Support circular references in Sync Streams (#608)

Author: simolus3

.changeset/pretty-ties-judge.md

@@ -0,0 +1,5 @@
+---
+'@powersync/service-sync-rules': patch
+---
+
+Support multiple references between the same tables as join conditions for Sync Streams (the compiler would previously crash due to an "internal circular reference error").

packages/sync-rules/src/compiler/querier_graph.ts

@@ -305,11 +305,28 @@ class PendingQuerierPath {
       } else {
         // Must be a match term.
         const partitionBy = (key: PartitionKey, otherRow: SingleDependencyExpression) => {
+          const otherResultSet = otherRow.resultSet;
+          if (
+            otherResultSet &&
+            this.resolveStack.find(
+              (s) =>
+                s === otherResultSet ||
+                (otherResultSet instanceof TableValuedResultSet && otherResultSet.inputResultSet == s)
+            )
+          ) {
+            // The other result set is already being resolved (so this is a circular reference). We will encounter this
+            // expression again as we go up the stack and the other result set would visit this expression (where the
+            // parameters are essentially swapped, the key here would be the otherRow from that perspective). Since this
+            // parameter lookup would have been resolved at that point, we can simply add the key as an output
+            // expression at that stage.
+            return;
+          }
+
           this.removePendingExpression(expression);
           const values = state.partition.putIfAbsent(key, () => []);
 
-          if (otherRow.resultSet != null) {
-            const lookup = this.resolveExpandingLookup(otherRow.resultSet);
+          if (otherResultSet != null) {
+            const lookup = this.resolveExpandingLookup(otherResultSet);
             const index = lookup.addOutput(new RowExpression(otherRow));
             const value = new LookupResultParameterValue(index);
             lookup.dependents.push(value);