magic-proxy: return 404 when InRelease file cannot be found "by hash"

When the magic-proxy script could not find a valid InRelease file for the
configured timestamp, it would fall back to serving the canonical version
of it. This meant that builds would succeed, even though snap-shotting the
repository failed.

This update makes the script return HTTP 404 when an InRelease by-hash
link for a given combination of mirror, suite and timestamp cannot be
found.

debian/changelog

@@ -1,8 +1,19 @@
 livecd-rootfs (2.566) UNRELEASED; urgency=medium
 
+  [ Julian Andres Klode ]
   * Do not mark direct dependencies of ubiquity as auto installed. This caused
     cryptsetup to remain auto on the installed system (LP: #1801629)
 
+  [ Tobias Koch ]
+  * When the magic-proxy script could not find a valid InRelease file for the
+    configured timestamp, it would fall back to serving the canonical version
+    of it. This meant that builds would succeed, even though snap-shotting the
+    repository failed.
+    .
+    This update makes the script return HTTP 404 when an InRelease by-hash
+    link for a given combination of mirror, suite and timestamp cannot be
+    found.
+
  -- Julian Andres Klode <juliank@ubuntu.com>  Fri, 22 Feb 2019 10:58:48 +0100
 
 livecd-rootfs (2.565) disco; urgency=medium

magic-proxy

@@ -771,14 +771,14 @@ class ProxyingHTTPRequestHandler(http.server.BaseHTTPRequestHandler):
 
             index = LPInReleaseIndex(mirror, suite,
                         cache=self.server.inrelease_cache)
-
-            try:
             inrelease = index.get_inrelease_for_timestamp(
                     self.server.snapshot_stamp)
-            except LPInReleaseIndexError as e:
-                inrelease = None
 
-            if inrelease is not None:
+            if inrelease is None:
+                self.__send_error(404, "No InRelease file found for given "
+                                       "mirror, suite and timestamp.")
+                return
+
             if target == "InRelease":
                 # If target is InRelease, send back contents directly.
                 data = inrelease.data.encode("utf-8")
@@ -839,6 +839,12 @@ class ProxyingHTTPRequestHandler(http.server.BaseHTTPRequestHandler):
         self.end_headers()
         shutil.copyfileobj(response, self.wfile)
 
+    def __send_error(self, status, message):
+        """Return an HTTP error status and a message in the response body."""
+        self.send_response(status)
+        self.send_header("Content-Type", "text/plain; charset=utf-8")
+        self.wfile.write(message.encode("utf-8"))
+
 
 class MagicHTTPProxy(socketserver.ThreadingMixIn, http.server.HTTPServer):
     """Tiny HTTP server using ProxyingHTTPRequestHandler instances to provide