nix-store --serve: Suppress log output on stderr when repeating a build

src/libstore/build.cc

@@ -3047,7 +3047,8 @@ void DerivationGoal::handleEOF(int fd)
 
 void DerivationGoal::flushLine()
 {
-    if (settings.verboseBuild)
+    if (settings.verboseBuild &&
+        (settings.printRepeatedBuilds || curRound == 1))
         printError(filterANSIEscapes(currentLogLine, true));
     else {
         logTail.push_back(currentLogLine);

src/libstore/globals.hh

@@ -149,6 +149,11 @@ struct Settings {
        before being killed (0 means no limit). */
     unsigned long maxLogSize;
 
+    /* When build-repeat > 0 and verboseBuild == true, whether to
+       print repeated builds (i.e. builds other than the first one) to
+       stderr. Hack to prevent Hydra logs from being polluted. */
+    bool printRepeatedBuilds = true;
+
     /* How often (in seconds) to poll for locks. */
     unsigned int pollInterval;
 

src/nix-store/nix-store.cc

@@ -842,6 +842,7 @@ static void opServe(Strings opFlags, Strings opArgs)
             settings.maxLogSize = readInt(in);
         if (GET_PROTOCOL_MINOR(clientVersion) >= 3)
             settings.set("build-repeat", std::to_string(readInt(in)));
+        settings.printRepeatedBuilds = false;
     };
 
     while (true) {