add empty/dummy fields expected by vscode

src/gc-heap-snapshot.cpp

@@ -606,7 +606,9 @@ void _record_gc_just_edge(const char *edge_type, size_t from_idx, size_t to_idx,
 void final_serialize_heap_snapshot(ios_t *json, ios_t *strings, HeapSnapshot &snapshot, char all_one)
 {
     // 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, "\"meta\":{");
     ios_printf(json, "\"node_fields\":[\"type\",\"name\",\"id\",\"self_size\",\"edge_count\",\"trace_node_id\",\"detachedness\"],");
     ios_printf(json, "\"node_types\":[");
@@ -617,10 +619,25 @@ void final_serialize_heap_snapshot(ios_t *json, ios_t *strings, HeapSnapshot &sn
     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\"],");
+    // 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\"]");
+    // end not used
     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, "}\n"); // end "snapshot"
+    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_tree\":[],");
+    ios_printf(json, "\"samples\":[],");
+    ios_printf(json, "\"locations\":[]");
+    // end not used
+
     ios_printf(json, "}");
 }

stdlib/Profile/src/heapsnapshot_reassemble.jl

@@ -92,7 +92,7 @@ function assemble_snapshot(in_prefix, io::IO)
     node_count = parse(Int, String(@view preamble[pos:endpos]))
 
     pos = last(findnext("edge_count\":", preamble, endpos)) + 1
-    endpos = findnext(==('}'), preamble, pos) - 1
+    endpos = findnext(==(','), preamble, pos) - 1
     edge_count = parse(Int, String(@view preamble[pos:endpos]))
 
     nodes = Nodes(node_count, edge_count)
@@ -137,7 +137,8 @@ function assemble_snapshot(in_prefix, io::IO)
     end
 
     _digits_buf = zeros(UInt8, ndigits(typemax(UInt)))
-    println(io, @view(preamble[1:end-2]), ",") # remove trailing "}\n", we don't end the snapshot here
+    println(io, @view(preamble[1:end-1]), ",") # remove trailing "}" to reopen the object
+
     println(io, "\"nodes\":[")
     for i in 1:length(nodes)
         i > 1 && println(io, ",")