Avoid stack overflow in cachekeycomputer

I managed to trigger a stack overflow in Morph, by having two chunks with
the same name and same build instructions in different strata (by mistake,
obviously).

The error before was this:

    ....
      File "/src/morph/morphlib/cachekeycomputer.py", line 77, in get_cache_id
        cacheid = self._calculate(source)
      File "/src/morph/morphlib/cachekeycomputer.py", line 86, in _calculate
        for a in source.dependencies],
      File "/src/morph/morphlib/cachekeycomputer.py", line 38, in compute_key
        ret = self._hash_id(self.get_cache_id(source))
      File "/src/morph/morphlib/cachekeycomputer.py", line 77, in get_cache_id
        cacheid = self._calculate(source)
      File "/src/morph/morphlib/cachekeycomputer.py", line 83, in _calculate
        'env': self._filterenv(self._build_env.env),
      File "/src/morph/morphlib/cachekeycomputer.py", line 32, in _filterenv
        return dict([(k, env[k]) for k in keys])
    RuntimeError: maximum recursion depth exceeded while calling a Python object

The error afterwards is this:

    2016-02-18 13:53:14 Deciding on task order
    ERROR: There are multiple versions of component 'cython'

Change-Id: I96a41cabe4d9aa81bddc5186c5248a636a0843c2

1 files changed, 12 insertions, 0 deletions

diff --git a/morphlib/sourceresolver.py b/morphlib/sourceresolver.py
index ce395645..d7c9707a 100644
--- a/morphlib/sourceresolver.py
+++ b/morphlib/sourceresolver.py
@@ -491,6 +491,18 @@ def create_source_pool(lrc, rrc, repo, ref, filenames, cachedir,
 
     def add_to_pool(reponame, ref, filename, absref, tree, morphology,
                     predefined_split_rules):
+        # If there are duplicate chunks which have the same 'name' and the
+        # same build instructions, we might cause a stack overflow in
+        # cachekeycomputer.py when trying to hash the build graph. The
+        # _find_duplicate_chunks() function doesn't handle this case, it
+        # is checking for duplicates with the same name but different build
+        # instructions.
+        if morphology['kind'] != 'stratum':
+            if pool.lookup(reponame, ref, filename):
+                raise morphlib.Error(
+                    "There are multiple versions of component '%s'" %
+                    morphology['name'])
+
         sources = morphlib.source.make_sources(
             reponame, ref, filename, absref, tree, morphology,
             predefined_split_rules)