %%pml magics honor the SPOT_TMPDIR and
+    TMPDIR environment variables.  This especially helps
+    when the current directory is read-only.
+
   Documentation:
 
   * A new example page shows how to test the equivalence of

python/spot/aux.py

@@ -26,6 +26,7 @@ import subprocess
 import sys
 import os
 import errno
+import contextlib
 
 def extend(*classes):
     """
@@ -80,3 +81,13 @@ def rm_f(filename):
     except OSError as e:
         if e.errno != errno.ENOENT:
             raise
+
+@contextlib.contextmanager
+def tmpdir():
+    cwd = os.getcwd()
+    tmpdir = os.environ.get('SPOT_TMPDIR') or os.environ.get('TMPDIR') or '.'
+    try:
+        os.chdir(tmpdir)
+        yield
+    finally:
+        os.chdir(cwd)

python/spot/ltsmin.i

@@ -149,8 +149,7 @@ try:
         def dve(self, line, cell):
             if not line:
                raise ValueError("missing variable name for %%dve")
-            # DiViNe prefers when files are in the current directory
-            # so write cell into local file
+            with spot.aux.tmpdir():
                with tempfile.NamedTemporaryFile(dir='.', suffix='.dve') as t:
                    t.write(cell.encode('utf-8'))
                    t.flush()
@@ -179,6 +178,7 @@ try:
         def pml(self, line, cell):
             if not line:
                raise ValueError("missing variable name for %%pml")
+            with spot.aux.tmpdir():
                with tempfile.NamedTemporaryFile(dir='.', suffix='.pml') as t:
                    t.write(cell.encode('utf-8'))
                    t.flush()