feat: add --apply flag

Allow applying every eval'ed drv to a provided nix expression to evaluate arbitrary fields.

EXAMPLE

```
% ./result/bin/nix-eval-jobs --flake github:NixOS/patchelf#hydraJobs \
  --apply \
  'drv: {
     version = if drv ? version then drv.version else null;
   }'
warning: `--gc-roots-dir' not specified
{"attr":"coverage","attrPath":["coverage"],"version":null}
{"attr":"patchelf-win32","attrPath":["patchelf-win32"],"version":"0.18.0"}
{"attr":"patchelf-win64","attrPath":["patchelf-win64"],"version":"0.18.0"}
{"attr":"release","attrPath":["release"],"version":null}
{"attr":"tarball","attrPath":["tarball"],"version":"0.18.0"}
```

README.md

@@ -43,6 +43,7 @@ executable.
 $ nix-eval-jobs --help
 USAGE: nix-eval-jobs [options] expr
 
+  --apply                apply the derivation to the provided Nix expression
   --arg                  Pass the value *expr* as the argument *name* to Nix functions.
   --argstr               Pass the string *string* as the argument *name* to Nix functions.
   --check-cache-status   Check if the derivations are present locally or in any configured substituters (i.e. binary cache). The information will be exposed in the `cacheStatus` field of the JSON output.

src/eval-args.cc

@@ -102,6 +102,12 @@ MyArgs::MyArgs() : MixCommonArgs("nix-eval-jobs") {
              .description = "treat the argument as a Nix expression",
              .handler = {&fromArgs, true}});
 
+    addFlag(
+        {.longName = "apply",
+         .description = "apply the derivation to the provided Nix expression",
+         .labels = {"expr"},
+         .handler = {&applyExpr}});
+
     // usually in MixFlakeOptions
     addFlag({
         .longName = "override-input",

src/eval-args.hh

@@ -14,6 +14,7 @@ class MyArgs : virtual public nix::MixEvalArgs,
   public:
     virtual ~MyArgs() = default;
     std::string releaseExpr;
+    std::string applyExpr;
     nix::Path gcRootsDir;
     bool flake = false;
     bool fromArgs = false;

src/worker.cc

@@ -9,6 +9,7 @@
 #include <nix/attr-path.hh>
 #include <nix/local-fs-store.hh>
 #include <nix/installable-flake.hh>
+#include <nix/value-to-json.hh>
 #include <sys/resource.h>
 #include <nlohmann/json.hpp>
 #include <cstdio>
@@ -147,6 +148,7 @@ void worker(
 
             if (v->type() == nix::nAttrs) {
                 if (auto packageInfo = nix::getDerivation(*state, *v, false)) {
+
                     std::optional<Constituents> maybeConstituents;
                     if (args.constituents) {
                         std::vector<std::string> constituents;
@@ -203,6 +205,26 @@ void worker(
                         }
                         maybeConstituents =
                             Constituents(constituents, namedConstituents);
+                    } else if (args.applyExpr != "") {
+                        auto applyExpr = state->parseExprFromString(
+                            args.applyExpr, state->rootPath("."));
+
+                        nix::Value vApply;
+                        nix::Value vRes;
+
+                        state->eval(applyExpr, vApply);
+
+                        state->callFunction(vApply, *v, vRes, nix::noPos);
+                        state->forceAttrs(
+                            vRes, nix::noPos,
+                            "apply needs to evaluate to an attrset");
+
+                        nix::NixStringContext context;
+                        std::stringstream ss;
+                        nix::printValueAsJSON(*state, true, vRes, nix::noPos,
+                                              ss, context);
+
+                        reply.update(nlohmann::json::parse(ss.str()));
                     }
                     auto drv = Drv(attrPathS, *state, *packageInfo, args,
                                    maybeConstituents);

tests/test_eval.py

@@ -212,6 +212,51 @@ def test_constituents_error() -> None:
         assert aggregate["constituents"] == []
 
 
+def test_apply() -> None:
+    with TemporaryDirectory() as tempdir:
+        applyExpr = """drv: {
+            the-name = drv.name;
+            version = drv.version or null;
+        }"""
+
+        cmd = [
+            str(BIN),
+            "--gc-roots-dir",
+            tempdir,
+            "--workers",
+            "1",
+            "--apply",
+            applyExpr,
+            "--flake",
+            ".#hydraJobs",
+        ]
+        res = subprocess.run(
+            cmd,
+            cwd=TEST_ROOT.joinpath("assets"),
+            text=True,
+            check=True,
+            stdout=subprocess.PIPE,
+        )
+
+        print(res.stdout)
+        results = [json.loads(r) for r in res.stdout.split("\n") if r]
+
+        assert len(results) == 5 # sanity check that we assert against all jobs
+
+        # Check that nix-eval-jobs applied the expression correctly
+        # and extracted 'version' as 'version' and 'name' as 'the-name'
+        assert results[0]["the-name"] == "job1"
+        assert results[0]["version"] is None
+        assert results[1]["the-name"].startswith("nix-")
+        assert results[1]["version"] is not None
+        assert results[2]["the-name"] == "package-with-deps"
+        assert results[2]["version"] is None
+        assert results[3]["the-name"] == "drvB"
+        assert results[3]["version"] is None
+        assert results[4]["the-name"].startswith("nix-")
+        assert results[4]["version"] is not None
+
+
 @pytest.mark.infiniterecursion
 def test_recursion_error() -> None:
     with TemporaryDirectory() as tempdir: