Use thread safe interior mutability for persist

Sadly there's not really a safe way to do this otherwise, but if you
really want to have persistence this way and absolutely need it to be
lock-free (because you're going to use it from the GUI thread), then you
can implement your own PersistentField.

nih_plug_derive/src/lib.rs

@@ -102,16 +102,15 @@ pub fn derive_params(input: TokenStream) -> TokenStream {
                 // function we get type erasure for free since we only need to worry about byte
                 // vectors
                 field_serialize_tokens.push(quote! {
-                    match ::nih_plug::params::serialize_field(&self.#field_name) {
+                    match ::nih_plug::params::PersistentField::map(
+                        &self.#field_name,
+                        ::nih_plug::params::serialize_field,
+                    ) {
                         Ok(data) => {
                             serialized.insert(String::from(#stable_name), data);
                         }
                         Err(err) => {
-                            ::nih_plug::nih_log!(
+                            ::nih_plug::nih_log!("Could not serialize '{}': {}", #stable_name, err)
-                                "Could not serialize '{}': {}",
-                                #stable_name,
-                                err
-                            )
                         }
                     };
                 });
@@ -119,7 +118,10 @@ pub fn derive_params(input: TokenStream) -> TokenStream {
                     #stable_name => {
                         match ::nih_plug::params::deserialize_field(&data) {
                             Ok(deserialized) => {
-                                 self.#field_name = deserialized;
+                                ::nih_plug::params::PersistentField::set(
+                                    &self.#field_name,
+                                    deserialized,
+                                );
                             }
                             Err(err) => {
                                 ::nih_plug::nih_log!(
@@ -164,9 +166,9 @@ pub fn derive_params(input: TokenStream) -> TokenStream {
                 serialized
             }
 
-            fn deserialize_fields(&mut self, serialized: ::std::collections::HashMap<&str, Vec<u8>>) {
+            fn deserialize_fields(&self, serialized: &::std::collections::HashMap<String, Vec<u8>>) {
                 for (field_name, data) in serialized {
-                    match field_name {
+                    match field_name.as_str() {
                         #(#field_deserialize_tokens)*
                         _ => nih_log!("Unknown field name: {}", field_name),
                     }

plugins/gain/src/lib.rs

@@ -24,6 +24,7 @@ use nih_plug::{
     util,
 };
 use std::pin::Pin;
+use std::sync::RwLock;
 
 struct Gain {
     params: Pin<Box<GainParams>>,
@@ -38,7 +39,7 @@ struct GainParams {
     /// together with a preset/state file saved for this plugin. This can be useful for storign
     /// things like sample data.
     #[persist = "industry_secrets"]
-    pub random_data: Vec<f32>,
+    pub random_data: RwLock<Vec<f32>>,
 }
 
 impl Default for Gain {
@@ -63,7 +64,7 @@ impl Default for GainParams {
                 value_to_string: formatters::f32_rounded(2),
                 string_to_value: None,
             },
-            random_data: Vec::new(),
+            random_data: RwLock::new(Vec::new()),
         }
     }
 }

src/params.rs

@@ -17,6 +17,7 @@
 use std::collections::HashMap;
 use std::fmt::Display;
 use std::pin::Pin;
+use std::sync::{Mutex, RwLock};
 
 pub type FloatParam = PlainParam<f32>;
 pub type IntParam = PlainParam<i32>;
@@ -26,6 +27,48 @@ pub use serde_json::from_slice as deserialize_field;
 /// Re-export for use in the [Params] proc-macro.
 pub use serde_json::to_vec as serialize_field;
 
+/// The functinoality needed for persisting a field to the plugin's state, and for restoring values
+/// when loading old state.
+pub trait PersistentField<'a, T>: Send + Sync
+where
+    T: serde::Serialize + serde::Deserialize<'a>,
+{
+    fn set(&self, new_value: T);
+    fn map<F, R>(&self, f: F) -> R
+    where
+        F: Fn(&T) -> R;
+}
+
+impl<'a, T> PersistentField<'a, T> for RwLock<T>
+where
+    T: serde::Serialize + serde::Deserialize<'a> + Send + Sync,
+{
+    fn set(&self, new_value: T) {
+        *self.write().expect("Poisoned RwLock on write") = new_value;
+    }
+    fn map<F, R>(&self, f: F) -> R
+    where
+        F: Fn(&T) -> R,
+    {
+        f(&self.read().expect("Poisoned RwLock on read"))
+    }
+}
+
+impl<'a, T> PersistentField<'a, T> for Mutex<T>
+where
+    T: serde::Serialize + serde::Deserialize<'a> + Send + Sync,
+{
+    fn set(&self, new_value: T) {
+        *self.lock().expect("Poisoned Mutex") = new_value;
+    }
+    fn map<F, R>(&self, f: F) -> R
+    where
+        F: Fn(&T) -> R,
+    {
+        f(&self.lock().expect("Poisoned Mutex"))
+    }
+}
+
 /// A distribution for a parameter's range. Probably need to add some forms of skewed ranges and
 /// maybe a callback based implementation at some point.
 #[derive(Debug)]
@@ -285,12 +328,15 @@ impl NormalizebleRange<i32> for Range<i32> {
     }
 }
 
-/// Describes a struct containing parameters. The idea is that we can have a normal struct
+/// Describes a struct containing parameters and other persistent fields. The idea is that we can
-/// containing [FloatParam] and other parameter types with attributes describing a unique identifier
+/// have a normal struct containing [FloatParam] and other parameter types with attributes assigning
-/// for each parameter. We can then build a mapping from those parameter IDs to the parameters using
+/// a unique identifier to each parameter. We can then build a mapping from those parameter IDs to
-/// the [Params::param_map] function. That way we can have easy to work with JUCE-style parameter
+/// the parameters using the [Params::param_map] function. That way we can have easy to work with
-/// objects in the plugin without needing to manually register each parameter, like you would in
+/// JUCE-style parameter objects in the plugin without needing to manually register each parameter,
-/// JUCE.
+/// like you would in JUCE.
+///
+/// The other persistent parameters should be [PersistentField]s containing types that can be
+/// serialized and deserialized with Serde.
 ///
 /// # Safety
 ///
@@ -308,9 +354,10 @@ pub trait Params {
     fn serialize_fields(&self) -> HashMap<String, Vec<u8>>;
 
     /// Restore all fields marked with `#[persist = "stable_name"]` from a hashmap created by
-    /// [Self::serialize_fields]. This gets called when the plugin's state is being restored. This
+    /// [Self::serialize_fields]. All of thse fields should be wrapped in a [PersistentFieldq] with
-    /// uses [deserialize_field] under the hood.
+    /// thread safe interior mutability, like an `RwLock` or a `Mutex`. This gets called when the
-    fn deserialize_fields(&mut self, serialized: HashMap<&str, Vec<u8>>);
+    /// plugin's state is being restored. This uses [deserialize_field] under the hood.
+    fn deserialize_fields(&self, serialized: &HashMap<String, Vec<u8>>);
 }
 
 /// Internal pointers to parameters. This is an implementation detail used by the wrappers.