serde_json adding an extra '}' - rust

Question

I have a problme serializing/desserializing an struct with serde_json in rust, i had used the impl's (dont used derive) that are on serde_json:ser and serde_json:de, but something is happening when i try to write it in a file, i hava thread (this is the unique thread that open that file) that is in a loop saving some data to a file named "config.json", but some times it saves an extra '}' at the end of the file and i don't know why.
{"Alive":true,...,"Threads":20}}
pub fn write_config(data: &Struct_object) -> () {
let file = OpenOptions::new().read(true).write(true).create(true).open("config.json");
let mut refval: Struct_object = data.clone();
match to_string::<Struct_object>(&mut refval) {
Ok(u) => {
match file {
Ok(mut s) => match s.write(u.as_bytes()) {
Ok(_) => (),
Err(u) => warn!("Config.json couldn't be writen, data: {{ {} }}, Error: {{ {} }}", data, u),
},
Err(t) => {
warn!("Config.json couldn't be writen, data: {{ {} }}, Error: {{ {} }}", data, t);
},
}
},
Err(v) => warn!("Config.json couldn't be writen, data: {{ {} }}, Error: {{ {} }}", data, v),
}
}
pub fn read_config() -> Struct_object{
if file_exist() {
let file = OpenOptions::new().read(true).open("config.json");
let mut buf = String::new();
match file {
Ok(mut s) => {
let _ = s.read_to_string(&mut buf);
match from_str::<Struct_object>(&buf) {
Ok(t) => t,
Err(u) => {
warn!("config.json couldn't be loaded, loading default configuration, Error: {}", u);
Struct_object::default()
}
}
},
Err(v) => {warn!("config.json couldn't be readed, loading default configuration, Error: {}", v);
Struct_object::default()},
}
} else {
//write_config(&Struct_object::default());
Struct_object::default()
}
}
there is the code to read and write the data, and the loop is something like this:
pub fn daemon_variable_set(val: Struct_object) {
loop {
let data = read_config();
val.write_rpc_url(data.read_rpc_url());
thread::sleep(Duration::from_secs(1));
write_config(&val);
thread::sleep(Duration::from_secs(1));
if !val.read_alive() {
break;
}
}
info!("Thread variable_set stop");
}
the information in the Struct_object are Arc<RwLock>, but in the serde is saving just the information of the primitive, and at read, i create the Arc to encapsulate the Primitives, so, i dont understand why is adding this extra '}' at the end of the line
Also i tried with the next, but was worst
use serde_json::de::from_reader;
use serde_json::ser::to_writer;

Answer 1

You're overwriting the config.txt file but you aren't truncating it before writing to it, so data from previous writes can be seen if the data being written is shorter.
Before:
{"foo":12}
After writing {"foo":5} without truncating the file (since its 1 character shorter):
{"foo":5}}
You should simply use File::create:
Opens a file in write-only mode.
This function will create a file if it does not exist, and will truncate it if it does.
It will internally use .write(true).create(true).truncate(true) which is usually what you want when writing to a file like this.

Answer 2

You are missing a call to std::fs::OpenOptions::truncate(true) in your write_config() function.
Without that, the file will be written to, overwriting the previous content, but leaving the previous content in place. If the file's conent from the previous iteration of write_config happens to be longer by exactly one character (for instance: some value changes from 123 to 99), that will show up as an "extra" } during the next read.