[format] allow leading slash in field names

Fixes #852

docs/source/formats.rst

@@ -101,10 +101,18 @@ should be another object with the following fields:
 
         [ { "field": "ts" }, " ", { "field": "msg" } ]
 
-    :field: The name of the message field that should be inserted at this
-      point in the message.  The special "__timestamp__" field name can be
-      used to insert a human-readable timestamp.  The "__level__" field can
-      be used to insert the level name as defined by lnav.
+    :field: The name or `JSON-Pointer <https://tools.ietf.org/html/rfc6901>`_
+      of the message field that should be inserted at this point in the
+      message.  The special :code:`__timestamp__` field name can be used to
+      insert a human-readable timestamp.  The :code:`__level__` field can be
+      used to insert the level name as defined by lnav.
+
+      .. tip::
+
+        Use a JSON-Pointer to reference nested fields.  For example, to include
+        a "procname" property that is nested in a "details" object, you would
+        write the field reference as :code:`/details/procname`.
+
     :min-width: The minimum width for the field.  If the value for the field
       in a given log message is shorter, padding will be added as needed to
       meet the minimum-width requirement. (v0.8.2+)

src/log_format.cc

@@ -1881,6 +1881,9 @@ void external_log_format::build(std::vector<std::string> &errors) {
         static const intern_string_t level_field = intern_string::lookup("__level__");
         json_format_element &jfe = *iter;
 
+        if (startswith(jfe.jfe_value.get(), "/")) {
+            jfe.jfe_value = intern_string::lookup(jfe.jfe_value.get() + 1);
+        }
         if (!jfe.jfe_ts_format.empty()) {
             if (!jfe.jfe_value.empty() && jfe.jfe_value != ts) {
                 log_warning("%s:line-format[%d]:ignoring field '%s' since "

src/log_format_loader.cc

@@ -216,8 +216,10 @@ static int read_format_int(yajlpp_parse_context *ypc, long long val)
 static int read_format_field(yajlpp_parse_context *ypc, const unsigned char *str, size_t len)
 {
     auto elf = (external_log_format *) ypc->ypc_obj_stack.top();
-    string value = string((const char *)str, len);
-    string field_name = ypc->get_path_fragment(1);
+    auto leading_slash = len > 0 && str[0] == '/';
+    auto value = string((const char *) (leading_slash ? str + 1 : str),
+                        leading_slash ? len - 1 : len);
+    auto field_name = ypc->get_path_fragment(1);
 
     if (field_name == "file-pattern") {
         elf->elf_file_pattern = value;

test/formats/nestedjson/format.json

@@ -7,11 +7,11 @@
         "line-format" : [
             { "field" : "ts" },
             " ",
-            { "field" : "@fields/lvl" },
+            { "field" : "/@fields/lvl" },
             " ",
             { "field" : "@fields/msg" }
         ],
-        "level-field" : "@fields/lvl",
+        "level-field" : "/@fields/lvl",
         "timestamp-field": "ts",
         "body-field" : "@fields/msg",
         "value" : {