From 623f3d321e91ea5426f604a4aeeaffcf95a23a25 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
Date: Mon, 8 Dec 2025 18:20:36 +0000
Subject: [PATCH] turn 'derivation has incorrect deferred output' into warning

this breaks nix develop when using a stable nix version

Update src/libstore/derivations.cc

Co-authored-by: John Ericson <git@JohnEricson.me>
---
 src/libstore-tests/derivation/invariants.cc |  2 ++
 src/libstore/derivations.cc                 | 27 ++++++++++++++-------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/src/libstore-tests/derivation/invariants.cc b/src/libstore-tests/derivation/invariants.cc
index 6d7cee968..cacdca0cd 100644
--- a/src/libstore-tests/derivation/invariants.cc
+++ b/src/libstore-tests/derivation/invariants.cc
@@ -174,6 +174,7 @@ TEST_F(FillInOutputPathsTest, throwsOnIncorrectInputAddressedPath)
     ASSERT_THROW(drv.fillInOutputPaths(*store), Error);
 }
 
+#if 0
 TEST_F(FillInOutputPathsTest, throwsOnIncorrectEnvVar)
 {
     auto wrongPath = StorePath{"c015dhfh5l0lp6wxyvdn7bmwhbbr6hr9-wrong-name"};
@@ -195,6 +196,7 @@ TEST_F(FillInOutputPathsTest, throwsOnIncorrectEnvVar)
 
     ASSERT_THROW(drv.fillInOutputPaths(*store), Error);
 }
+#endif
 
 TEST_F(FillInOutputPathsTest, preservesDeferredWithInputDrvs)
 {
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 842ef9056..67ab6ba62 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -1230,7 +1230,7 @@ static void processDerivationOutputPaths(Store & store, auto && drv, std::string
     std::optional<DrvHash> hashesModulo;
 
     for (auto & [outputName, output] : drv.outputs) {
-        auto envHasRightPath = [&](const StorePath & actual) {
+        auto envHasRightPath = [&](const StorePath & actual, bool isDeferred = false) {
             if constexpr (fillIn) {
                 auto j = drv.env.find(outputName);
                 /* Fill in mode: fill in missing or empty environment
@@ -1248,12 +1248,20 @@ static void processDerivationOutputPaths(Store & store, auto && drv, std::string
                     "derivation has missing environment variable '%s', should be '%s' but is not present",
                     outputName,
                     store.printStorePath(actual));
-            if (j->second != store.printStorePath(actual))
-                throw Error(
-                    "derivation has incorrect environment variable '%s', should be '%s' but is actually '%s'",
-                    outputName,
-                    store.printStorePath(actual),
-                    j->second);
+            if (j->second != store.printStorePath(actual)) {
+                if (isDeferred)
+                    warn(
+                        "derivation has incorrect environment variable '%s', should be '%s' but is actually '%s'\nThis will be an error in future versions of Nix; compatibility of CA derivations will be broken.",
+                        outputName,
+                        store.printStorePath(actual),
+                        j->second);
+                else
+                    throw Error(
+                        "derivation has incorrect environment variable '%s', should be '%s' but is actually '%s'",
+                        outputName,
+                        store.printStorePath(actual),
+                        j->second);
+            }
         };
         auto hash = [&]<typename Output>(const Output & outputVariant) {
             if (!hashesModulo) {
@@ -1287,8 +1295,9 @@ static void processDerivationOutputPaths(Store & store, auto && drv, std::string
                     else
                         /* Validation mode: deferred outputs
                            should have been filled in */
-                        throw Error(
-                            "derivation has incorrect deferred output, should be '%s'", store.printStorePath(outPath));
+                        warn(
+                            "derivation has incorrect deferred output, should be '%s'.\nThis will be an error in future versions of Nix; compatibility of CA derivations will be broken.",
+                            store.printStorePath(outPath));
                 } else {
                     /* Will never happen, based on where
                        `hash` is called. */