More verbose error when merging sessions

Author: dcherian

icechunk/src/session.rs

@@ -109,8 +109,11 @@ pub enum SessionErrorKind {
     InvalidIndex { coords: ChunkIndices, path: Path },
     #[error("invalid chunk index for splitting manifests: {coords:?}")]
     InvalidIndexForSplitManifests { coords: ChunkIndices },
-    #[error("incompatible manifest splitting config when merging sessions")]
-    IncompatibleSplits,
+    #[error("incompatible manifest splitting config when merging two sessions")]
+    IncompatibleSplittingConfig {
+        ours: ManifestSplittingConfig,
+        theirs: ManifestSplittingConfig,
+    },
     #[error("`to` snapshot ancestry doesn't include `from`")]
     BadSnapshotChainForDiff,
     #[error("failed to create manifest from chunk stream")]
@@ -920,7 +923,11 @@ impl Session {
                 .get(node)
                 .is_some_and(|their_splits| !our_splits.compatible_with(their_splits))
         }) {
-            return Err(SessionErrorKind::IncompatibleSplits.into());
+            let ours = self.config().manifest().splitting().clone();
+            let theirs = self.config().manifest().splitting().clone();
+            return Err(
+                SessionErrorKind::IncompatibleSplittingConfig { ours, theirs }.into()
+            );
         }
 
         // Session.splits is _complete_ in that it will include every possible split.