add more validation to shard backend url map

parsing will error in cases of:
* unordered shards
* multiple shards for same endblock

config/config.go

@@ -245,6 +245,7 @@ func ParseRawShardRoutingBackendHostURLMap(raw string) (map[string]IntervalURLMa
 			)
 		}
 
+		prevMaxHeight := uint64(0)
 		backendByEndHeight := make(map[uint64]*url.URL, len(endpointBackendValues)/2)
 		for i := 0; i < len(endpointBackendValues); i += 2 {
 			endHeight, err := strconv.ParseUint(endpointBackendValues[i], 10, 64)
@@ -253,6 +254,19 @@ func ParseRawShardRoutingBackendHostURLMap(raw string) (map[string]IntervalURLMa
 					endpointBackendValues[i], host, err,
 				)
 			}
+			// ensure this is the only shard defined with this endBlock for this host
+			if _, exists := backendByEndHeight[endHeight]; exists {
+				return parsed, fmt.Errorf("multiple shards defined for %s with end block %d", host, endHeight)
+			}
+			// require height definitions to be ordered
+			// this is enforced because the shards are expected to cover the entire range
+			// from the previous shard's endBlock to the current shard's endBlock
+			if endHeight < prevMaxHeight {
+				return parsed, fmt.Errorf(
+					"shard map expects end blocks to be ordered. for host %s, shard for height %d found after shard for height %d",
+					host, endHeight, prevMaxHeight,
+				)
+			}
 
 			backendRoute, err := url.Parse(endpointBackendValues[i+1])
 			if err != nil || backendRoute.String() == "" {
@@ -261,6 +275,7 @@ func ParseRawShardRoutingBackendHostURLMap(raw string) (map[string]IntervalURLMa
 				)
 			}
 			backendByEndHeight[endHeight] = backendRoute
+			prevMaxHeight = endHeight
 		}
 
 		parsed[host] = NewIntervalURLMap(backendByEndHeight)

config/config_test.go

@@ -79,6 +79,12 @@ func TestUnitTestParseRawShardRoutingBackendHostURLMap(t *testing.T) {
 
 	_, err = config.ParseRawShardRoutingBackendHostURLMap("invalid-backend-host>100|")
 	require.ErrorContains(t, err, "invalid shard backend route () for height 100 of host invalid-backend-host")
+
+	_, err = config.ParseRawShardRoutingBackendHostURLMap("unsorted-shards>100|backend-100|50|backend-50")
+	require.ErrorContains(t, err, "shard map expects end blocks to be ordered")
+
+	_, err = config.ParseRawShardRoutingBackendHostURLMap("multiple-shards-for-same-height>10|magic|20|dino|20|dinosaur")
+	require.ErrorContains(t, err, "multiple shards defined for multiple-shards-for-same-height with end block 20")
 }
 
 func setDefaultEnv() {