for filesystems not supporting mmap'ing a file we need to use a

SyncToFd dummy just as we did for compressed files in 0.9.5

apt-pkg/contrib/mmap.cc

@@ -102,6 +102,7 @@ bool MMap::Map(FileFd &Fd)
 	 {
 	    // for readonly, we don't need sync, so make it simple
 	    Base = malloc(iSize);
+	    SyncToFd = new FileFd();
 	    return Fd.Read(Base, iSize);
 	 }
 	 // FIXME: Writing to compressed fd's ?

debian/changelog

@@ -18,6 +18,8 @@ apt (0.9.7.3) UNRELEASED; urgency=low
       second (filebased) constructor of DynamicMMap (Closes: #677704)
     - refer to APT::Cache-Start in case the growing failed as if -Limit is
       really the offender it will be noted in a previous error message.
+    - for filesystems not supporting mmap'ing a file we need to use a
+      SyncToFd dummy just as we did for compressed files in 0.9.5
 
  -- David Kalnischkies <kalnischkies@gmail.com>  Wed, 18 Jul 2012 11:45:57 +0200