From c4bf219b55c76329988671d6b383d073373919f0 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Tue, 15 Sep 2020 14:26:56 +0000
Subject: [PATCH] Don't link deriver until after any delayed exception is
 thrown

Otherwise, we will associate fixed-output derivations with outputs that
they did indeed produce, but which had the wrong hash. That's no good.
---
 src/libstore/build.cc | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 9eff03f7b..d6a775ae9 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -4251,22 +4251,28 @@ void DerivationGoal::registerOutputs()
     /* Register each output path as valid, and register the sets of
        paths referenced by each of them.  If there are cycles in the
        outputs, this will fail. */
-    {
-        ValidPathInfos infos2;
-        for (auto & [outputName, newInfo] : infos) {
-            /* FIXME: we will want to track this mapping in the DB whether or
-               not we have a drv file. */
-            if (useDerivation)
-                worker.store.linkDeriverToPath(drvPath, outputName, newInfo.path);
-            infos2.push_back(newInfo);
-        }
-        worker.store.registerValidPaths(infos2);
+    ValidPathInfos infos2;
+    for (auto & [outputName, newInfo] : infos) {
+        infos2.push_back(newInfo);
     }
+    worker.store.registerValidPaths(infos2);
 
     /* In case of a fixed-output derivation hash mismatch, throw an
        exception now that we have registered the output as valid. */
     if (delayedException)
         std::rethrow_exception(delayedException);
+
+    /* If we made it this far, we are sure the output matches the derivation
+       (since the delayedException would be a fixed output CA mismatch). That
+       means it's safe to link the derivation to the output hash. We must do
+       that for floating CA derivations, which otherwise couldn't be cached,
+       but it's fine to do in all cases. */
+    for (auto & [outputName, newInfo] : infos) {
+        /* FIXME: we will want to track this mapping in the DB whether or
+           not we have a drv file. */
+        if (useDerivation)
+            worker.store.linkDeriverToPath(drvPath, outputName, newInfo.path);
+    }
 }