nixguix: Deal with manifest entries without an integrity field

In that case, this fallbacks to use the "outputHash" which is an equivalent field of the
integrity one except it's for "recursive" outputHashMode. This adds the necessary
assertions around this case so correct data is sent to loaders as well.

Related to T3781

swh/lister/nixguix/lister.py

@@ -16,6 +16,7 @@ Artifacts can be of types:
 """
 
 import base64
+import binascii
 from dataclasses import dataclass
 from enum import Enum
 import logging
@@ -362,11 +363,20 @@ class NixGuixLister(StatelessLister[PageResult]):
                     yield built_artifact
                     continue
 
+                outputHash = artifact.get("outputHash")
                 integrity = artifact.get("integrity")
-                if integrity is None:
-                    logger.warning("Skipping url <%s>: missing integrity field", origin)
+                if integrity is None and outputHash is None:
+                    logger.warning(
+                        "Skipping url <%s>: missing integrity and outputHash field",
+                        origin,
+                    )
                     continue
 
+                # Falls back to outputHash field if integrity is missing
+                if integrity is None and outputHash:
+                    # We'll deal with outputHash as integrity field
+                    integrity = outputHash
+
                 try:
                     is_tar, origin = is_tarball(urls, self.session)
                 except ArtifactNatureMistyped:
@@ -396,10 +406,18 @@ class NixGuixLister(StatelessLister[PageResult]):
                 # convert into a dict of checksums. This only parses the
                 # `hash-expression` (hash-<b64-encoded-checksum>) as defined in
                 # https://w3c.github.io/webappsec-subresource-integrity/#the-integrity-attribute
-                chksum_algo, chksum_b64 = integrity.split("-")
-                checksums: Dict[str, str] = {
-                    chksum_algo: base64.decodebytes(chksum_b64.encode()).hex()
-                }
+                try:
+                    chksum_algo, chksum_b64 = integrity.split("-")
+                    checksums: Dict[str, str] = {
+                        chksum_algo: base64.decodebytes(chksum_b64.encode()).hex()
+                    }
+                except binascii.Error:
+                    logger.exception(
+                        "Skipping url: <%s>: integrity computation failure for <%s>",
+                        url,
+                        artifact,
+                    )
+                    continue
 
                 # The 'outputHashMode' attribute determines how the hash is computed. It
                 # must be one of the following two values:

swh/lister/nixguix/tests/data/guix-swh_sources.json

@@ -27,6 +27,16 @@
       ],
       "integrity": "sha256-lV3xiWUZmSnt4LW0ni/sUyC/bbtaxkTzvFLFtJKLuI4="
     },
+    {
+      "outputHash": "sha256-9uF0fYl4Zz/Ia2UKx7CBi8ZU8jfWoBfy2QSgTSwXo5A",
+      "outputHashAlgo": null,
+      "outputHashMode": "recursive",
+      "type": "url",
+      "urls": [
+        "https://github.com/figiel/hosts/archive/v1.0.0.tar.gz"
+      ],
+      "inferredFetcher": "fetchzip"
+    },
     {
       "type": "url",
       "urls": [ "unknown://example.org/wrong-scheme-so-skipped.txt" ],

swh/lister/nixguix/tests/data/nixpkgs-swh_sources.json

@@ -79,6 +79,16 @@
       "svn_url": "https://code.call-cc.org/svn/chicken-eggs/release/5/iset/tags/2.2",
       "svn_revision": 39057
     },
+    {
+      "outputHash": "sha256-LxVcYj2WKHbhNu5x/DFkxQPOYrVkNvwiE/qcODq52Lc=",
+      "outputHashAlgo": null,
+      "outputHashMode": "recursive",
+      "type": "url",
+      "urls": [
+        "https://github.com/julian-klode/triehash/archive/debian/0.3-3.tar.gz"
+      ],
+      "inferredFetcher": "fetchzip"
+    },
     {
       "type": "url",
       "urls": [