fix(tvix): Use copy constructor to add strings to protos r/1886

Passing a string directly to add_paths like this causes the proto class
to take ownership over the string, meaning when it is destructed it
will *explicitly* free the string. When the string's actual owner (the
derivation struct) then goes out of scope it'll get freed again, causing
a double-free. This fixes that to instead use the copy constructor to
assign to a pointer to a new path, and covers the whole to_proto method
with a rapidcheck test.

Fixes: b/64
Change-Id: I84235bed9104ff430a0acf686d4a96f1e2e9a897
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2106
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

2 files changed, 29 insertions, 1 deletions

diff --git a/third_party/nix/src/libstore/derivations.cc b/third_party/nix/src/libstore/derivations.cc
index 0b7f5d092c..ed184b6d9d 100644
--- a/third_party/nix/src/libstore/derivations.cc
+++ b/third_party/nix/src/libstore/derivations.cc
@@ -72,7 +72,7 @@ nix::proto::Derivation BasicDerivation::to_proto() const {
     result.mutable_outputs()->insert({key, output.to_proto()});
   }
   for (const auto& input_src : inputSrcs) {
-    result.mutable_input_sources()->add_paths(input_src);
+    *result.mutable_input_sources()->add_paths() = input_src;
   }
   result.set_platform(platform);
   result.mutable_builder()->set_path(builder);
diff --git a/third_party/nix/src/tests/derivations_test.cc b/third_party/nix/src/tests/derivations_test.cc
index 63e2c3070e..50a540e84a 100644
--- a/third_party/nix/src/tests/derivations_test.cc
+++ b/third_party/nix/src/tests/derivations_test.cc
@@ -114,6 +114,34 @@ RC_GTEST_FIXTURE_PROP(DerivationsTest, UnparseParseRoundTrip,
   AssertDerivationsEqual(drv, parsed);
 }
 
+// NOLINTNEXTLINE
+RC_GTEST_FIXTURE_PROP(DerivationsTest, ToProtoPreservesInput,
+                      (Derivation && drv)) {
+  auto proto = drv.to_proto();
+
+  RC_ASSERT(proto.outputs_size() == drv.outputs.size());
+  RC_ASSERT(proto.input_sources().paths_size() == drv.inputSrcs.size());
+  auto paths = proto.input_sources().paths();
+  for (const auto& input_src : drv.inputSrcs) {
+    RC_ASSERT(std::find(paths.begin(), paths.end(), input_src) != paths.end());
+  }
+
+  RC_ASSERT(proto.platform() == drv.platform);
+  RC_ASSERT(proto.builder().path() == drv.builder);
+
+  RC_ASSERT(proto.args_size() == drv.args.size());
+  auto args = proto.args();
+  for (const auto& arg : drv.args) {
+    RC_ASSERT(std::find(args.begin(), args.end(), arg) != args.end());
+  }
+
+  RC_ASSERT(proto.env_size() == drv.env.size());
+  auto env = proto.env();
+  for (const auto& [key, value] : drv.env) {
+    RC_ASSERT(env.at(key) == value);
+  }
+}
+
 class ParseDrvPathWithOutputsTest : public DerivationsTest {};
 
 TEST(ParseDrvPathWithOutputsTest, ParseDrvPathWithOutputs) {