Bug#891867: diffoscope: improve .changes diffs

[Helmut Grohne]

Package: diffoscope
Version: 91
Severity: wishlist
Tags: patch

Hi awesome reproducible team!

I'm working on adding more build profiles and diffoscope + reproducible
builds turn out to be an awesome tool in validating that my profiles
only drop packages and never change package contents. :)

Now I've run into a strange corner case with diffing .changes files. If
the last entry in a Checksums-Sha256 field is removed, new last entry is
removed and added. For example:

├── Files
│ @@ -1,12 +1,8 @@
│  
│   a6434caf7853f0e84013e6004b934351 10496 debug optional cracklib-runtime-dbgsym_2.9.2-5.1_amd64.deb
│   93d547da959c0080bb45cc05d354539f 148972 admin optional cracklib-runtime_2.9.2-5.1_amd64.deb
│ - 608dbc69affb8158f7f203e3c83e5595 11084 libs optional cracklib2_2.9.2-5.1_amd64.buildinfo
│ + 1941a0fd2dbf8b3df8536caab1895993 9042 libs optional cracklib2_2.9.2-5.1_amd64.buildinfo
│   b2010c8e3498d469ae3b5bf5e5469da1 21416 debug optional libcrack2-dbgsym_2.9.2-5.1_amd64.deb
│   f099f5629a8b0cad9e5fba5ded9f88d9 32052 libdevel extra libcrack2-dev_2.9.2-5.1_amd64.deb
│   e3b100bcf99561c965cc5da8ef58e206 120108 debian-installer optional libcrack2-udeb_2.9.2-5.1_amd64.udeb
│ - e04c3f5f14a2c8eb2022ba8eb1d5d915 54788 libs optional libcrack2_2.9.2-5.1_amd64.deb
│ - 85fa278ff743cf93ead1075664df42f2 12880 debug optional python-cracklib-dbgsym_2.9.2-5.1_amd64.deb
│ - 674e3f26f4301f5c253fddb0366a3832 23392 python optional python-cracklib_2.9.2-5.1_amd64.deb
│ - c1bceeb542a7dbed7d2989fd93bba60b 14160 debug optional python3-cracklib-dbgsym_2.9.2-5.1_amd64.deb
│ - b9868203bdda021286129428b5eb3e55 23424 python optional python3-cracklib_2.9.2-5.1_amd64.deb
│ + e04c3f5f14a2c8eb2022ba8eb1d5d915 54788 libs optional libcrack2_2.9.2-5.1_amd64.deb

The DotChangesFile comaprator performs a super call into DebControlFile,
which pulls the Checksums-Sha256 field out of the parsed deb822 file. In
doing so it strips the trailing newline and this what makes for the
difference above. I propose working around the issue with the following
patch:

--- a/diffoscope/comparators/debian.py
+++ b/diffoscope/comparators/debian.py
@@ -142,16 +142,16 @@
         # Compare Files as string
         if self.deb822.get('Files'):
             differences.append(Difference.from_text(
-                self.deb822.get_as_string('Files'),
-                other.deb822.get_as_string('Files'),
+                self.deb822.get_as_string('Files') + "\n",
+                other.deb822.get_as_string('Files') + "\n",
                 self.path,
                 other.path,
                 source='Files',
             ))
         else:
             differences.append(Difference.from_text(
-                self.deb822.get_as_string('Checksums-Sha256'),
-                other.deb822.get_as_string('Checksums-Sha256'),
+                self.deb822.get_as_string('Checksums-Sha256') + "\n",
+                other.deb822.get_as_string('Checksums-Sha256') + "\n",
                 self.path,
                 other.path,
                 source='Checksums-Sha256',

Many thanks to Mattia Rizzolo for pointing precisely at the relevant
source code.

Helmut