eval: use a work directory instead of an environment variable to cont…

…rol the Nix expression

No test used the `NIX_CHECK_BY_NAME_EXPR_PATH` to change the Nix file that was read, and it's simple
to rebuild in order to test out changes to the `src/eval.nix`.

default.nix

@@ -14,7 +14,6 @@ let
   };
   inherit (pkgs) lib;
 
-  runtimeExprPath = ./src/eval.nix;
   testNixpkgsPath = ./tests/mock-nixpkgs.nix;
   nixpkgsLibPath = nixpkgs + "/lib";
 
@@ -53,15 +52,13 @@ let
       inherit
         nixpkgsLibPath
         initNix
-        runtimeExprPath
         testNixpkgsPath
         version
         ;
       nix = defaultNixPackage;
     };
 
     shell = pkgs.mkShell {
-      env.NIX_CHECK_BY_NAME_EXPR_PATH = toString runtimeExprPath;
       env.NIX_CHECK_BY_NAME_NIX_PACKAGE = lib.getBin defaultNixPackage;
       env.NIX_PATH = "test-nixpkgs=${toString testNixpkgsPath}:test-nixpkgs/lib=${toString nixpkgsLibPath}";
       env.RUST_SRC_PATH = "${pkgs.rustPlatform.rustLibSrc}";

package.nix

@@ -14,7 +14,6 @@
 
   nixpkgsLibPath,
   initNix,
-  runtimeExprPath,
   testNixpkgsPath,
   version,
 }:
@@ -38,7 +37,6 @@ rustPlatform.buildRustPackage {
     clippy
     makeWrapper
   ];
-  env.NIX_CHECK_BY_NAME_EXPR_PATH = "${runtimeExprPath}";
   env.NIX_CHECK_BY_NAME_NIX_PACKAGE = lib.getBin nix;
   env.NIX_PATH = "test-nixpkgs=${testNixpkgsPath}:test-nixpkgs/lib=${nixpkgsLibPath}";
   checkPhase = ''
@@ -62,7 +60,6 @@ rustPlatform.buildRustPackage {
   '';
   postInstall = ''
     wrapProgram $out/bin/nixpkgs-check-by-name \
-      --set NIX_CHECK_BY_NAME_EXPR_PATH "$NIX_CHECK_BY_NAME_EXPR_PATH" \
       --set NIX_CHECK_BY_NAME_NIX_PACKAGE ${lib.getBin nix}
   '';
 }

src/eval.rs

@@ -11,13 +11,16 @@ use crate::validation::ResultIteratorExt as _;
 use crate::validation::{self, Validation::Success};
 use crate::NixFileStore;
 use relative_path::RelativePathBuf;
+use std::fs;
 use std::path::Path;
 
 use anyhow::Context;
 use serde::Deserialize;
 use std::path::PathBuf;
 use std::process;
-use tempfile::NamedTempFile;
+use tempfile::Builder;
+
+const EVAL_NIX: &[u8] = include_bytes!("eval.nix");
 
 /// Attribute set of this structure is returned by `./eval.nix`
 #[derive(Deserialize)]
@@ -106,26 +109,29 @@ pub fn check_values(
     package_names: Vec<String>,
     is_test: bool,
 ) -> validation::Result<ratchet::Nixpkgs> {
-    // Write the list of packages we need to check into a temporary JSON file.
-    // This can then get read by the Nix evaluation.
-    let attrs_file = NamedTempFile::new().with_context(|| "Failed to create a temporary file")?;
+    let work_dir = Builder::new()
+        .prefix("nixpkgs-check-by-name")
+        .tempdir()
+        .with_context(|| "Failed to create a working directory")?;
 
-    // We need to canonicalise this path. If it's a symlink (which can be the case on Darwin),
-    // Nix would need to read both the symlink and the target path, and therefore need two NIX_PATH
-    // entries for restrict-eval. If we resolve the symlinks, we only need one predictable entry.
-    let attrs_file_path = attrs_file.path().canonicalize()?;
+    // Canonicalize the path so that if a symlink were returned, we wouldn't ask Nix to follow it.
+    let work_dir_path = work_dir.path().canonicalize()?;
 
-    serde_json::to_writer(&attrs_file, &package_names).with_context(|| {
+    // Write the list of packages we need to check into a temporary JSON file.
+    let package_names_path = work_dir_path.join("package-names.json");
+    let package_names_file = fs::File::create(&package_names_path)?;
+    serde_json::to_writer(&package_names_file, &package_names).with_context(|| {
         format!(
-            "Failed to serialise the package names to the temporary path {}",
-            attrs_file_path.display()
+            "Failed to serialise the package names to the work dir {}",
+            work_dir_path.display()
         )
     })?;
 
-    let expr_path = std::env::var("NIX_CHECK_BY_NAME_EXPR_PATH")
-        .with_context(|| "Could not get environment variable NIX_CHECK_BY_NAME_EXPR_PATH")?;
+    // Write the Nix file into the work directory.
+    let eval_nix_path = work_dir_path.join("eval.nix");
+    fs::write(&eval_nix_path, EVAL_NIX)?;
 
-    // Pinning nix in this way makes the tool more reproducible
+    // Pinning Nix in this way makes the tool more reproducible
     let nix_package = std::env::var("NIX_CHECK_BY_NAME_NIX_PACKAGE")
         .with_context(|| "Could not get environment variable NIX_CHECK_BY_NAME_NIX_PACKAGE")?;
 
@@ -141,13 +147,13 @@ pub fn check_values(
             "--readonly-mode",
             "--restrict-eval",
         ])
-        // Pass the path to the attrs_file as an argument and add it to the NIX_PATH so it can be
-        // accessed in restrict-eval mode.
-        .args(["--arg", "attrsPath"])
-        .arg(&attrs_file_path)
+        // Add the work directory to the NIX_PATH so that it can be accessed in restrict-eval mode.
         .arg("-I")
-        .arg(&attrs_file_path)
-        // Same for the nixpkgs to test.
+        .arg(&work_dir_path)
+        .args(["--arg", "attrsPath"])
+        .arg(&package_names_path)
+        // Same for the nixpkgs to test, adding it to the NIX_PATH so it can be accessed in
+        // restrict-eval mode.
         .args(["--arg", "nixpkgsPath"])
         .arg(nixpkgs_path)
         .arg("-I")
@@ -162,8 +168,7 @@ pub fn check_values(
         command.arg("--show-trace");
     }
 
-    command.args(["-I", &expr_path]);
-    command.arg(expr_path);
+    command.arg(eval_nix_path);
 
     let result = command
         .output()
@@ -174,6 +179,7 @@ pub fn check_values(
         let stderr = String::from_utf8_lossy(&result.stderr).to_string();
         return Ok(NixpkgsProblem::NixEval(NixEvalError { stderr }).into());
     }
+
     // Parse the resulting JSON value
     let attributes: Vec<(String, Attribute)> = serde_json::from_slice(&result.stdout)
         .with_context(|| {

src/main.rs

@@ -272,12 +272,23 @@ mod tests {
     }
 
     fn test_nixpkgs(name: &str, path: &Path, expected_errors: &str) -> anyhow::Result<()> {
+        // Match the expected errors almost verbatim -- `@REDACTED@` turns into `.*`.
+        let pattern = format!(
+            "^{}$",
+            regex::escape(expected_errors).replace("@REDACTED@", ".*")
+        );
+
+        let expected_errors_regex = regex::RegexBuilder::new(&pattern)
+            .dot_matches_new_line(true)
+            .build()?;
+
         let base_path = path.join("base");
         let base_nixpkgs = if base_path.exists() {
             base_path.as_path()
         } else {
             Path::new("tests/empty-base")
         };
+
         // Empty dir, needed so that no warnings are printed when testing older Nix versions
         // that don't recognise certain newer keys in nix.conf
         let nix_conf_dir = tempdir()?;
@@ -297,21 +308,7 @@ mod tests {
             },
         )?;
 
-        let expr_path = std::env::var("NIX_CHECK_BY_NAME_EXPR_PATH")
-            .with_context(|| "Could not get environment variable NIX_CHECK_BY_NAME_EXPR_PATH")?;
-
-        // We end up with a small Nix trace that includes the absolute path to src/eval.nix
-        // on the output, which we need to relativise for the tests to succeed everywhere
-        let actual_errors = String::from_utf8_lossy(&writer).replace(&expr_path, "src/eval.nix");
-
-        let pattern = format!(
-            "^{}$",
-            regex::escape(expected_errors).replace("@REDACTED@", ".*")
-        );
-
-        let expected_errors_regex = regex::RegexBuilder::new(&pattern)
-            .dot_matches_new_line(true)
-            .build()?;
+        let actual_errors = String::from_utf8_lossy(&writer);
 
         if !expected_errors_regex.is_match(&actual_errors) {
             panic!(