ssh-ng: also store build logs that are accessible by nix log

Right now when building a derivation remotely via

    $ nix build -j0 -f . hello -L --builders 'ssh://builder'

it's possible later to read through the entire build-log by running
`nix log -f . hello`. This isn't possible however when using `ssh-ng`
rather than `ssh`.

The reason for that is that there are two different ways to transfer
logs in Nix through e.g. an SSH tunnel (that are used by `ssh`/`ssh-ng`
respectively):

* `ssh://` receives its logs from the fd pointing to `builderOut`. This
  is directly passed to the "log-sink" (and to the logger on each `\n`),
  hence `nix log` works here.
* `ssh-ng://` however expects JSON-like messages (i.e. `@nix {log data
  in here}`) and passes it directly to the logger without doing anything
  with the `logSink`. However it's certainly possible to extract
  log-lines from this format as these have their own message-type in the
  JSON payload (i.e. `resBuildLogLine`).

  This is basically what I changed in this patch: if the code-path for
  `builderOut` is not reached and a `logSink` is initialized, the
  message was successfully processed by the JSON logger (i.e. it's in
  the expected format) and the line is of the expected type (i.e.
  `resBuildLogLine`), the line will be written to the log-sink as well.

Closes #5079

src/libstore/build/derivation-goal.cc

@@ -1175,10 +1175,10 @@ bool DerivationGoal::isReadDesc(int fd)
     return fd == hook->builderOut.readSide.get();
 }
 
-
 void DerivationGoal::handleChildOutput(int fd, const string & data)
 {
-    if (isReadDesc(fd))
+    auto isWrittenToLog = isReadDesc(fd);
+    if (isWrittenToLog)
     {
         logSize += data.size();
         if (settings.maxLogSize && logSize > settings.maxLogSize) {
@@ -1207,7 +1207,14 @@ void DerivationGoal::handleChildOutput(int fd, const string & data)
     if (hook && fd == hook->fromHook.readSide.get()) {
         for (auto c : data)
             if (c == '\n') {
-                handleJSONLogMessage(currentHookLine, worker.act, hook->activities, true);
+                auto s = handleJSONLogMessage(currentHookLine, worker.act, hook->activities, true);
+                if (s && !isWrittenToLog && logSink) {
+                    auto json = nlohmann::json::parse(std::string(currentHookLine, 5));
+                    if (json["type"] == resBuildLogLine) {
+                        auto f = json["fields"];
+                        (*logSink)((f.size() > 0 ? f.at(0).get<std::string>() : "") + "\n");
+                    }
+                }
                 currentHookLine.clear();
             } else
                 currentHookLine += c;