maint: Update get_uri to be callback based

(cherry picked from commit 1aaf9b4d764da5aa8c1297d87ad1002bf1d7a7ea)

rust/nix-util/src/lib.rs

@@ -1 +1,2 @@
 pub mod context;
+pub mod string_return;

rust/nix-util/src/string_return.rs

@@ -0,0 +1,54 @@
+/// Callback for nix_store_get_uri and other functions that return a string.
+///
+/// This function is used by the other nix_* crates, and you should never need to call it yourself.
+pub unsafe extern "C" fn callback_get_vec_u8(
+    start: *const ::std::os::raw::c_char,
+    n: std::os::raw::c_uint,
+    user_data: *mut std::os::raw::c_void,
+) {
+    let ret = user_data as *mut Vec<u8>;
+    let slice = std::slice::from_raw_parts(start as *const u8, n as usize);
+    if !(*ret).is_empty() {
+        panic!("callback_get_vec_u8: slice must be empty. Were we called twice?");
+    }
+    (*ret).extend_from_slice(slice);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use nix_c_raw as raw;
+
+    /// Typecheck the function signature against the generated bindings in nix_c_raw.
+    static _CALLBACK_GET_VEC_U8: raw::nix_get_string_callback = Some(callback_get_vec_u8);
+
+    #[test]
+    fn test_callback_get_vec_u8_empty() {
+        let mut ret: Vec<u8> = Vec::new();
+        let start: *const std::os::raw::c_char = std::ptr::null();
+        let n: std::os::raw::c_uint = 0;
+        let user_data: *mut std::os::raw::c_void =
+            &mut ret as *mut Vec<u8> as *mut std::os::raw::c_void;
+
+        unsafe {
+            callback_get_vec_u8(start, n, user_data);
+        }
+
+        assert_eq!(ret, vec![]);
+    }
+
+    #[test]
+    fn test_callback_get_vec_u8() {
+        let mut ret: Vec<u8> = Vec::new();
+        let start: *const std::os::raw::c_char = b"helloGARBAGE".as_ptr() as *const i8;
+        let n: std::os::raw::c_uint = 5;
+        let user_data: *mut std::os::raw::c_void =
+            &mut ret as *mut Vec<u8> as *mut std::os::raw::c_void;
+
+        unsafe {
+            callback_get_vec_u8(start, n, user_data);
+        }
+
+        assert_eq!(ret, b"hello".to_vec());
+    }
+}