feat: builtins.readSymlink primop

src/libexpr/primops.cc

@@ -2376,6 +2376,26 @@ static RegisterPrimOp primop_readFileType({
     .fun = prim_readFileType,
 });
 
+/* Read the target of a symlink. */
+static void prim_readSymlink(EvalState & state, const PosIdx pos, Value ** args, Value & v)
+{
+    const auto path = state.realisePath(pos, *args[0], SymlinkResolution::Ancestors);
+    const auto target = path.readLink();
+    const auto parent = path.parent();
+    v.mkPath(SourcePath(path.accessor, CanonPath(target, parent.path)), state.mem);
+}
+
+static RegisterPrimOp primop_readSymlink({
+    .name = "__readSymlink",
+    .args = {"path"},
+    .doc = R"(
+      Return the target of the symlink at *path*.
+
+      If *path* does not refer to a symlink, an error is thrown.
+    )",
+    .fun = prim_readSymlink,
+});
+
 /* Read a directory (without . or ..) */
 static void prim_readDir(EvalState & state, const PosIdx pos, Value ** args, Value & v)
 {

tests/functional/lang/eval-fail-readSymlink-not-a-symlink.err.exp

@@ -0,0 +1 @@
+error: filesystem error: read_symlink: not a symlink [/pwd/lang/readDir/bar]

tests/functional/lang/eval-fail-readSymlink-not-a-symlink.nix

@@ -0,0 +1 @@
+builtins.readSymlink ./readDir/bar

tests/functional/lang/eval-fail-readSymlink-not-a-symlink.postprocess

@@ -0,0 +1,11 @@
+# shellcheck shell=bash
+set -euo pipefail
+testcaseBasename=$1
+
+# Normalize platform-specific filesystem error messages
+sed -i "$testcaseBasename.err" \
+  -e 's/filesystem error: in read_symlink:/filesystem error: read_symlink:/' \
+  -e 's/Invalid argument/not a symlink/' \
+  -e 's/Not a symbolic link/not a symlink/' \
+  -e 's/\["\([^"]*\)"\]/[\1]/' \
+  ;

tests/functional/lang/eval-okay-readSymlink.exp

@@ -0,0 +1 @@
+{ ldir = /pwd/lang/readDir/foo; linked = /pwd/lang/readDir/foo/git-hates-directories; }

tests/functional/lang/eval-okay-readSymlink.nix

@@ -0,0 +1,4 @@
+{
+  ldir = builtins.readSymlink ./readDir/ldir;
+  linked = builtins.readSymlink ./readDir/linked;
+}