libutil: handle json builder log messages with unexpected format

Before this change, expressions like:

with import <nixpkgs> {};
runCommand "foo" {} ''
  echo '@nix {}' >&$NIX_LOG_FD
''

would result in Lix crashing, because accessing nonexistent fields of
a JSON object throws an exception.

Rather than handling each field individually, we just catch JSON
exceptions wholesale. Since these log messages are an unusual
circumstance, log a warning when this happens.

Fixes #544.

Change-Id: Idc2d8acf6e37046b3ec212f42e29269163dca893
(cherry picked from commit e55cd3beea710db727fd966f265a1b715b7285f3)

src/libutil/logging.cc

@@ -295,6 +295,7 @@ bool handleJSONLogMessage(nlohmann::json & json,
     const Activity & act, std::map<ActivityId, Activity> & activities,
     bool trusted)
 {
+    try {
         std::string action = json["action"];
 
         if (action == "start") {
@@ -326,6 +327,13 @@ bool handleJSONLogMessage(nlohmann::json & json,
         }
 
         return true;
+    } catch (nlohmann::json::exception &e) {
+        warn(
+            "warning: Unable to handle a JSON message from the builder: %s",
+            e.what()
+        );
+        return false;
+    }
 }
 
 bool handleJSONLogMessage(const std::string & msg,

tests/functional/dependencies.nix

@@ -33,6 +33,18 @@ let
     outputHash = "1dq9p0hnm1y75q2x40fws5887bq1r840hzdxak0a9djbwvx0b16d";
   };
 
+  unusual-logging = mkDerivation {
+    name = "unusual-logging";
+    buildCommand = ''
+      {
+        echo "@nix 1"
+        echo "@nix {}"
+        echo '@nix {"action": null}'
+      } >&$NIX_LOG_FD
+      touch $out
+    '';
+  };
+
 in
 mkDerivation {
   name = "dependencies-top";
@@ -42,6 +54,7 @@ mkDerivation {
   input1_drv = input1;
   input2_drv = input2;
   input0_drv = input0;
+  unusual_logging_drv = unusual-logging;
   fod_input_drv = fod_input;
   meta.description = "Random test package";
 }