From 0712339912a4b139efa87768fd3a38713e0e13ac Mon Sep 17 00:00:00 2001
From: Cole Helbling <cole.helbling@determinate.systems>
Date: Mon, 2 Jun 2025 14:28:43 -0700
Subject: [PATCH 1/2] `--keep-failed` with remote builders will keep the failed
 build directory on that builder

---
 src/build-remote/build-remote.cc | 9 ++++++++-
 tests/functional/build-remote.sh | 1 +
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 60247b735..e79ee6a45 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -329,8 +329,15 @@ connected:
                 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({
diff --git a/tests/functional/build-remote.sh b/tests/functional/build-remote.sh
index 3231341cb..2d3e874b1 100644
--- a/tests/functional/build-remote.sh
+++ b/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".* ]]
 
 build_dir="$(grep "note: keeping build" <<< "$out" | sed -E "s/^(.*)note: keeping build directory '(.*)'(.*)$/\2/")"
 [[ "foo" = $(<"$build_dir"/bar) ]]

From 54aa73b19bd3bb3f86c1cfae7eaca669923c9313 Mon Sep 17 00:00:00 2001
From: Cole Helbling <cole.helbling@determinate.systems>
Date: Tue, 3 Jun 2025 08:38:04 -0700
Subject: [PATCH 2/2] fixup: only show "you can rerun" message if the
 derivation's platform is supported on this machine

---
 src/build-remote/build-remote.cc | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index e79ee6a45..e0a6b2d8e 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -332,8 +332,10 @@ connected:
             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."
+                        "The failed build directory was kept on the remote builder due to `--keep-failed`.%s",
+                        (settings.thisSystem == drv.platform || settings.extraPlatforms.get().count(drv.platform) > 0)
+                            ? " 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);