Explicit snapshot prefix (#243)

* Add an explicit prefix to Snapshot

* Update SDKs

* Extend test case

Author: comnik

meta/src/meta/grammar.y

@@ -1261,9 +1261,11 @@ snapshot_mapping
         $2: logic.RelationId = $$.source_relation
 
 snapshot
-    : "(" "snapshot" snapshot_mapping* ")"
-      construct: $$ = transactions.Snapshot(mappings=$3)
-      deconstruct: $3: Sequence[transactions.SnapshotMapping] = $$.mappings
+    : "(" "snapshot" edb_path snapshot_mapping* ")"
+      construct: $$ = transactions.Snapshot(prefix=$3, mappings=$4)
+      deconstruct:
+        $3: Sequence[String] = $$.prefix
+        $4: Sequence[transactions.SnapshotMapping] = $$.mappings
 
 epoch_reads
     : "(" "reads" read* ")"

proto/relationalai/lqp/v1/transactions.proto

@@ -64,16 +64,20 @@ message Context {
   repeated RelationId relations = 1;
 }
 
-// A single (destination, source) pair within a Snapshot action.
+// A single (destination, source) pair within a Snapshot action. The destination_path is
+// relative to the Snapshot's prefix.
 message SnapshotMapping {
   repeated string destination_path = 1;
   RelationId source_relation = 2;
 }
 
 // Demand the source IDBs, take immutable snapshots, and turn them into EDBs under the
-// given paths (specified as sequences of strings, see EDB).
+// given paths (specified as sequences of strings, see EDB). If a prefix is specified,
+// the engine should treat this as a complete snapshot of all EDBs under that prefix,
+// removing any pre-existing EDBs for the same prefix that are not in the mappings.
 message Snapshot {
   repeated SnapshotMapping mappings = 1;
+  repeated string prefix = 2;
 }
 
 //