feat: Add nix-util crate with Context wrapper

(cherry picked from commit 6c8e116e8bbaecce2b77fd9e0db89f366b57f9b6)

rust/Cargo.lock

@@ -11,6 +11,12 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "anyhow"
+version = "1.0.79"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "080e9890a082662b09c1ad45f567faeeb47f22b5fb23895fbe1e651e718e25ca"
+
 [[package]]
 name = "bindgen"
 version = "0.69.4"
@@ -166,6 +172,14 @@ dependencies = [
  "pkg-config",
 ]
 
+[[package]]
+name = "nix-util"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "nix-c-raw",
+]
+
 [[package]]
 name = "nixops4"
 version = "0.1.0"

rust/Cargo.toml

@@ -1,5 +1,6 @@
 [workspace]
 members = [
     "nix-c-raw",
+    "nix-util",
 ]
 resolver = "2"

rust/nix-util/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "nix-util"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+
+[dependencies]
+anyhow = "1.0.79"
+nix-c-raw = { path = "../nix-c-raw" }

rust/nix-util/src/context.rs

@@ -0,0 +1,54 @@
+use anyhow::{bail, Result};
+use nix_c_raw as raw;
+use std::ptr::null_mut;
+use std::ptr::NonNull;
+
+pub struct Context {
+    inner: NonNull<raw::nix_c_context>,
+}
+
+impl Context {
+    pub fn new() -> Self {
+        let ctx = unsafe { raw::nix_c_context_create() };
+        if ctx.is_null() {
+            panic!("nix_c_context_create returned a null pointer");
+        }
+        let ctx = Context {
+            inner: NonNull::new(ctx).unwrap(),
+        };
+        ctx
+    }
+    pub fn ptr(&self) -> *mut raw::nix_c_context {
+        self.inner.as_ptr()
+    }
+    pub fn check_err(&self) -> Result<()> {
+        let err = unsafe { raw::nix_err_code(self.inner.as_ptr()) };
+        if err != raw::NIX_OK.try_into().unwrap() {
+            // msgp is a borrowed pointer, so we don't need to free it
+            let msgp = unsafe { raw::nix_err_msg(null_mut(), self.inner.as_ptr(), null_mut()) };
+            // Turn the i8 pointer into a Rust string by copying
+            let msg: &str = unsafe { core::ffi::CStr::from_ptr(msgp).to_str()? };
+            bail!("{}", msg);
+        }
+        Ok(())
+    }
+}
+
+impl Drop for Context {
+    fn drop(&mut self) {
+        unsafe {
+            raw::nix_c_context_free(self.inner.as_ptr());
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn context_new_and_drop() {
+        // don't crash
+        let _c = Context::new();
+    }
+}

rust/nix-util/src/lib.rs

@@ -0,0 +1 @@
+pub mod context;