linux: show mountinfo if unable to write core file

Find out what kind of weird situation prevents this

Author: mdavidsaver

ci_core_dumper/linux.py

@@ -337,14 +337,18 @@ def dump2(pid, gdb, extra_cmds):
     # write the core file into some temporary storage in the target mount NS
     for tmpdir in ('/tmp', '/var/tmp', '/dev/shm'):
         corefile = os.path.join(tmpdir, 'core.ccd.%d'%pid)
+        assert not os.path.exists(corefile), corefile
         try:
             OF = open(corefile, 'wb')
             break
-        except:
-            print('Unable to write core file to %s'%corefile)
+        except Exception as e:
+            print('Unable to write core file to %s : %r'%(corefile, e))
             continue
     else:
         print('Unable to store core file in target FS')
+        with open('/proc/{}/mountinfo'.format(pid), 'r') as F:
+            for L in F.readlines():
+                print('  ', L.rstrip())
         sys.exit(1)
 
     print('Wrote core file to %s'%corefile)