55. Now it's failing because SBT index is saving all signatures

(instead of only the one it was used to build the tree).
This was actually a feature (see #198) but it broke the SBT code
(it wasn't ready for that!)

sourmash/signature.py

@@ -22,6 +22,7 @@
 
 
 sig_refs = weakref.WeakKeyDictionary()
+mhs_refs = weakref.WeakKeyDictionary()
 
 
 class SourmashSignature(RustObject):
@@ -61,6 +62,19 @@ def __str__(self):
         return "SourmashSignature({})".format(md5pref)
     __repr__ = __str__
 
+    def minhashes(self):
+        size = ffi.new("uintptr_t *")
+        mhs_ptr = self._methodcall(lib.signature_get_mhs, size)
+        size = ffi.unpack(size, 1)[0]
+
+        mhs = []
+        for i in range(size):
+            mh = MinHash._from_objptr(mhs_ptr[i], shared=True)
+            mhs.append(mh)
+#            mhs_refs[mh] = mh
+
+        return mhs
+
     def md5sum(self):
         "Calculate md5 hash of the bottom sketch, specifically."
         m = hashlib.md5()
@@ -276,10 +290,11 @@ def load_signatures(data, ksize=None, select_moltype=None,
             sig_refs[sig] = sigs
 
         for sig in sigs:
-            if not ksize or ksize == sig.minhash.ksize:
-                if not select_moltype or \
-                     sig.minhash.is_molecule_type(select_moltype):
-                    yield sig
+            for minhash in sig.minhashes():
+                if not ksize or ksize == minhash.ksize:
+                    if not select_moltype or \
+                         minhash.is_molecule_type(select_moltype):
+                        yield sig
     except Exception as e:
         error("Error in parsing signature; quitting.")
         error("Exception: {}", str(e))