Allow restricting possible runtime references

Adds a new derivation attribute `__visibleReferences` which restricts the
list of paths that are searched in build outputs in order to establish
runtime references.

Setting this attribute requires the experimental feature
`invisible-references` to be enabled.

Useful when creating filesystem images containing their own embedded Nix
store: setting `__visibleReferences = [];` makes them self-contained
blobs of data with no runtime dependencies.

doc/manual/src/language/advanced-attributes.md

@@ -55,6 +55,25 @@ Derivations can declare some infrequently used optional attributes.
     dependency on `foobar` or any other derivation depending recursively
     on `foobar`.
 
+  - [`__visibleReferences`]{#adv-attr-__visibleReferences}\
+    The **experimental** attribute `__visibleReferences` restricts the set of
+    store paths that Nix will consider when scanning for runtime
+    dependencies. By default, this set includes build inputs, output
+    paths, and paths built by recursive Nix invocations. Setting this
+    attribute will restrict the set to the given list.
+    For example,
+
+    ```nix
+    __visibleReferences = [];
+    ```
+
+    effectively produces a blob of data without any runtime dependencies.
+    This is useful when generating self-contained filesystem images with
+    their own embedded Nix store: hashes found inside such an image refer
+    to the embedded store and not to the host's Nix store.
+
+    This is only allowed if the `invisible-references` experimental feature is enabled.
+
   - [`exportReferencesGraph`]{#adv-attr-exportReferencesGraph}\
     This attribute allows builders access to the references graph of
     their inputs. The attribute is a list of inputs in the Nix store
@@ -207,13 +226,13 @@ Derivations can declare some infrequently used optional attributes.
     the hash in either hexadecimal or base-32 notation. (See the
     [`nix-hash` command](../command-ref/nix-hash.md) for information
     about converting to and from base-32 notation.)
-    
+
   - [`__contentAddressed`]{#adv-attr-__contentAddressed}
     If this **experimental** attribute is set to true, then the derivation
     outputs will be stored in a content-addressed location rather than the
     traditional input-addressed one.
-    This only has an effect if the `ca-derivation` experimental feature is enabled.
-    
+    This only has an effect if the `ca-derivations` experimental feature is enabled.
+
     Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for *fixed-output derivations* (see above).
 
   - [`passAsFile`]{#adv-attr-passAsFile}\

src/libstore/build/local-derivation-goal.cc

@@ -2138,12 +2138,31 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
 
     /* The paths that can be referenced are the input closures, the
        output paths, and any paths that have been built via recursive
-       Nix calls. */
+       Nix calls; this list is restricted to the paths in the
+       __visibleReferences derivation attribute, if present. */
     StorePathSet referenceablePaths;
     for (auto & p : inputPaths) referenceablePaths.insert(p);
     for (auto & i : scratchOutputs) referenceablePaths.insert(i.second);
     for (auto & p : addedPaths) referenceablePaths.insert(p);
 
+    if (auto visibleReferences = parsedDrv->getStringsAttr("__visibleReferences")) {
+        settings.requireExperimentalFeature(Xp::InvisibleReferences);
+
+        StorePathSet spec;
+        for (auto & i : *visibleReferences) {
+            if (worker.store.isStorePath(i))
+                spec.insert(worker.store.parseStorePath(i));
+            else
+                throw BuildError("derivation attribute __visibleReferences contains an illegal reference '%s'", i);
+        }
+
+        StorePathSet tmp;
+        std::set_intersection(referenceablePaths.begin(), referenceablePaths.end(),
+                              spec.begin(), spec.end(),
+                              std::inserter(tmp, tmp.begin()));
+        referenceablePaths = std::move(tmp);
+    }
+
     /* FIXME `needsHashRewrite` should probably be removed and we get to the
        real reason why we aren't using the chroot dir */
     auto toRealPathChroot = [&](const Path & p) -> Path {

src/libutil/experimental-features.cc

@@ -14,6 +14,7 @@ std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::NoUrlLiterals, "no-url-literals" },
     { Xp::FetchClosure, "fetch-closure" },
     { Xp::ReplFlake, "repl-flake" },
+    { Xp::InvisibleReferences, "invisible-references" },
 };
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)

src/libutil/experimental-features.hh

@@ -23,6 +23,7 @@ enum struct ExperimentalFeature
     NoUrlLiterals,
     FetchClosure,
     ReplFlake,
+    InvisibleReferences,
 };
 
 /**

src/nix/develop.cc

@@ -192,6 +192,7 @@ static StorePath getDerivationEnvironment(ref<Store> store, ref<Store> evalStore
     drv.env.erase("allowedRequisites");
     drv.env.erase("disallowedReferences");
     drv.env.erase("disallowedRequisites");
+    drv.env.erase("__visibleReferences");
 
     /* Rehash and write the derivation. FIXME: would be nice to use
        'buildDerivation', but that's privileged. */

tests/check-refs.nix

@@ -67,4 +67,15 @@ rec {
     disallowedReferences = [test5];
   };
 
+  test11 = makeTest 11 {
+    buildCommand = "echo ${dep} > $out";
+    __visibleReferences = [];
+    disallowedReferences = [dep];
+  };
+
+  test12 = makeTest 12 {
+    buildCommand = "echo ${dep} ${src} > $out";
+    __visibleReferences = [dep];
+  };
+
 }

tests/check-refs.sh

@@ -40,3 +40,17 @@ nix-build -o $RESULT check-refs.nix -A test7
 
 # test10 should succeed (no disallowed references).
 nix-build -o $RESULT check-refs.nix -A test10
+
+if isDaemonNewer 2.12pre20220922; then
+    enableFeatures invisible-references
+    restartDaemon
+
+    # test11 should succeed (no referenceable paths).
+    test11=$(nix-build -o $RESULT check-refs.nix -A test11)
+    [[ -z $(nix-store -q --references "$test11") ]]
+
+    # test12 should succeed.
+    test12=$(nix-build -o $RESULT check-refs.nix -A test12)
+    nix-store -q --references "$test12" | grep -q "$dep"
+    ! nix-store -q --references "$test12" | grep -q aux-ref
+fi