From 027fd45230b74c67e65d06e7073c04b62c60eb4e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Fri, 27 May 2022 16:15:28 +0200
Subject: [PATCH] Fix a segfault in the git fetcher

The git fetcher code used to dereference the (potentially empty) `ref`
input attribute. This was magically working, probably because the
compiler somehow outsmarted us, but is now blowing up with newer nixpkgs
versions.

Fix that by not trying to access this field while we don't know for sure
that it has been defined.

Fix #6554
---
 src/libfetchers/git.cc | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index a71bff76f57..9cbd392477a 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -449,11 +449,10 @@ struct GitInputScheme : InputScheme
             }
         }
 
-        const Attrs unlockedAttrs({
+        Attrs unlockedAttrs({
             {"type", cacheType},
             {"name", name},
             {"url", actualUrl},
-            {"ref", *input.getRef()},
         });
 
         Path repoDir;
@@ -466,6 +465,7 @@ struct GitInputScheme : InputScheme
                     head = "master";
                 }
                 input.attrs.insert_or_assign("ref", *head);
+                unlockedAttrs.insert_or_assign("ref", *head);
             }
 
             if (!input.getRev())
@@ -482,6 +482,7 @@ struct GitInputScheme : InputScheme
                     head = "master";
                 }
                 input.attrs.insert_or_assign("ref", *head);
+                unlockedAttrs.insert_or_assign("ref", *head);
             }
 
             if (auto res = getCache()->lookup(store, unlockedAttrs)) {