From a175c67def61f68b8b1ac860ea7b0672badd1d6e Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 16 Jun 2025 19:34:28 +0200
Subject: [PATCH] Fix rootFS fingerprint in the impure case

---
 src/libutil/union-source-accessor.cc | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/libutil/union-source-accessor.cc b/src/libutil/union-source-accessor.cc
index 9950f6049..69cf04c18 100644
--- a/src/libutil/union-source-accessor.cc
+++ b/src/libutil/union-source-accessor.cc
@@ -72,6 +72,18 @@ struct UnionSourceAccessor : SourceAccessor
         }
         return std::nullopt;
     }
+
+    std::pair<CanonPath, std::optional<std::string>> getFingerprint(const CanonPath & path) override
+    {
+        if (fingerprint)
+            return {path, fingerprint};
+        for (auto & accessor : accessors) {
+            auto [subpath, fingerprint] = accessor->getFingerprint(path);
+            if (fingerprint)
+                return {subpath, fingerprint};
+        }
+        return {path, std::nullopt};
+    }
 };
 
 ref<SourceAccessor> makeUnionSourceAccessor(std::vector<ref<SourceAccessor>> && accessors)