refact: Add Context::check_one_call(f)

This makes it easier to stick to a regular pattern and not forget
to check the context afterwards.

(cherry picked from commit 966ef3769a23075f5d660a48cf5a7307df925dbf)

rust/nix-util/src/context.rs

@@ -34,6 +34,31 @@ impl Context {
         }
         Ok(())
     }
+
+    pub fn check_err_and_clear(&self) -> Result<()> {
+        let r = self.check_err();
+        if r.is_err() {
+            unsafe {
+                // TODO (https://github.com/NixOS/nix/pull/10910): raw::err_clear
+                raw::set_err_msg(
+                    self.inner.as_ptr(),
+                    raw::NIX_OK.try_into().unwrap(),
+                    b"\0".as_ptr() as *const i8,
+                );
+            }
+        }
+        r
+    }
+
+    /// Run the function, and check the error, then reset the error.
+    /// Make at most one call to a Nix function in `f`.
+    /// Do not use if the context isn't fresh or cleared (e.g. with `check_err_and_clear`).
+    pub fn check_one_call<T, F: FnOnce(*mut raw::c_context) -> T>(&self, f: F) -> Result<T> {
+        let t = f(self.ptr());
+        self.check_err_and_clear()?;
+        Ok(t)
+    }
+
     /// NIX_ERR_KEY is returned when e.g. an attribute is missing. Return true if the error is of this type.
     pub fn is_key_error(&self) -> bool {
         unsafe { raw::err_code(self.inner.as_ptr()) == raw::NIX_ERR_KEY }