fix(model-client): endless merge in VersionMerger

model-server and model-client were both doing the same merge creating
two new versions that were merged again into another version.
Both sides were creating new merges forever without any actual changes
to the model.

Author: slisson

model-datastructure/src/commonMain/kotlin/org/modelix/model/VersionMerger.kt

@@ -45,11 +45,32 @@ class VersionMerger(private val storeCache: IDeserializingKeyValueStore, private
         }
     }
 
+    private fun collectLatestNonMerges(version: CLVersion?, visited: MutableSet<String>, result: MutableSet<Long>) {
+        if (version == null) return
+        if (!visited.add(version.getContentHash())) return
+        if (version.isMerge()) {
+            collectLatestNonMerges(version.getMergedVersion1(), visited, result)
+            collectLatestNonMerges(version.getMergedVersion2(), visited, result)
+        } else {
+            result.add(version.id)
+            collectLatestNonMerges(version.baseVersion, visited, result)
+        }
+    }
+
     protected fun mergeHistory(leftVersion: CLVersion, rightVersion: CLVersion): CLVersion {
         if (leftVersion.hash == rightVersion.hash) return leftVersion
         val commonBase = Companion.commonBaseVersion(leftVersion, rightVersion)
         if (commonBase?.hash == leftVersion.hash) return rightVersion
         if (commonBase?.hash == rightVersion.hash) return leftVersion
+
+        val leftNonMerges = HashSet<Long>().also { collectLatestNonMerges(leftVersion, HashSet(), it) }
+        val rightNonMerges = HashSet<Long>().also { collectLatestNonMerges(rightVersion, HashSet(), it) }
+        if (leftNonMerges == rightNonMerges) {
+            // If there is no actual change on both sides, but they just did the same merge, we have to pick one
+            // of them, otherwise both sides will continue creating merges forever.
+            return if (leftVersion.id < rightVersion.id) leftVersion else rightVersion
+        }
+
         val versionsToApply = LinearHistory(commonBase?.hash).load(leftVersion, rightVersion)
 
 //        println("merge ${getVersion(leftVersionHash).id.toString(16)} ${LinearHistory(storeCache, commonBase).load(leftVersion).map { it.id.toString(16) }} and ${getVersion(rightVersionHash).id.toString(16)} ${LinearHistory(storeCache, commonBase).load(rightVersion).map { it.id.toString(16) }}: ${commonBase?.let{getVersion(it)}?.id?.toString(16)} + ${versionsToApply.map { it.id.toString(16) }}")