copyPathToStore(): Preserve symlinks

E.g. in a derivation attribute `foo = ./bar`, if ./bar is a symlink,
we should copy the symlink to the store, not its target. This restores
the behaviour of Nix <= 2.19.

src/libexpr/eval.cc

@@ -2384,7 +2384,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
         : [&]() {
             auto dstPath = fetchToStore(
                 *store,
-                path.resolveSymlinks(),
+                path.resolveSymlinks(SymlinkResolution::Ancestors),
                 settings.readOnlyMode ? FetchMode::DryRun : FetchMode::Copy,
                 path.baseName(),
                 ContentAddressMethod::Raw::NixArchive,

tests/functional/meson.build

@@ -164,6 +164,7 @@ suites = [
       'debugger.sh',
       'extra-sandbox-profile.sh',
       'help.sh',
+      'symlinks.sh',
     ],
     'workdir': meson.current_source_dir(),
   },

tests/functional/simple.sh

@@ -15,7 +15,7 @@ echo "output path is $outPath"
 [[ ! -w $outPath ]]
 
 text=$(cat "$outPath/hello")
-if test "$text" != "Hello World!"; then exit 1; fi
+[[ "$text" = "Hello World!" ]]
 
 TODO_NixOS
 

tests/functional/symlinks.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+# Check that when we have a derivation attribute that refers to a
+# symlink, we copy the symlink, not its target.
+# shellcheck disable=SC2016
+nix build --impure --no-link --expr '
+  with import ./config.nix;
+
+  mkDerivation {
+    name = "simple";
+    builder = builtins.toFile "builder.sh" "[[ -L \"$symlink\" ]]; mkdir $out";
+    symlink = ./lang/symlink-resolution/foo/overlays;
+  }
+'