config: treat empty strings as None

This is preparing for saving config from within web interface.
All lines even with empty values should be in TOML to avoid
creating new values at the end of file...

Changed default `config.toml` and added generic deserializer.

Author: manio

contrib/config.toml

@@ -15,13 +15,13 @@
   # Enable wired USB connection to phone (VID:PID should be specified, zero is wildcard and can be used for single or both fields)
   # you can obtain it e.g. using `lsusb` after connecting phone,
   # and then use e.g. "18d1:0" which will handle specified phone vendor ID (Google Pixel in this example)
-  # wired = "0000:0000"
+  wired = ""
 
   # Use a Google Android Auto Desktop Head Unit emulator instead of real HU device (will listen on TCP 5277 port)
   dhu = false
 
   # UDC Controller name (used in special configurations)
-  # udc = "dummy_udc.0"
+  udc = ""
 
 ### DEBUGGING
   # Enable debug info
@@ -38,9 +38,11 @@
   advertise = false
 
   # BLE device name (how your bluetooth device is visible)
-  # btalias = "WirelessAADongle"
+  # by default it is WirelessAADongle-CPUSERIAL
+  btalias = ""
 
   # Auto-connect to phone and initiate connection
+  # empty string/no value: don't auto-connect,
   # zeros/wildcard: iterate over all previously connected devices,
   # set it to phone BT MAC address for connecting to this specific phone
   connect = "00:00:00:00:00:00"
@@ -87,7 +89,7 @@
 
   # Path to script executed when Android Auto session starts/stops and EV battery data is needed
   # Argument "start" or "stop" is appended automatically when invoked
-  # ev_battery_logger = "/usr/bin/some_script"
+  ev_battery_logger = ""
 
   # Traction battery capacity [Wh]
   ev_battery_capacity = 22000

src/main.rs

@@ -17,9 +17,9 @@ use simplelog::*;
 use usb_gadget::uevent_listener;
 use usb_gadget::UsbGadgetState;
 
-use serde::de::{self, Deserializer, Visitor};
+use serde::de::{self, Deserializer, Error as DeError, Visitor};
 use serde::Deserialize;
-use std::fmt;
+use std::fmt::{self, Display};
 use std::fs::OpenOptions;
 use std::path::PathBuf;
 use std::str::FromStr;
@@ -112,6 +112,20 @@ struct Args {
     config: PathBuf,
 }
 
+pub fn empty_string_as_none<'de, T, D>(deserializer: D) -> Result<Option<T>, D::Error>
+where
+    T: FromStr,
+    T::Err: Display,
+    D: Deserializer<'de>,
+{
+    let s: String = Deserialize::deserialize(deserializer)?;
+    if s.trim().is_empty() {
+        Ok(None)
+    } else {
+        T::from_str(&s).map(Some).map_err(DeError::custom)
+    }
+}
+
 #[derive(Debug, Clone, Deserialize)]
 #[serde(default)]
 pub struct AppConfig {
@@ -120,12 +134,15 @@ pub struct AppConfig {
     hexdump_level: HexdumpLevel,
     disable_console_debug: bool,
     legacy: bool,
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     connect: Option<Address>,
     logfile: PathBuf,
     stats_interval: u16,
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     udc: Option<String>,
     iface: String,
     hostapd_conf: PathBuf,
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     btalias: Option<String>,
     keepalive: bool,
     timeout_secs: u16,
@@ -137,9 +154,11 @@ pub struct AppConfig {
     disable_media_sink: bool,
     disable_tts_sink: bool,
     developer_mode: bool,
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     wired: Option<UsbId>,
     dhu: bool,
     ev: bool,
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     ev_battery_logger: Option<PathBuf>,
     ev_battery_capacity: u64,
     ev_factor: f32,