JSON reactor: improve handling of nested containers

Call the parent container's item method before calling the child
item's start method so we can easily know the current nesting level
when nested items are added.

include/qpdf/JSON.hh

@@ -220,7 +220,10 @@ class JSON
         // The start/end methods are called when parsing of a
         // dictionary or array is started or ended. The item methods
         // are called when an item is added to a dictionary or array.
-        // See important notes in "Item methods" below.
+        // When adding a container to another container, the item
+        // method is called with an empty container before the lower
+        // container's start method is called. See important notes in
+        // "Item methods" below.
 
         // During parsing of a JSON string, the parser is operating on
         // a single object at a time. When a dictionary or array is
@@ -230,10 +233,10 @@ class JSON
         // following method calls
         //
         // dictionaryStart -- current object is the top-level dictionary
+        // dictionaryItem  -- called with "a" and an empty array
         // arrayStart      -- current object is the array
         // arrayItem       -- called with the "1" object
         // containerEnd    -- now current object is the dictionary again
-        // dictionaryItem  -- called with "a" and the just-completed array
         // containerEnd    -- current object is undefined
         //
         // If the top-level item in a JSON string is a scalar, the
@@ -261,8 +264,12 @@ class JSON
         // NOTE: When a dictionary or an array is added to a
         // container, the dictionaryItem or arrayItem method is called
         // when the child item's start delimiter is encountered, so
-        // the JSON object passed in at that time will always be
-        // in its initial, empty state.
+        // the JSON object passed in at that time will always be in
+        // its initial, empty state. Additionally, the child item's
+        // start method is not called until after the parent item's
+        // item method is called. This makes it possible to keep track
+        // of the current depth level by incrementing level on start
+        // methods and decrementing on end methods.
 
         QPDF_DLL
         virtual bool

libqpdf/JSON.cc

@@ -949,17 +949,11 @@ JSONParser::handleToken()
         case '{':
             item = std::make_shared<JSON>(JSON::makeDictionary());
             item->setStart(offset - token.length());
-            if (reactor) {
-                reactor->dictionaryStart();
-            }
             break;
 
         case '[':
             item = std::make_shared<JSON>(JSON::makeArray());
             item->setStart(offset - token.length());
-            if (reactor) {
-                reactor->arrayStart();
-            }
             break;
 
         default:
@@ -1187,6 +1181,18 @@ JSONParser::handleToken()
             "JSONParser::handleToken: unexpected null item in transition");
     }
 
+    if (reactor && item.get()) {
+        // Calling container start method is postponed until after
+        // adding the containers to their parent containers, if any.
+        // This makes it much easier to keep track of the current
+        // nesting level.
+        if (item->isDictionary()) {
+            reactor->dictionaryStart();
+        } else if (item->isArray()) {
+            reactor->arrayStart();
+        }
+    }
+
     // Prepare for next token
     if (item.get()) {
         if (item->isDictionary()) {

libtests/qtest/json_parse/good-01-react.out

@@ -1,14 +1,14 @@
 dictionary start
 dictionary item: a -> [6, 11): "bcd"
-array start
 dictionary item: e -> [18, 0): []
+array start
 array item: [19, 20): 1
 array item: [41, 42): 2
 array item: [44, 45): 3
 array item: [46, 47): 4
 array item: [48, 54): "five"
-dictionary start
 array item: [56, 0): {}
+dictionary start
 dictionary item: six -> [64, 65): 7
 dictionary item: 8 -> [72, 73): 9
 container end: [56, 74): {}

libtests/qtest/json_parse/good-04-react.out

@@ -1,16 +1,16 @@
 array start
-array start
 array item: [1, 0): []
 array start
 array item: [2, 0): []
-dictionary start
+array start
 array item: [3, 0): {}
+dictionary start
 container end: [3, 5): {}
 container end: [2, 6): []
-dictionary start
 array item: [8, 0): {}
 dictionary start
 dictionary item:  -> [13, 0): {}
+dictionary start
 container end: [13, 15): {}
 container end: [8, 16): {}
 container end: [1, 17): []

libtests/qtest/json_parse/good-10-react.out

@@ -1,30 +1,30 @@
 dictionary start
-array start
 dictionary item: a -> [9, 0): []
+array start
 array item: [10, 11): 1
 array item: [13, 14): 2
-dictionary start
 array item: [16, 0): {}
+dictionary start
 dictionary item: x -> [22, 25): "y"
 container end: [16, 26): {}
 array item: [28, 29): 3
-dictionary start
 array item: [31, 0): {}
+dictionary start
 dictionary item: keep -> [40, 61): "not in final output"
 container end: [31, 62): {
   "keep": "not in final output"
 }
 container end: [9, 63): []
-array start
 dictionary item: keep -> [75, 0): []
+array start
 array item: [76, 77): 1
 array item: [79, 83): null
 array item: [85, 86): 2
 array item: [88, 93): false
 array item: [95, 101): "keep"
 array item: [103, 104): 3
-array start
 array item: [106, 0): []
+array start
 array item: [107, 113): "this"
 array item: [115, 121): "keep"
 array item: [123, 128): "not"