Improve code stability is checksum checking

- use context manager for dealing with the checksum file
- use loop that can tolerate zero, one, or more items return rather than the previous expression which would break if anything other than exactly one item was returned

(cherry picked from commit 03dbb1d9c4)

squash

changelogs/fragments/get_url-remove-fragile-code.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - get_url - improve code that parses checksums from a file so it is not fragile and reports a helpful error when no matching checksum is found

lib/ansible/modules/net_tools/basics/get_url.py

@@ -445,11 +445,22 @@ def main():
                 checksum_url = checksum
                 # download checksum file to checksum_tmpsrc
                 checksum_tmpsrc, checksum_info = url_get(module, checksum_url, dest, use_proxy, last_mod_time, force, timeout, headers, tmp_dest)
-                lines = [line.rstrip('\n') for line in open(checksum_tmpsrc)]
+                with open(checksum_tmpsrc) as f:
+                    lines = [line.rstrip('\n') for line in f]
                 os.remove(checksum_tmpsrc)
                 lines = dict(s.split(None, 1) for s in lines)
                 filename = url_filename(url)
-                [checksum] = (k for (k, v) in lines.items() if v.strip('./') == filename)
+
+                # Look through each line in the checksum file for a hash corresponding to
+                # the filename in the url, returning the first hash that is found.
+                for cksum in (s for (s, f) in lines.items() if f.strip('./') == filename):
+                    checksum = cksum
+                    break
+                else:
+                    checksum = None
+
+                if checksum is None:
+                    module.fail_json("Unable to find a checksum for file '%s' in '%s'" % (filename, checksum_url))
             # Remove any non-alphanumeric characters, including the infamous
             # Unicode zero-width space
             checksum = re.sub(r'\W+', '', checksum).lower()