libexpr/primops: reject __json in structured attrs derivations

In structured attrs derivations, __json is used to serialize the `env`
attribute into a JSON object.

If the derivation enables structured attributes AND specify its own
`__json`, one of the field will take precedence (the reader can have fun
by guessing which one).

To reduce underspecification, we disallow `__json` in structured
attributes derivations.

Fixes #380.

Change-Id: I51c3b2af1ff9449471ba81d4c72df6a7a263eef7
Signed-off-by: Raito Bezarius <raito@lix.systems>

doc/manual/rl-next/reject-json-structured-attrs.md

@@ -0,0 +1,19 @@
+---
+synopsis: "Reject `__json` in structured attributes derivations"
+cls: [5286]
+issues: [fj#380]
+category: "Improvements"
+credits: [raito]
+---
+
+In structured attributes derivations, `__json` is used internally to store the
+JSON representation of the `env` attribute field that users can set.
+
+Unfortunately, a user can set `__json` *and* enable structured attributes,
+resulting in a broken derivation from a semantic point of view.
+
+As no user can benefit from setting `__json` *and* enable structured attributes,
+we disallow that possibility and throw an error from now on.
+
+This is not seen as a breaking change because there's no user code that can
+benefit from this behavior, hence, it's an improvement to user experience.

lix/libexpr/primops.cc

@@ -903,6 +903,14 @@ drvName, Bindings * attrs, Value & v)
             "attribute passed to builtins.derivationStrict"
         ))
     {
+        if (attrs->get(state.ctx.symbols.sym___json)) {
+            state.ctx.errors
+                .make<EvalError>(
+                    "a `__json` attribute cannot be passed to builtins.derivationStrict when structured "
+                    "attributes are enabled"
+                )
+                .debugThrow();
+        }
         jsonObject = JSON::object();
     }
 

lix/libexpr/symbol-table.hh

@@ -217,6 +217,7 @@ public:
     const Symbol sym___structuredAttrs = create("__structuredAttrs");
     const Symbol sym___contentAddressed = create("__contentAddressed");
     const Symbol sym___impure = create("__impure");
+    const Symbol sym___json = create("__json");
     /* Derivation */
     const Symbol sym_outPath = create("outPath");
     const Symbol sym_drvPath = create("drvPath");

tests/functional2/build/test_structured_attrs.py

@@ -0,0 +1,19 @@
+from testlib.fixtures.nix import Nix
+
+
+evil_drv = """
+  builtins.derivation {
+    name = "foo";
+    builder = "/bin/sh";
+    system = builtins.currentSystem;
+    __structuredAttrs = true;
+
+    __json.hello = "1";
+    env.hello = "2";
+  }
+"""
+
+
+def test_improper_structured_attrs_drv(nix: Nix):
+    result = nix.nix_build(["--expr", evil_drv]).run().expect(1)
+    assert "a `__json` attribute cannot be passed" in result.stderr_s