feat: expose building a CompiledPlugin with a fuel limit (#883)

Expose a new method `extism_compiled_plugin_new_with_fuel_limit` to
build a CompiledPlugin with a fuel limit.

Fixes #882

.github/workflows/release-dotnet-native.yaml

@@ -20,10 +20,13 @@ jobs:
         uses: actions/setup-dotnet@v3.0.3
         with:
            dotnet-version: 7.x
-      - uses: dawidd6/action-download-artifact@v6
-        with:
-          workflow: release.yml
-          name: release-artifacts
+      - name: download release
+        run: |
+          tag='${{ github.ref }}'
+          tag="${tag/refs\/tags\//}"
+          gh release download "$tag" -p 'libextism-*.tar.gz'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: Extract Archive
         run: |      
           extract_archive() {

convert/Cargo.toml

@@ -13,7 +13,7 @@ description = "Traits to make Rust types usable with Extism"
 anyhow = "1.0.75"
 base64 = "~0.22"
 bytemuck = {version = "1.14.0", optional = true }
-prost = { version = "0.13.1", optional = true }
+prost = { version = "0.14.1", optional = true }
 protobuf = { version = "3.2.0", optional = true }
 rmp-serde = { version = "1.1.2", optional = true }
 serde = "1.0.186"

runtime/Cargo.toml

@@ -9,35 +9,54 @@ repository.workspace = true
 version.workspace = true
 
 [dependencies]
-wasmtime = {version = ">= 27.0.0, < 31.0.0"}
-wasi-common = {version = ">= 27.0.0, < 31.0.0"}
-wiggle = {version = ">= 27.0.0, < 31.0.0"}
+wasmtime = { version="37", default-features = false, features = [
+  'cache',
+  'gc',
+  'gc-drc',
+  'cranelift',
+  'coredump',
+  'wat',
+  'parallel-compilation',
+  'pooling-allocator',
+  'demangle',
+] }
+wasi-common = "37"
+wiggle = "37"
 anyhow = "1"
-serde = {version = "1", features = ["derive"]}
+serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-toml = "0.8"
+toml = "0.9"
 sha2 = "0.10"
 tracing = "0.1"
-tracing-subscriber = {version = "0.3.18", features = ["std", "env-filter", "fmt"]}
+tracing-subscriber = { version = "0.3.18", features = [
+  "std",
+  "env-filter",
+  "fmt",
+] }
 url = "2"
 glob = "0.3"
-ureq = {version = "3.0", optional=true}
+ureq = { version = "3.0", optional = true }
 extism-manifest = { workspace = true }
 extism-convert = { workspace = true, features = ["extism-path"] }
 uuid = { version = "1", features = ["v4"] }
 libc = "0.2"
 
 [features]
-default = ["http", "register-http", "register-filesystem"]
-register-http = ["ureq"] # enables wasm to be downloaded using http
-register-filesystem = [] # enables wasm to be loaded from disk
-http = ["ureq"]          # enables extism_http_request
+default = ["http", "register-http", "register-filesystem", "wasmtime-default-features"]
+register-http = ["ureq"]           # enables wasm to be downloaded using http
+register-filesystem = []           # enables wasm to be loaded from disk
+http = ["ureq"]                    # enables extism_http_request
+wasmtime-exceptions = []           # enables exception-handling proposal in wasmtime (requires wasmtime gc feature)
+wasmtime-default-features = [
+  'wasmtime/default',
+]
+
 
 [build-dependencies]
 cbindgen = { version = "0.29", default-features = false }
 
 [dev-dependencies]
-criterion = "0.6.0"
+criterion = "0.7.0"
 quickcheck = "1"
 rand = "0.9.0"
 

runtime/README.md

@@ -24,7 +24,7 @@ There are a few environment variables that can be used for debugging purposes:
 - `EXTISM_COREDUMP=extism.core`: write [coredump](https://github.com/WebAssembly/tool-conventions/blob/main/Coredump.md) to a file when a WebAssembly function traps
 - `EXTISM_DEBUG=1`: generate debug information
 - `EXTISM_PROFILE=perf|jitdump|vtune`: enable Wasmtime profiling
-- `EXTISM_CACHE_CONFIG=path/to/config.toml`: enable Wasmtime cache, see [the docs](https://docs.wasmtime.dev/cli-cache.html) for details about configuration. Setting this to an empty string will disable caching.
+- `EXTISM_CACHE_CONFIG=path/to/config.toml`: enable Wasmtime cache, details [here](#wasmtime-caching)
 
 > *Note*: The debug and coredump info will only be written if the plug-in has an error.
 
@@ -112,7 +112,8 @@ let mut plugin = Plugin::new(&manifest, [], true);
 let res = plugin.call::<&str, &str>("count_vowels", "Yellow, world!").unwrap();
 println!("{}", res);
 # => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}
-let mut plugin = Plugin::new(&manifest, [], true).with_config_key("vowels", "aeiouyAEIOUY");
+let manifest = Manifest::new([url]).with_config_key("vowels", "aeiouyAEIOUY");
+let mut plugin = Plugin::new(&manifest, [], true).unwrap();
 let res = plugin.call::<&str, &str>("count_vowels", "Yellow, world!").unwrap();
 println!("{}", res);
 # => {"count": 4, "total": 4, "vowels": "aeiouyAEIOUY"}
@@ -229,3 +230,42 @@ Inside your host application, the rust-sdk emits these as [tracing](https://gith
 tracing_subscriber::fmt::init();
 ```
 
+### Wasmtime Caching
+
+To enable or disable caching for plugin compilation, you need to provide a configuration file that will be used by the [wasmtime crate](https://github.com/bytecodealliance/wasmtime).
+
+For more information and values that can be used for configuring caching, take a look at [the docs](https://docs.wasmtime.dev/cli-cache.html).
+
+> *Note*: As of now extism uses wasmtime [`version = ">= 27.0.0, < 31.0.0"`](https://github.com/extism/extism/blob/v1.11.1/runtime/Cargo.toml#L12), but the `enabled` key requirement [was removed](https://github.com/bytecodealliance/wasmtime/pull/10859) from `wasmtime` and its documentation, this could explain the `failed to parse config file` error you might encounter without it.
+
+An example configuration for caching would be:
+
+```toml
+[cache]
+enabled = true # This value is required
+directory = "/some/path"
+```
+
+You can :
+- [Create a global `wasmtime` configuration file](#using-a-configuration-file) in `$HOME/.config/wasmtime/config.toml`.
+- [Set the `EXTISM_CACHE_CONFIG` environment variable](#using-an-environment-variable)
+- [Set the configuration file path using `PluginBuilder`](#using-pluginbuilder)
+
+#### Using a configuration file
+
+The [wasmtime](https://github.com/bytecodealliance/wasmtime) crate, by default, will look for a configuration file in your systems' default configuration directory (for example on UNIX systems: `$HOME/.config/wasmtime/config.toml`),
+for more [information on this behaviour](`https://docs.rs/wasmtime/31.0.0/wasmtime/struct.Config.html#method.cache_config_load_default`).
+
+#### Using an environment variable
+
+You can set the `EXTISM_CACHE_CONFIG=path/to/config.toml` environment variable to set the path of the configuration file used by [wasmtime](https://github.com/bytecodealliance/wasmtime).
+Setting the variable to an empty string will disable caching (it won't load any configuration file).
+
+> *Note*: If the environment variable is not set, `wasmtime` will still try to read from a configuration file that may exist in your system's default configuration folder (e.g. `$HOME/.config/wasmtime/config.toml`).
+
+The environment variable does not override the path you might have set using `PluginBuilder`. will only be checked for if you did not specify a cache configuration path in `PluginBuilder`.
+
+#### Using PluginBuilder
+
+If you use a [PluginBuilder](https://docs.rs/extism/latest/extism/struct.PluginBuilder.html), you can set the `wasmtime` configuration path using the [with_cache_config](https://docs.rs/extism/latest/extism/struct.PluginBuilder.html#method.with_cache_config) method.
+This will override the `EXTISM_CACHE_CONFIG` environment variable if it's set, so you could have a "global" and per plugin configuration if needed.

runtime/examples/fs.rs

@@ -16,7 +16,7 @@ fn main() {
 
     let res = plugin.call::<&str, &str>("try_read", "").unwrap();
 
-    println!("{:?}", res);
+    println!("{res:?}");
 
     println!("-----------------------------------------------------");
 
@@ -30,7 +30,7 @@ fn main() {
     );
     let res2 = plugin.call::<&str, &str>("try_write", &line).unwrap();
 
-    println!("{:?}", res2);
+    println!("{res2:?}");
 
     println!("done!");
 }

runtime/examples/linking.rs

@@ -30,6 +30,6 @@ fn main() {
         let res = plugin
             .call::<&str, &str>("reflect", "Hello, world!")
             .unwrap();
-        println!("{}", res);
+        println!("{res}");
     }
 }

runtime/examples/log_callback.rs

@@ -25,6 +25,6 @@ fn main() {
     println!("Dumping logs");
 
     for line in LOGS.lock().unwrap().iter() {
-        print!("{}", line);
+        print!("{line}");
     }
 }

runtime/examples/readme.rs

@@ -52,6 +52,6 @@ fn main() {
         let res = plugin
             .call::<&str, &str>("count_vowels", "Hello, world!")
             .unwrap();
-        println!("{}", res);
+        println!("{res}");
     }
 }

runtime/extism.h

@@ -191,6 +191,17 @@ ExtismCompiledPlugin *extism_compiled_plugin_new(const uint8_t *wasm,
                                                  bool with_wasi,
                                                  char **errmsg);
 
+/**
+ * Pre-compile an Extism plugin and set the number of instructions a plugin is allowed to execute
+ */
+ExtismCompiledPlugin *extism_compiled_plugin_new_with_fuel_limit(const uint8_t *wasm,
+                                                                 ExtismSize wasm_size,
+                                                                 const ExtismFunction **functions,
+                                                                 ExtismSize n_functions,
+                                                                 bool with_wasi,
+                                                                 uint64_t fuel_limit,
+                                                                 char **errmsg);
+
 /**
  * Free `ExtismCompiledPlugin`
  */

runtime/src/lib.rs

@@ -29,6 +29,7 @@ pub(crate) mod manifest;
 pub(crate) mod pdk;
 mod plugin;
 mod plugin_builder;
+mod pool;
 mod readonly_dir;
 mod timer;
 
@@ -43,6 +44,7 @@ pub use plugin::{
     CancelHandle, CompiledPlugin, Plugin, WasmInput, EXTISM_ENV_MODULE, EXTISM_USER_MODULE,
 };
 pub use plugin_builder::{DebugOptions, PluginBuilder};
+pub use pool::{Pool, PoolBuilder, PoolPlugin};
 
 pub(crate) use internal::{Internal, Wasi};
 pub(crate) use timer::{Timer, TimerAction};
@@ -96,7 +98,7 @@ pub fn set_log_callback<F: 'static + Clone + Fn(&str)>(
         let x = tracing_subscriber::EnvFilter::builder()
             .with_default_directive(tracing::Level::ERROR.into());
         if is_level {
-            x.parse_lossy(format!("extism={}", filter))
+            x.parse_lossy(format!("extism={filter}"))
         } else {
             x.parse_lossy(filter)
         }

runtime/src/manifest.rs

@@ -10,7 +10,7 @@ use crate::*;
 fn hex(data: &[u8]) -> String {
     let mut s = String::new();
     for &byte in data {
-        write!(&mut s, "{:02x}", byte).unwrap();
+        write!(&mut s, "{byte:02x}").unwrap();
     }
     s
 }

runtime/src/pdk.rs

@@ -258,12 +258,16 @@ pub(crate) fn http_request(
             };
             let buf: &[u8] = data.memory_bytes(handle)?;
             let agent = ureq::agent();
-            let config = agent.configure_request(r.body(buf)?);
+            let config = agent
+                .configure_request(r.body(buf)?)
+                .http_status_as_error(false);
             let req = config.timeout_global(timeout).build();
             ureq::run(req)
         } else {
             let agent = ureq::agent();
-            let config = agent.configure_request(r.body(())?);
+            let config = agent
+                .configure_request(r.body(())?)
+                .http_status_as_error(false);
             let req = config.timeout_global(timeout).build();
             ureq::run(req)
         };

runtime/src/plugin.rs

@@ -1,6 +1,7 @@
 use std::{
     any::Any,
     collections::{BTreeMap, BTreeSet},
+    path::PathBuf,
     sync::TryLockError,
 };
 
@@ -60,26 +61,16 @@ impl CompiledPlugin {
             .wasm_tail_call(true)
             .wasm_function_references(true)
             .wasm_gc(true);
+        #[cfg(feature = "wasmtime-exceptions")]
+        {
+            config.wasm_exceptions(true);
+        }
 
         if builder.options.fuel.is_some() {
             config.consume_fuel(true);
         }
 
-        match &builder.options.cache_config {
-            Some(None) => (),
-            Some(Some(path)) => {
-                config.cache_config_load(path)?;
-            }
-            None => {
-                if let Ok(env) = std::env::var("EXTISM_CACHE_CONFIG") {
-                    if !env.is_empty() {
-                        config.cache_config_load(&env)?;
-                    }
-                } else {
-                    config.cache_config_load_default()?;
-                }
-            }
-        }
+        config.cache(Self::configure_cache(&builder.options.cache_config)?);
 
         let engine = Engine::new(&config)?;
 
@@ -97,6 +88,43 @@ impl CompiledPlugin {
             engine,
         })
     }
+
+    /// Return optional cache according to builder options.
+    fn configure_cache(
+        cache_opt: &Option<Option<std::path::PathBuf>>,
+    ) -> Result<Option<wasmtime::Cache>, Error> {
+        match cache_opt {
+            // Explicitly disabled
+            Some(None) => Ok(None),
+
+            // Explicit path
+            Some(Some(p)) => {
+                let cache = wasmtime::Cache::from_file(Some(p.as_path()))?;
+                Ok(Some(cache))
+            }
+
+            // Unspecified, try environment, then system fallback
+            None => {
+                match std::env::var_os("EXTISM_CACHE_CONFIG") {
+                    Some(val) => {
+                        if val.is_empty() {
+                            // Disable cache if env var exists but is empty
+                            Ok(None)
+                        } else {
+                            let p = PathBuf::from(val);
+                            let cache = wasmtime::Cache::from_file(Some(p.as_path()))?;
+                            Ok(Some(cache))
+                        }
+                    }
+                    None => {
+                        // load cache configuration from the system default path
+                        let cache = wasmtime::Cache::from_file(None)?;
+                        Ok(Some(cache))
+                    }
+                }
+            }
+        }
+    }
 }
 
 /// Plugin contains everything needed to execute a WASM function
@@ -191,6 +219,7 @@ pub(crate) fn profiling_strategy() -> ProfilingStrategy {
 /// Defines an input type for Wasm data.
 ///
 /// Types that implement `Into<WasmInput>` can be passed directly into `Plugin::new`
+#[derive(Clone)]
 pub enum WasmInput<'a> {
     /// Raw Wasm module
     Data(std::borrow::Cow<'a, [u8]>),
@@ -343,6 +372,7 @@ fn relink(
                 let (kind, ty) = match import.ty() {
                     ExternType::Func(t) => ("function", t.to_string()),
                     ExternType::Global(t) => ("global", t.content().to_string()),
+                    ExternType::Tag(t) => ("tag", t.ty().to_string()),
                     ExternType::Table(t) => ("table", t.element().to_string()),
                     ExternType::Memory(_) => ("memory", String::new()),
                 };
@@ -962,8 +992,7 @@ impl Plugin {
                     }
                     Err(msg) => {
                         res = Err(Error::msg(format!(
-                            "unable to load error message from memory: {}",
-                            msg,
+                            "unable to load error message from memory: {msg}",
                         )));
                     }
                 }

runtime/src/plugin_builder.rs

@@ -33,6 +33,7 @@ impl Default for DebugOptions {
 }
 
 /// PluginBuilder is used to configure and create `Plugin` instances
+#[derive(Clone)]
 pub struct PluginBuilder<'a> {
     pub(crate) source: WasmInput<'a>,
     pub(crate) config: Option<wasmtime::Config>,

runtime/src/pool.rs

@@ -0,0 +1,195 @@
+use crate::{Error, FromBytesOwned, Plugin, ToBytes};
+use std::collections::HashMap;
+use std::sync::Arc;
+use std::sync::RwLock;
+
+// `PoolBuilder` is used to configure and create `Pool`s
+#[derive(Debug, Clone)]
+pub struct PoolBuilder {
+    /// Max number of concurrent instances for a plugin - by default this is set to
+    /// the output of `std::thread::available_parallelism`
+    pub max_instances: usize,
+}
+
+impl PoolBuilder {
+    /// Create a `PoolBuilder` with default values
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Set the max number of parallel instances
+    pub fn with_max_instances(mut self, n: usize) -> Self {
+        self.max_instances = n;
+        self
+    }
+
+    /// Create a new `Pool` with the given configuration
+    pub fn build<F: 'static + Fn() -> Result<Plugin, Error>>(self, source: F) -> Pool {
+        Pool::new_from_builder(source, self)
+    }
+}
+
+impl Default for PoolBuilder {
+    fn default() -> Self {
+        PoolBuilder {
+            max_instances: std::thread::available_parallelism()
+                .expect("available parallelism")
+                .into(),
+        }
+    }
+}
+
+/// `PoolPlugin` is used by the pool to track the number of live instances of a particular plugin
+#[derive(Clone, Debug)]
+pub struct PoolPlugin(std::rc::Rc<std::cell::RefCell<Plugin>>);
+
+impl PoolPlugin {
+    fn new(plugin: Plugin) -> Self {
+        Self(std::rc::Rc::new(std::cell::RefCell::new(plugin)))
+    }
+
+    /// Access the underlying plugin
+    pub fn plugin(&self) -> std::cell::RefMut<'_, Plugin> {
+        self.0.borrow_mut()
+    }
+
+    /// Helper to call a plugin function on the underlying plugin
+    pub fn call<'a, Input: ToBytes<'a>, Output: FromBytesOwned>(
+        &self,
+        name: impl AsRef<str>,
+        input: Input,
+    ) -> Result<Output, Error> {
+        self.plugin().call(name.as_ref(), input)
+    }
+
+    /// Helper to get the underlying plugin's ID
+    pub fn id(&self) -> uuid::Uuid {
+        self.plugin().id
+    }
+}
+
+type PluginSource = dyn Fn() -> Result<Plugin, Error>;
+
+struct PoolInner {
+    plugin_source: Box<PluginSource>,
+    instances: Vec<PoolPlugin>,
+}
+
+unsafe impl Send for PoolInner {}
+unsafe impl Sync for PoolInner {}
+
+/// `Pool` manages threadsafe access to a limited number of instances of multiple plugins
+#[derive(Clone)]
+pub struct Pool {
+    config: PoolBuilder,
+    inner: Arc<std::sync::Mutex<PoolInner>>,
+    existing_functions: Arc<RwLock<HashMap<String, bool>>>,
+}
+
+unsafe impl Send for Pool {}
+unsafe impl Sync for Pool {}
+
+impl Pool {
+    /// Create a new pool with the default configuration
+    pub fn new<F: 'static + Fn() -> Result<Plugin, Error>>(source: F) -> Self {
+        Self::new_from_builder(Box::new(source), PoolBuilder::default())
+    }
+
+    /// Create a new pool configured using a `PoolBuilder`
+    pub fn new_from_builder<F: 'static + Fn() -> Result<Plugin, Error>>(
+        source: F,
+        builder: PoolBuilder,
+    ) -> Self {
+        Pool {
+            config: builder,
+            inner: Arc::new(std::sync::Mutex::new(PoolInner {
+                plugin_source: Box::new(source),
+                instances: Default::default(),
+            })),
+            existing_functions: RwLock::new(HashMap::default()).into(),
+        }
+    }
+
+    fn find_available(&self) -> Result<Option<PoolPlugin>, Error> {
+        let pool = self.inner.lock().unwrap();
+        for instance in pool.instances.iter() {
+            if std::rc::Rc::strong_count(&instance.0) == 1 {
+                return Ok(Some(instance.clone()));
+            }
+        }
+        Ok(None)
+    }
+
+    /// Get the number of live instances for a plugin
+    pub fn count(&self) -> usize {
+        self.inner.lock().unwrap().instances.len()
+    }
+
+    /// Get access to a plugin, this will create a new instance if needed (and allowed by the specified
+    /// max_instances). `Ok(None)` is returned if the timeout is reached before an available plugin could be
+    /// acquired
+    pub fn get(&self, timeout: std::time::Duration) -> Result<Option<PoolPlugin>, Error> {
+        let start = std::time::Instant::now();
+        let max = self.config.max_instances;
+        if let Some(avail) = self.find_available()? {
+            return Ok(Some(avail));
+        }
+
+        {
+            let mut pool = self.inner.lock().unwrap();
+            if pool.instances.len() < max {
+                let plugin = (*pool.plugin_source)()?;
+                let instance = PoolPlugin::new(plugin);
+                pool.instances.push(instance);
+                return Ok(Some(pool.instances.last().unwrap().clone()));
+            }
+        }
+
+        loop {
+            if let Ok(Some(x)) = self.find_available() {
+                return Ok(Some(x));
+            }
+            if std::time::Instant::now() - start > timeout {
+                return Ok(None);
+            }
+
+            std::thread::sleep(std::time::Duration::from_millis(100));
+        }
+    }
+
+    /// Access a plugin in a callback function. This calls `Pool::get` then the provided
+    /// callback. `Ok(None)` is returned if the timeout is reached before an available
+    /// plugin could be acquired
+    pub fn with_plugin<T>(
+        &self,
+        timeout: std::time::Duration,
+        f: impl FnOnce(&mut Plugin) -> Result<T, Error>,
+    ) -> Result<Option<T>, Error> {
+        if let Some(plugin) = self.get(timeout)? {
+            return f(&mut plugin.plugin()).map(Some);
+        }
+        Ok(None)
+    }
+
+    /// Returns `true` if the given function exists, otherwise `false`. Results are cached
+    /// after the first call.
+    pub fn function_exists(&self, name: &str, timeout: std::time::Duration) -> Result<bool, Error> {
+        // read current value if any
+        let read = self.existing_functions.read().unwrap();
+        let exists_opt = read.get(name).cloned();
+        drop(read);
+        if let Some(exists) = exists_opt {
+            Ok(exists)
+        } else {
+            // load plugin and call function_exists
+            let plugin = self.get(timeout)?;
+            let exists = plugin.unwrap().0.borrow().function_exists(name);
+
+            // write result to hashmap
+            let mut write = self.existing_functions.write().unwrap();
+            write.insert(name.to_string(), exists);
+
+            Ok(exists)
+        }
+    }
+}

runtime/src/sdk.rs

@@ -363,6 +363,70 @@ pub unsafe extern "C" fn extism_compiled_plugin_new(
         })
 }
 
+/// Pre-compile an Extism plugin and set the number of instructions a plugin is allowed to execute
+#[no_mangle]
+pub unsafe extern "C" fn extism_compiled_plugin_new_with_fuel_limit(
+    wasm: *const u8,
+    wasm_size: Size,
+    functions: *mut *const ExtismFunction,
+    n_functions: Size,
+    with_wasi: bool,
+    fuel_limit: u64,
+    errmsg: *mut *mut std::ffi::c_char,
+) -> *mut CompiledPlugin {
+    trace!("Call to extism_plugin_new with wasm pointer {:?}", wasm);
+    let data = std::slice::from_raw_parts(wasm, wasm_size as usize);
+
+    let mut builder = PluginBuilder::new(data)
+        .with_wasi(with_wasi)
+        .with_fuel_limit(fuel_limit);
+
+    if !functions.is_null() {
+        let funcs = (0..n_functions)
+            .map(|i| unsafe { *functions.add(i as usize) })
+            .map(|ptr| {
+                if ptr.is_null() {
+                    return Err("Cannot pass null pointer");
+                }
+
+                let ExtismFunction(func) = &*ptr;
+                let Some(func) = func.take() else {
+                    return Err("Function cannot be registered with multiple different Plugins");
+                };
+
+                Ok(func)
+            })
+            .collect::<Result<Vec<_>, _>>()
+            .unwrap_or_else(|e| {
+                if !errmsg.is_null() {
+                    let e = std::ffi::CString::new(e.to_string()).unwrap();
+                    *errmsg = e.into_raw();
+                }
+                Vec::new()
+            });
+
+        if funcs.len() != n_functions as usize {
+            return std::ptr::null_mut();
+        }
+
+        builder = builder.with_functions(funcs);
+    }
+
+    CompiledPlugin::new(builder)
+        .map(|v| Box::into_raw(Box::new(v)))
+        .unwrap_or_else(|e| {
+            if !errmsg.is_null() {
+                let e = std::ffi::CString::new(format!(
+                    "Unable to compile Extism plugin: {}",
+                    e.root_cause(),
+                ))
+                .unwrap();
+                *errmsg = e.into_raw();
+            }
+            std::ptr::null_mut()
+        })
+}
+
 /// Free `ExtismCompiledPlugin`
 #[no_mangle]
 pub unsafe extern "C" fn extism_compiled_plugin_free(plugin: *mut CompiledPlugin) {
@@ -871,7 +935,7 @@ fn set_log_file(log_file: impl Into<std::path::PathBuf>, filter: &str) -> Result
         let x = tracing_subscriber::EnvFilter::builder()
             .with_default_directive(tracing::Level::ERROR.into());
         if is_level {
-            x.parse_lossy(format!("extism={}", filter))
+            x.parse_lossy(format!("extism={filter}"))
         } else {
             x.parse_lossy(filter)
         }
@@ -926,7 +990,7 @@ unsafe fn set_log_buffer(filter: &str) -> Result<(), Error> {
         let x = tracing_subscriber::EnvFilter::builder()
             .with_default_directive(tracing::Level::ERROR.into());
         if is_level {
-            x.parse_lossy(format!("extism={}", filter))
+            x.parse_lossy(format!("extism={filter}"))
         } else {
             x.parse_lossy(filter)
         }

runtime/src/tests/issues.rs

@@ -16,7 +16,7 @@ fn test_issue_620() {
     // Call test method, this does not work
     let p = plugin.call::<(), String>("test", ()).unwrap();
 
-    println!("{}", p);
+    println!("{p}");
 }
 
 // https://github.com/extism/extism/issues/619
@@ -53,5 +53,5 @@ fn test_issue_775() {
         Ok(code) => Err(code),
     }
     .unwrap();
-    println!("{}", p);
+    println!("{p}");
 }

runtime/src/tests/mod.rs

@@ -1,3 +1,4 @@
 mod issues;
 mod kernel;
+mod pool;
 mod runtime;

runtime/src/tests/pool.rs

@@ -0,0 +1,63 @@
+use crate::*;
+use std::time::Duration;
+
+fn run_thread(p: Pool, i: u64) -> std::thread::JoinHandle<()> {
+    std::thread::spawn(move || {
+        std::thread::sleep(Duration::from_millis(i));
+        let s: String = p
+            .get(Duration::from_secs(1))
+            .unwrap()
+            .unwrap()
+            .call("count_vowels", "abc")
+            .unwrap();
+        println!("{s}");
+    })
+}
+
+fn init(max_instances: usize) -> Pool {
+    let data = include_bytes!("../../../wasm/code.wasm");
+    let plugin_builder =
+        extism::PluginBuilder::new(extism::Manifest::new([extism::Wasm::data(data)]))
+            .with_wasi(true);
+    PoolBuilder::new()
+        .with_max_instances(max_instances)
+        .build(move || plugin_builder.clone().build())
+}
+
+#[test]
+fn test_threads() {
+    for i in 1..=3 {
+        let pool = init(i);
+        let threads = vec![
+            run_thread(pool.clone(), 1000),
+            run_thread(pool.clone(), 1000),
+            run_thread(pool.clone(), 1000),
+            run_thread(pool.clone(), 1000),
+            run_thread(pool.clone(), 1000),
+            run_thread(pool.clone(), 1000),
+            run_thread(pool.clone(), 500),
+            run_thread(pool.clone(), 500),
+            run_thread(pool.clone(), 500),
+            run_thread(pool.clone(), 500),
+            run_thread(pool.clone(), 500),
+            run_thread(pool.clone(), 0),
+        ];
+
+        for t in threads {
+            t.join().unwrap();
+        }
+
+        assert!(pool.count() <= i);
+    }
+}
+
+#[test]
+fn test_exists() -> Result<(), Error> {
+    let pool = init(1);
+    let timeout = Duration::from_secs(1);
+    assert!(pool.function_exists("count_vowels", timeout)?);
+    assert!(pool.function_exists("count_vowels", timeout)?);
+    assert!(!pool.function_exists("not_existing", timeout)?);
+    assert!(!pool.function_exists("not_existing", timeout)?);
+    Ok(())
+}

runtime/src/tests/runtime.rs

@@ -133,10 +133,7 @@ fn it_works() {
         .unwrap();
     let native_avg: std::time::Duration = native_sum / native_num_tests as u32;
 
-    println!(
-        "native function call (avg, N = {}): {:?}",
-        native_num_tests, native_avg
-    );
+    println!("native function call (avg, N = {native_num_tests}): {native_avg:?}");
 
     let num_tests = test_times.len();
     let sum: std::time::Duration = test_times
@@ -145,7 +142,7 @@ fn it_works() {
         .unwrap();
     let avg: std::time::Duration = sum / num_tests as u32;
 
-    println!("wasm function call (avg, N = {}): {:?}", num_tests, avg);
+    println!("wasm function call (avg, N = {num_tests}): {avg:?}");
 
     // Check that log file was written to
     if log {
@@ -212,7 +209,7 @@ fn test_cancel() {
         let _output: Result<&[u8], Error> = plugin.call("loop_forever", "abc123");
         let end = std::time::Instant::now();
         let time = end - start;
-        println!("Cancelled plugin ran for {:?}", time);
+        println!("Cancelled plugin ran for {time:?}");
     }
 }
 
@@ -271,7 +268,7 @@ fn test_fuel_consumption() {
     assert!(output.is_err());
 
     let fuel_consumed = plugin.fuel_consumed().unwrap();
-    println!("Fuel consumed: {}", fuel_consumed);
+    println!("Fuel consumed: {fuel_consumed}");
     assert!(fuel_consumed > 0);
 }
 
@@ -440,7 +437,7 @@ fn test_memory_max() {
     assert!(output.is_err());
 
     let err = output.unwrap_err().root_cause().to_string();
-    println!("{:?}", err);
+    println!("{err:?}");
     assert_eq!(err, "oom");
 
     // Should pass with memory.max set to a large enough number
@@ -503,7 +500,7 @@ fn test_extism_error() {
     let mut plugin = Plugin::new(&manifest, [f], true).unwrap();
     let output: Result<String, Error> = plugin.call("count_vowels", "a".repeat(1024));
     assert!(output.is_err());
-    println!("{:?}", output);
+    println!("{output:?}");
     assert_eq!(output.unwrap_err().root_cause().to_string(), "TEST");
 }
 
@@ -823,7 +820,7 @@ fn test_http_response_headers() {
     .unwrap();
     let req = HttpRequest::new("https://extism.org");
     let Json(res): Json<HashMap<String, String>> = plugin.call("http_get", Json(req)).unwrap();
-    println!("{:?}", res);
+    println!("{res:?}");
     assert_eq!(res["content-type"], "text/html; charset=utf-8");
 }
 
@@ -838,6 +835,6 @@ fn test_http_response_headers_disabled() {
     .unwrap();
     let req = HttpRequest::new("https://extism.org");
     let Json(res): Json<HashMap<String, String>> = plugin.call("http_get", Json(req)).unwrap();
-    println!("{:?}", res);
+    println!("{res:?}");
     assert!(res.is_empty());
 }