Fix crash in CPAchecker's tool-info module

We have a heuristic that checks if CPAchecker is used from source
but misses a rebuild. This would crash if broken symlinks or other
file-access problems occur.
As it is just an optional feature, we can ignore such problems
and print a warning.

An example where this happened was for example
system-wide installation of CPAchecker
with the executable being /usr/bin/cpachecker
and /usr/src existing.

benchexec/tools/cpachecker.py

@@ -69,16 +69,21 @@ def executable(self, tool_locator):
 
         # If this is a source checkout of CPAchecker, we heuristically check that
         # sources are not newer than binaries (cpachecker.jar or files in bin/).
-        if os.path.isdir(src_dir):
-            src_mtime = self._find_newest_mtime(src_dir)
-
-            if os.path.isfile(jar_file):
-                if src_mtime > os.stat(jar_file).st_mtime:
-                    sys.exit("CPAchecker JAR is not uptodate, run 'ant jar'!")
-
-            elif os.path.isdir(cls_dir):
-                if src_mtime > self._find_newest_mtime(cls_dir):
-                    sys.exit("CPAchecker build is not uptodate, run 'ant'!")
+        try:
+            if os.path.isdir(src_dir):
+                src_mtime = self._find_newest_mtime(src_dir)
+
+                if os.path.isfile(jar_file):
+                    if src_mtime > os.stat(jar_file).st_mtime:
+                        sys.exit("CPAchecker JAR is not uptodate, run 'ant jar'!")
+
+                elif os.path.isdir(cls_dir):
+                    if src_mtime > self._find_newest_mtime(cls_dir):
+                        sys.exit("CPAchecker build is not uptodate, run 'ant'!")
+        except OSError as e:
+            logging.warning(
+                "Could not determine whether CPAchecker needs to be rebuilt: %s", e
+            )
 
         return executable