Correctly handle layers with multiple XML files

Some data layers can legitimately contain more than one XML file. We
need to be able to differentiate between them in order to locate the
FGDC metadata. This provides some basic support to do this. A more
robust implementation should incrementally parse the beginning of an XML
file to find the root element.

slingshot/app.py

@@ -1,5 +1,6 @@
 import base64
 import os
+import re
 import shutil
 import uuid
 from zipfile import ZipFile
@@ -179,6 +180,7 @@ def commit(self):
 class GeoBag(object):
     def __init__(self, bag):
         self.bag = bag
+        self._fgdc = None
         try:
             self.record = MitRecord.from_file(self.gbl_record)
         except Exception:
@@ -197,7 +199,21 @@ def gbl_record(self):
 
     @property
     def fgdc(self):
-        return self._file_by_ext('.xml')
+        """Full path to FGDC file."""
+        # There can sometimes be multiple XML files in a package. We will
+        # try to find an FGDC file among them.
+        if not self._fgdc:
+            files = self._files_by_ext('.xml')
+            if len(files) > 1:
+                for f in files:
+                    with open(f, encoding="utf-8") as fp:
+                        head = fp.read(1024)
+                    if re.match('\s*<metadata>', head, re.I):
+                        self._fgdc = f
+                        break
+            else:
+                self._fgdc = files[0]
+        return self._fgdc
 
     def save(self):
         self.record.to_file(self.gbl_record)
@@ -206,15 +222,29 @@ def save(self):
     def is_valid(self):
         return self.bag.is_valid()
 
-    def _file_by_ext(self, ext):
+    def _files_by_ext(self, ext):
+        """Return list of full paths to files with given extension.
+
+        Raises :class:`slingshot.app.MissingFile` exception when no
+        file with the given extension is found.
+        """
         fnames = [f for f in self.bag.payload_files()
                   if f.lower().endswith(ext.lower())]
+        if not fnames:
+            raise MissingFile('Could not find file with extension {}'
+                              .format(ext))
+        return [os.path.join(str(self.bag), name) for name in fnames]
+
+    def _file_by_ext(self, ext):
+        """Return full path to a single file with the given extension.
+
+        Raises :class:`slingshot.app.TooManyFiles` exception when more
+        than one file with the specified extension is found.
+        """
+        fnames = self._files_by_ext(ext)
         if len(fnames) > 1:
-            raise Exception('Multiple files with extension {}'.format(ext))
-        elif not fnames:
-            raise Exception('Could not find file with extension {}'
-                            .format(ext))
-        return os.path.join(str(self.bag), fnames.pop())
+            raise TooManyFiles('Multiple files with extension {}'.format(ext))
+        return fnames[0]
 
 
 class GeoTiffBag(GeoBag):
@@ -228,7 +258,7 @@ def name(self):
     def tif(self):
         try:
             return self._file_by_ext('.tif')
-        except Exception:
+        except MissingFile:
             return self._file_by_ext('.tiff')
 
 
@@ -337,3 +367,11 @@ def close(self):
             self.shx.close()
         if self.dbf:
             self.dbf.close()
+
+
+class MissingFile(Exception):
+    """Required file in spatial data package is missing."""
+
+
+class TooManyFiles(Exception):
+    """Too many files of the same extension found in spatial data package."""