Import symlink resolution regression

[edolstra]

Describe the bug

Nix 2.19.8:

$ nix eval https://flakehub.com/f/DeterminateSystems/nix/2.26.1 --raw --system x86_64-linux --apply '(x: x.drvPath)'
/nix/store/v7v8pli2n4biwwgbh9csc5jlk6f3b4kx-nix-2.26.1.drv

Nix 2.27:

$ nix eval https://flakehub.com/f/DeterminateSystems/nix/2.26.1 --raw --system x86_64-linux --apply '(x: x.drvPath)'
/nix/store/rv615sv2w4wad00cms2x7562krinz1ah-nix-2.26.1.drv

However 2.19.8 produces the same result as 2.27 when using --impure:

$ nix eval https://flakehub.com/f/DeterminateSystems/nix/2.26.1 --raw --system x86_64-linux --apply '(x: x.drvPath)' --impure
/nix/store/rv615sv2w4wad00cms2x7562krinz1ah-nix-2.26.1.drv

This issue is triggered by this line:

  nix-functional-tests = callPackage ../src/nix-functional-tests/package.nix {

where ../src/nix-functional-tests is a symlink to tests/functional. The latter has

    workDir = ./.;

which via

      sourceRoot = "${src.name}/" + workDirSubpath;

ends up in the derivation as source/./tests/functional (2.19) or source/./src/nix-functional-tests (2.27).

The root cause is that in Nix <= 2.19, we had a function checkSourcePath() that resolved symlinks depending on whether pure mode was enabled. This was removed in ea95327.

Steps To Reproduce

Expected behavior

Metadata

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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[edolstra]

Here is a "fix":

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 51d2991e7..2dc31c048 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -260,7 +260,8 @@ static void scopedImport(EvalState & state, const PosIdx pos, SourcePath & path,
    argument. */
 static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * vScope, Value & v)
 {
-    auto path = realisePath(state, pos, vPath, std::nullopt);
+    auto path = realisePath(state, pos, vPath,
+        state.settings.pureEval ? std::optional(SymlinkResolution::Full) : std::nullopt);
     auto path2 = path.path.abs();
 
     // FIXME

However, I'm inclined to close this as WILLNOTFIX since it's really 2.19 that was broken. We really don't want the behavior of symlink resolution to depend on whether --impure is enabled.

[roberth]

Agreed in meeting.