support nix_store_open's params argument

(cherry picked from commit 3dd5137e33709559ffa152feb7ab508246adfb54)

rust/nix-store/src/store.rs

@@ -4,7 +4,7 @@ use nix_c_raw as raw;
 use nix_util::context::Context;
 use nix_util::result_string_init;
 use nix_util::string_return::{callback_get_result_string, callback_get_result_string_data};
-use std::ffi::CString;
+use std::ffi::{c_char, CString};
 use std::ptr::null_mut;
 use std::ptr::NonNull;
 
@@ -41,7 +41,10 @@ pub struct Store {
     context: Context,
 }
 impl Store {
-    pub fn open(url: &str) -> Result<Self> {
+    pub fn open<'a, 'b>(
+        url: &str,
+        params: impl IntoIterator<Item = (&'a str, &'b str)>,
+    ) -> Result<Self> {
         let x = INIT.as_ref();
         match x {
             Ok(_) => {}
@@ -54,13 +57,27 @@ impl Store {
         let context: Context = Context::new();
 
         let uri_ptr = CString::new(url)?;
-        let store = unsafe {
-            raw::store_open(
-                context.ptr(),
-                uri_ptr.as_ptr(),
-                null_mut::<*mut *const i8>(),
-            )
-        };
+
+        // this intermediate value must be here and must not be moved
+        // because it owns the data the `*const c_char` pointers point to.
+        let params: Vec<(CString, CString)> = params
+            .into_iter()
+            .map(|(k, v)| Ok((CString::new(k)?, CString::new(v)?))) // to do. context
+            .collect::<Result<_>>()?;
+        // this intermediate value owns the data the `*mut *const c_char` pointer points to.
+        let mut params: Vec<_> = params
+            .iter()
+            .map(|(k, v)| [k.as_ptr(), v.as_ptr()])
+            .collect();
+        // this intermediate value owns the data the `*mut *mut *const c_char` pointer points to.
+        let mut params: Vec<*mut *const c_char> = params
+            .iter_mut()
+            .map(|t| t.as_mut_ptr())
+            .chain(std::iter::once(null_mut())) // signal the end of the array
+            .collect();
+
+        let store =
+            unsafe { raw::store_open(context.ptr(), uri_ptr.as_ptr(), params.as_mut_ptr()) };
         context.check_err()?;
         if store.is_null() {
             panic!("nix_c_store_open returned a null pointer without an error");