Standardize depth enforcement.

Sources/SwiftProtobuf/Google_Protobuf_ListValue+Extensions.swift

@@ -44,19 +44,19 @@ extension Google_Protobuf_ListValue: _CustomJSONCodable {
             return
         }
         try decoder.scanner.skipRequiredArrayStart()
-        // Since we override the JSON decoding, we can't rely
-        // on the default recursion depth tracking.
+        // When a JSON array becomes a google.protobuf.List, that means we're
+        // adding an "object" to the depth of objects modeled, so manually
+        // include that in the recursion usage.
         try decoder.scanner.incrementRecursionDepth()
+        defer { decoder.scanner.decrementRecursionDepth() }
         if decoder.scanner.skipOptionalArrayEnd() {
-            decoder.scanner.decrementRecursionDepth()
             return
         }
         while true {
             var v = Google_Protobuf_Value()
             try v.decodeJSON(from: &decoder)
             values.append(v)
             if decoder.scanner.skipOptionalArrayEnd() {
-                decoder.scanner.decrementRecursionDepth()
                 return
             }
             try decoder.scanner.skipRequiredComma()

Sources/SwiftProtobuf/Google_Protobuf_Value+Extensions.swift

@@ -76,6 +76,12 @@ extension Google_Protobuf_Value: _CustomJSONCodable {
     }
 
     internal mutating func decodeJSON(from decoder: inout JSONDecoder) throws {
+        // No matter what was in the JSON, when it becomes a
+        // google.protobuf.Value, that means we've added an "object" to the
+        // depth of objects modeled, so manually include that in the recursion
+        // usage.
+        try decoder.scanner.incrementRecursionDepth()
+        defer { decoder.scanner.decrementRecursionDepth() }
         let c = try decoder.scanner.peekOneCharacter()
         switch c {
         case "n":

Tests/SwiftProtobufTests/Test_Struct.swift

@@ -146,7 +146,8 @@ final class Test_JSON_ListValue: XCTestCase, PBTestHelpers {
     }
 
     func test_JSON_nested_list() throws {
-        let limit = JSONDecodingOptions().messageDepthLimit
+        // Every element in a List is a Value, so the max depth is 1/2 of the limit.
+        let limit = JSONDecodingOptions().messageDepthLimit / 2
         let depths = [
             // Small lists
             1, 2, 3, 4, 5,