From 459f9e01851cadb7f642cbce473ed7dff215f314 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Domen=20Ko=C5=BEar?= <domen@cachix.org>
Date: Wed, 22 Oct 2025 13:38:05 -0500
Subject: [PATCH] Fix misleading error messages for missing NARs due to stale
 cache
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When Nix's SQLite narinfo cache indicates a NAR exists, but the NAR
has been garbage collected from the binary cache, Nix displays error
messages even though the operation succeeds via fallback. This is
misleading because the cached narinfo is simply outdated.

This changes SubstituteGone exceptions to produce warnings instead of
errors, accurately reflecting that this is an expected cache coherency
issue, not an actual failure.

Fixes #11411

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
---
 src/libstore/build/substitution-goal.cc | 8 +++++---
 tests/functional/binary-cache.sh        | 8 +++++++-
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/libstore/build/substitution-goal.cc b/src/libstore/build/substitution-goal.cc
index d16e530a4..ac18de304 100644
--- a/src/libstore/build/substitution-goal.cc
+++ b/src/libstore/build/substitution-goal.cc
@@ -268,16 +268,18 @@ Goal::Co PathSubstitutionGoal::tryToRun(
     try {
         promise.get_future().get();
     } catch (std::exception & e) {
-        printError(e.what());
-
         /* Cause the parent build to fail unless --fallback is given,
            or the substitute has disappeared. The latter case behaves
            the same as the substitute never having existed in the
            first place. */
         try {
             throw;
-        } catch (SubstituteGone &) {
+        } catch (SubstituteGone & sg) {
+            /* Missing NARs are expected when they've been garbage collected.
+               This is not a failure, so log as a warning instead of an error. */
+            logWarning({.msg = sg.info().msg});
         } catch (...) {
+            printError(e.what());
             substituterFailed = true;
         }
 
diff --git a/tests/functional/binary-cache.sh b/tests/functional/binary-cache.sh
index 2c102df07..d801ac6aa 100755
--- a/tests/functional/binary-cache.sh
+++ b/tests/functional/binary-cache.sh
@@ -111,7 +111,13 @@ clearStore
 
 mv "$cacheDir/nar" "$cacheDir/nar2"
 
-nix-build --substituters "file://$cacheDir" --no-require-sigs dependencies.nix -o "$TEST_ROOT/result"
+nix-build --substituters "file://$cacheDir" --no-require-sigs dependencies.nix -o "$TEST_ROOT/result" 2>&1 | tee "$TEST_ROOT/log"
+
+# Verify that missing NARs produce warnings, not errors
+# The build should succeed despite the warnings
+grepQuiet "does not exist in binary cache" "$TEST_ROOT/log"
+# Ensure the message is not at error level by checking that the command succeeded
+[ -e "$TEST_ROOT/result" ]
 
 mv "$cacheDir/nar2" "$cacheDir/nar"