feat: Add manual testing of bindings with Valgrind

(cherry picked from commit 12fed067bf230d3258db172afe648ec690ba584c)

doc/hacking/test-ffi.md

@@ -0,0 +1,25 @@
+
+# Testing FFI code
+
+If `cargo-valgrind` is broken, you may run `valgrind` manually.
+
+1. `cd rust; cargo test -v`
+2. find the relevant test suite executable in the log
+    - example: `/home/user/src/nix-bindings-rust/rust/target/debug/deps/nix_util-036ec381a9e3fd6d`
+3. `valgrind --leak-check=full <paste the test exe>`
+4. check that
+    - `definitely lost: 0 bytes in 0 blocks`
+
+## Paranoid check
+
+Although normal valgrind tends to catch things, you may choose to enable `--show-leak-kinds=all`.
+This will print a few false positive.
+
+Acceptable leaks are those involving (and this may be Linux-specific)
+- `call_init`: static initializers
+  - `nix::GlobalConfig::Register::Register`
+  - `_GLOBAL__sub_I_logging.cc`
+  - ...
+- `new<test::test_main::{closure_env#0}>`: a leak in the rust test framework
+
+When in doubt, compare the log to a run with your new test case commented out.

flake.nix

@@ -55,6 +55,11 @@
               pkgs.rustfmt
               pkgs.pkg-config
               pkgs.clang-tools # clangd
+              pkgs.valgrind
+              # TODO: set up cargo-valgrind in shell and build
+              #       currently both this and `cargo install cargo-valgrind`
+              #       produce a binary that says ENOENT.
+              # pkgs.cargo-valgrind
             ];
             shellHook = ''
               ${config.pre-commit.installationScript}