ocibuilder: emit history entries with component info

Add OCI history entries for each layer we emit. AIUI, history is
not mandated by the OCI image spec and while most tools handle
images without it fine (buildah even has a `--omit-history` flag),
some currently break (e.g. bootc). We could fix issues where we
encounter them, but it's much easier to match the rest of the
ecosystem and have it.

As a bonus, this gives us a more reliable way to retain
component information than via layer annotations (see e.g.
https://github.com/containers/buildah/issues/6652). And there's tooling
like `podman history` built to display that info more easily.

Assisted-by: Claude Opus 4.5

src/ocibuilder.rs

@@ -134,10 +134,6 @@ impl Builder {
                 .with_context(|| format!("adding component {}", name))?;
         }
 
-        // Clear history - we don't want to emit it in the output image
-        // XXX: add a e.g. push_layer_without_history_annotated to ocidir
-        config.history_mut().take();
-
         Ok(())
     }
 
@@ -179,7 +175,28 @@ impl Builder {
             hm
         };
 
-        oci_dir.push_layer_with_history_annotated(manifest, config, layer, Some(annotations), None);
+        let mtime_i64 =
+            i64::try_from(component.mtime_clamp).context("mtime_clamp overflows i64")?;
+
+        let created = chrono::DateTime::from_timestamp(mtime_i64, 0)
+            .with_context(|| format!("invalid mtime_clamp: {}", component.mtime_clamp))?
+            .to_rfc3339_opts(chrono::SecondsFormat::Secs, true);
+
+        let history = oci_image::HistoryBuilder::default()
+            .created(created)
+            .author("chunkah".to_string())
+            .created_by("chunkah".to_string())
+            .comment(name.to_string())
+            .build()
+            .context("building history entry")?;
+
+        oci_dir.push_layer_with_history_annotated(
+            manifest,
+            config,
+            layer,
+            Some(annotations),
+            Some(history),
+        );
 
         Ok(())
     }
@@ -379,14 +396,49 @@ mod tests {
         assert_eq!(layer_b.1[0].1, tar::EntryType::Regular);
         assert_eq!(layer_b.1[0].2, 9); // "content b".len()
 
-        // Verify no history entries
+        // Verify history entries match layers
+        let history = result
+            .image_config
+            .history()
+            .as_ref()
+            .expect("image should have history entries");
+        assert_eq!(
+            history.len(),
+            result.manifest.layers().len(),
+            "history entries should match layer count"
+        );
+
+        // Check each history entry has expected fields
+        for entry in history {
+            assert_eq!(
+                entry.author().as_deref(),
+                Some("chunkah"),
+                "history author should be 'chunkah'"
+            );
+            assert_eq!(
+                entry.created_by().as_deref(),
+                Some("chunkah"),
+                "history created_by should be 'chunkah'"
+            );
+            assert!(
+                entry.comment().is_some(),
+                "history should have comment with component name"
+            );
+            assert!(
+                entry.created().is_some(),
+                "history should have created timestamp"
+            );
+        }
+
+        // Verify component names in history comments
+        let comments: Vec<_> = history.iter().filter_map(|h| h.comment().clone()).collect();
         assert!(
-            result
-                .image_config
-                .history()
-                .as_ref()
-                .is_none_or(|h| h.is_empty()),
-            "image should have no history entries"
+            comments.iter().any(|c| c == "component_a"),
+            "history comments should contain component_a"
+        );
+        assert!(
+            comments.iter().any(|c| c == "component_b"),
+            "history comments should contain component_b"
         );
     }