From 2e93272f1968aecb13795cc9b9c8b17437c10eae Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Wed, 10 Apr 2024 15:17:39 +0200
Subject: [PATCH 1/2] Add a test for depending on a symlink store path

Regression test for https://github.com/NixOS/nix/issues/9579

(cherry picked from commit 872d93eb13f22e8705e03903b65c7eba8b26a99b)
---
 tests/functional/linux-sandbox.sh       |  3 +++
 tests/functional/symlink-derivation.nix | 36 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+)
 create mode 100644 tests/functional/symlink-derivation.nix

diff --git a/tests/functional/linux-sandbox.sh b/tests/functional/linux-sandbox.sh
index ff7d257bd..04209277b 100644
--- a/tests/functional/linux-sandbox.sh
+++ b/tests/functional/linux-sandbox.sh
@@ -73,3 +73,6 @@ testCert missing fixed-output "$nocert"
 
 # Cert in sandbox when ssl-cert-file is set to an existing file
 testCert present fixed-output "$cert"
+
+# Symlinks should be added in the sandbox directly and not followed
+nix-sandbox-build symlink-derivation.nix
diff --git a/tests/functional/symlink-derivation.nix b/tests/functional/symlink-derivation.nix
new file mode 100644
index 000000000..17ba37424
--- /dev/null
+++ b/tests/functional/symlink-derivation.nix
@@ -0,0 +1,36 @@
+with import ./config.nix;
+
+let
+  foo_in_store = builtins.toFile "foo" "foo";
+  foo_symlink = mkDerivation {
+    name = "foo-symlink";
+    buildCommand = ''
+      ln -s ${foo_in_store} $out
+    '';
+  };
+  symlink_to_not_in_store = mkDerivation {
+    name = "symlink-to-not-in-store";
+    buildCommand = ''
+      ln -s ${builtins.toString ./.} $out
+    '';
+  };
+in
+mkDerivation {
+  name = "depends-on-symlink";
+  buildCommand = ''
+    (
+      set -x
+
+      # `foo_symlink` should be a symlink pointing to `foo_in_store`
+      [[ -L ${foo_symlink} ]]
+      [[ $(readlink ${foo_symlink}) == ${foo_in_store} ]]
+
+      # `symlink_to_not_in_store` should be a symlink pointing to `./.`, which
+      # is not available in the sandbox
+      [[ -L ${symlink_to_not_in_store} ]]
+      [[ $(readlink ${symlink_to_not_in_store}) == ${builtins.toString ./.} ]]
+      (! ls ${symlink_to_not_in_store}/)
+    )
+    echo "Success!" > $out
+  '';
+}

From 4c7f69f531f6efc5fb8e5da03059a02bb252c131 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Wed, 10 Apr 2024 15:19:18 +0200
Subject: [PATCH 2/2] Fix permission denied when building symlink derivation
 which points to a symlink out of the store

Bind-mounting symlinks is apparently not possible, which is why the
thing was failing.

Fortunately, symlinks are small, so we can fallback to copy them at no cost.

Fix https://github.com/NixOS/nix/issues/9579

Co-authored-by: Artturin <Artturin@artturin.com>
(cherry picked from commit 913db9f7385b8717d9eaf6269e9f319e78e4c564)
---
 src/libstore/build/local-derivation-goal.cc | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index a9b6a8dbf..9e4ab65a2 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -26,6 +26,7 @@
 #include <regex>
 #include <queue>
 
+#include <sys/stat.h>
 #include <sys/un.h>
 #include <fcntl.h>
 #include <termios.h>
@@ -396,20 +397,30 @@ void LocalDerivationGoal::cleanupPostOutputsRegisteredModeNonCheck()
 static void doBind(const Path & source, const Path & target, bool optional = false) {
     debug("bind mounting '%1%' to '%2%'", source, target);
     struct stat st;
-    if (stat(source.c_str(), &st) == -1) {
+
+    auto bindMount = [&]() {
+        if (mount(source.c_str(), target.c_str(), "", MS_BIND | MS_REC, 0) == -1)
+            throw SysError("bind mount from '%1%' to '%2%' failed", source, target);
+    };
+
+    if (lstat(source.c_str(), &st) == -1) {
         if (optional && errno == ENOENT)
             return;
         else
             throw SysError("getting attributes of path '%1%'", source);
     }
-    if (S_ISDIR(st.st_mode))
+    if (S_ISDIR(st.st_mode)) {
         createDirs(target);
-    else {
+        bindMount();
+    } else if (S_ISLNK(st.st_mode)) {
+        // Symlinks can (apparently) not be bind-mounted, so just copy it
+        createDirs(dirOf(target));
+        copyFile(source, target, /* andDelete */ false);
+    } else {
         createDirs(dirOf(target));
         writeFile(target, "");
+        bindMount();
     }
-    if (mount(source.c_str(), target.c_str(), "", MS_BIND | MS_REC, 0) == -1)
-        throw SysError("bind mount from '%1%' to '%2%' failed", source, target);
 };
 #endif