--keep-failed with remote builders will keep the failed build directory on that builder

Author: cole-h

src/build-remote/build-remote.cc

@@ -329,8 +329,15 @@ static int main_build_remote(int argc, char * * argv)
                 drv.inputSrcs = store->parseStorePathSet(inputs);
             optResult = sshStore->buildDerivation(*drvPath, (const BasicDerivation &) drv);
             auto & result = *optResult;
-            if (!result.success())
+            if (!result.success()) {
+                if (settings.keepFailed) {
+                    warn(
+                        "The failed build directory was kept on the remote builder due to `--keep-failed`. "
+                        "If the build's architecture matches your host, you can re-run the command with `--builders ''` to disable remote building for this invocation."
+                    );
+                }
                 throw Error("build of '%s' on '%s' failed: %s", store->printStorePath(*drvPath), storeUri, result.errorMsg);
+            }
         } else {
             copyClosure(*store, *sshStore, StorePathSet {*drvPath}, NoRepair, NoCheckSigs, substitute);
             auto res = sshStore->buildPathsWithResults({

tests/functional/build-remote.sh

@@ -85,6 +85,7 @@ out="$(nix-build 2>&1 failing.nix \
   --arg busybox "$busybox")" || true
 
 [[ "$out" =~ .*"note: keeping build directory".* ]]
+[[ "$out" =~ .*"The failed build directory was kept on the remote builder due to `--keep-failed`. ".* ]]
 
 build_dir="$(grep "note: keeping build" <<< "$out" | sed -E "s/^(.*)note: keeping build directory '(.*)'(.*)$/\2/")"
 [[ "foo" = $(<"$build_dir"/bar) ]]