api: Add dc_msg_save_file() which saves file copy at the provided path (#4309)

... and fails if file already exists. The UI should open the file saving dialog, defaulting to
Downloads and original filename, when asked to save the file. After confirmation it should call
dc_msg_save_file().

deltachat-ffi/deltachat.h

@@ -4050,6 +4050,19 @@ char*           dc_msg_get_subject            (const dc_msg_t* msg);
 char*           dc_msg_get_file               (const dc_msg_t* msg);
 
 
+/**
+ * Save file copy at the user-provided path.
+ *
+ * Fails if file already exists at the provided path.
+ *
+ * @memberof dc_msg_t
+ * @param msg The message object.
+ * @param path Destination file path with filename and extension.
+ * @return 0 on failure, 1 on success.
+ */
+int             dc_msg_save_file              (const dc_msg_t* msg, const char* path);
+
+
 /**
  * Get an original attachment filename, with extension but without the path. To get the full path,
  * use dc_msg_get_file().

deltachat-ffi/src/lib.rs

@@ -3362,6 +3362,34 @@ pub unsafe extern "C" fn dc_msg_get_file(msg: *mut dc_msg_t) -> *mut libc::c_cha
         .unwrap_or_else(|| "".strdup())
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_msg_save_file(
+    msg: *mut dc_msg_t,
+    path: *const libc::c_char,
+) -> libc::c_int {
+    if msg.is_null() || path.is_null() {
+        eprintln!("ignoring careless call to dc_msg_save_file()");
+        return 0;
+    }
+    let ffi_msg = &*msg;
+    let ctx = &*ffi_msg.context;
+    let path = to_string_lossy(path);
+    let r = block_on(
+        ffi_msg
+            .message
+            .save_file(ctx, &std::path::PathBuf::from(path)),
+    );
+    match r {
+        Ok(()) => 1,
+        Err(_) => {
+            r.context("Failed to save file from message")
+                .log_err(ctx)
+                .unwrap_or_default();
+            0
+        }
+    }
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_msg_get_filename(msg: *mut dc_msg_t) -> *mut libc::c_char {
     if msg.is_null() {