Enable FDB transaction tracingoptional-fdb-tracing

To trace FDB transactions:

 1. Enable tracing in erlfdb application environment:
     `network_options = [{trace_enable, ...}]`
   OR with an environment variable:
     `FDB_NETWORK_OPTION_TRACE_ENABLE = ""`

 2. Set `[fabric] fdb_trace=true` configuration value

 3. Add the `x-couchdb-fdb-trace:true` header to each request that should be
   traced.

The transaction name is set to the nonce value, which is already used by
CouchDB to track API requests.

Only transactions started from the main request process will be traced. So if a
process is spawned without inheriting the `erlfdb_trace` process dict key, that
transaction will not be traced.

1 files changed, 18 insertions, 0 deletions

diff --git a/src/chttpd/src/chttpd.erl b/src/chttpd/src/chttpd.erl
index 4d32c03c5..fb227aea3 100644
--- a/src/chttpd/src/chttpd.erl
+++ b/src/chttpd/src/chttpd.erl
@@ -238,6 +238,8 @@ handle_request_int(MochiReq) ->
     erlang:put(dont_log_request, true),
     erlang:put(dont_log_response, true),
 
+    maybe_trace_fdb(MochiReq:get_header_value("x-couchdb-fdb-trace")),
+
     {HttpReq2, Response} = case before_request(HttpReq0) of
         {ok, HttpReq1} ->
             process_request(HttpReq1);
@@ -1214,6 +1216,22 @@ get_user(#httpd{user_ctx = #user_ctx{name = User}}) ->
 get_user(#httpd{user_ctx = undefined}) ->
     "undefined".
 
+maybe_trace_fdb("true") ->
+    % Remember to also enable tracing in erlfdb application environment:
+    %   network_options = [{trace_enable, ...}]
+    % Or via the OS environment variable:
+    %   FDB_NETWORK_OPTION_TRACE_ENABLE = ""
+    case config:get_boolean("fabric", "fdb_trace", false) of
+        true ->
+            Nonce = erlang:get(nonce),
+            erlang:put(erlfdb_trace, list_to_binary(Nonce));
+        false ->
+            ok
+    end;
+maybe_trace_fdb(_) ->
+    ok.
+
+
 -ifdef(TEST).
 
 -include_lib("eunit/include/eunit.hrl").