Profile: fix order of fields in heapsnapshot & improve formatting (#55890)

Author: IanButterworth

src/gc-heap-snapshot.cpp

@@ -618,38 +618,32 @@ void final_serialize_heap_snapshot(ios_t *json, ios_t *strings, HeapSnapshot &sn
 {
     // mimicking https://github.com/nodejs/node/blob/5fd7a72e1c4fbaf37d3723c4c81dce35c149dc84/deps/v8/src/profiler/heap-snapshot-generator.cc#L2567-L2567
     // also https://github.com/microsoft/vscode-v8-heap-tools/blob/c5b34396392397925ecbb4ecb904a27a2754f2c1/v8-heap-parser/src/decoder.rs#L43-L51
-    ios_printf(json, "{\"snapshot\":{");
+    ios_printf(json, "{\"snapshot\":{\n");
 
-    ios_printf(json, "\"meta\":{");
-    ios_printf(json, "\"node_fields\":[\"type\",\"name\",\"id\",\"self_size\",\"edge_count\",\"trace_node_id\",\"detachedness\"],");
-    ios_printf(json, "\"node_types\":[");
+    ios_printf(json, "  \"meta\":{\n");
+    ios_printf(json, "    \"node_fields\":[\"type\",\"name\",\"id\",\"self_size\",\"edge_count\",\"trace_node_id\",\"detachedness\"],\n");
+    ios_printf(json, "    \"node_types\":[");
     snapshot.node_types.print_json_array(json, false);
     ios_printf(json, ",");
-    ios_printf(json, "\"string\", \"number\", \"number\", \"number\", \"number\", \"number\"],");
-    ios_printf(json, "\"edge_fields\":[\"type\",\"name_or_index\",\"to_node\"],");
-    ios_printf(json, "\"edge_types\":[");
+    ios_printf(json, "\"string\", \"number\", \"number\", \"number\", \"number\", \"number\"],\n");
+    ios_printf(json, "    \"edge_fields\":[\"type\",\"name_or_index\",\"to_node\"],\n");
+    ios_printf(json, "    \"edge_types\":[");
     snapshot.edge_types.print_json_array(json, false);
     ios_printf(json, ",");
-    ios_printf(json, "\"string_or_number\",\"from_node\"],");
+    ios_printf(json, "\"string_or_number\",\"from_node\"],\n");
     // not used. Required by microsoft/vscode-v8-heap-tools
-    ios_printf(json, "\"trace_function_info_fields\":[\"function_id\",\"name\",\"script_name\",\"script_id\",\"line\",\"column\"],");
-    ios_printf(json, "\"trace_node_fields\":[\"id\",\"function_info_index\",\"count\",\"size\",\"children\"],");
-    ios_printf(json, "\"sample_fields\":[\"timestamp_us\",\"last_assigned_id\"],");
-    ios_printf(json, "\"location_fields\":[\"object_index\",\"script_id\",\"line\",\"column\"]");
+    ios_printf(json, "    \"trace_function_info_fields\":[\"function_id\",\"name\",\"script_name\",\"script_id\",\"line\",\"column\"],\n");
+    ios_printf(json, "    \"trace_node_fields\":[\"id\",\"function_info_index\",\"count\",\"size\",\"children\"],\n");
+    ios_printf(json, "    \"sample_fields\":[\"timestamp_us\",\"last_assigned_id\"],\n");
+    ios_printf(json, "    \"location_fields\":[\"object_index\",\"script_id\",\"line\",\"column\"]\n");
     // end not used
-    ios_printf(json, "},\n"); // end "meta"
+    ios_printf(json, "  },\n"); // end "meta"
 
-    ios_printf(json, "\"node_count\":%zu,", snapshot.num_nodes);
-    ios_printf(json, "\"edge_count\":%zu,", snapshot.num_edges);
-    ios_printf(json, "\"trace_function_count\":0"); // not used. Required by microsoft/vscode-v8-heap-tools
-    ios_printf(json, "},\n"); // end "snapshot"
-
-    // not used. Required by microsoft/vscode-v8-heap-tools
-    ios_printf(json, "\"trace_function_infos\":[],");
-    ios_printf(json, "\"trace_tree\":[],");
-    ios_printf(json, "\"samples\":[],");
-    ios_printf(json, "\"locations\":[]");
-    // end not used
+    ios_printf(json, "  \"node_count\":%zu,\n", snapshot.num_nodes);
+    ios_printf(json, "  \"edge_count\":%zu,\n", snapshot.num_edges);
+    ios_printf(json, "  \"trace_function_count\":0\n"); // not used. Required by microsoft/vscode-v8-heap-tools
+    ios_printf(json, "}\n"); // end "snapshot"
 
+    // this } is removed by the julia reassembler in Profile
     ios_printf(json, "}");
 }

stdlib/Profile/src/heapsnapshot_reassemble.jl

@@ -155,7 +155,8 @@ function assemble_snapshot(in_prefix, io::IO)
         _write_decimal_number(io, nodes.edge_count[i], _digits_buf)
         print(io, ",0,0")
     end
-    print(io, "],\"edges\":[")
+    print(io, "],\n")
+    print(io, "\"edges\":[")
     e = 1
     for n in 1:length(nodes)
         count = nodes.edge_count[n]
@@ -177,6 +178,13 @@ function assemble_snapshot(in_prefix, io::IO)
     end
     println(io, "],")
 
+    # not used. Required by microsoft/vscode-v8-heap-tools
+    # This order of these fields is required by chrome dev tools otherwise loading fails
+    println(io, "\"trace_function_infos\":[],")
+    println(io, "\"trace_tree\":[],")
+    println(io, "\"samples\":[],")
+    println(io, "\"locations\":[],")
+
     println(io, "\"strings\":[")
     open(string(in_prefix, ".strings"), "r") do strings_io
         first = true