Add a wasmtime::Linker type (#1384)

* Add a `wasmtime::Linker` type

This commit adds a new type to the `wasmtime` crate, a `Linker`. This
linker is intended to vastly simplify calling `Instance::new` by easily
performing name resolution and incrementally defining state over time.
The goal here is to start down a path of making linking wasm modules in
`wasmtime` a first-class and ergonomic operation. This is highly likely
to evolve over time and get tweaked through releases as we iterate
towards a design well-suited for `wasmtime`, but this is intended to at
least be the initial foundation for such functionality.

This commit additionally also adds a C API for the linker and switches
the existing linking examples to using this linker in both Rust and C.

One piece of future work I'd like to tackle next is to integrate WASI
into the `wasmtime` crate in a more first-class manner. This [`Linker`]
type provides a great location to hook into the instantiation process to
easily instantiate modules with WASI imports. That's a relatively large
refactoring for now though and I figured it'd be best left for a
different time.

Closes #727

crates/api/src/lib.rs

@@ -12,6 +12,7 @@ mod externals;
 mod frame_info;
 mod func;
 mod instance;
+mod linker;
 mod module;
 mod r#ref;
 mod runtime;
@@ -24,6 +25,7 @@ pub use crate::externals::*;
 pub use crate::frame_info::FrameInfo;
 pub use crate::func::*;
 pub use crate::instance::Instance;
+pub use crate::linker::*;
 pub use crate::module::Module;
 pub use crate::r#ref::{AnyRef, HostInfo, HostRef};
 pub use crate::runtime::*;

crates/api/src/linker.rs

@@ -0,0 +1,368 @@
+use crate::{
+    Extern, ExternType, Func, FuncType, GlobalType, ImportType, Instance, IntoFunc, Module, Store,
+};
+use anyhow::{bail, Result};
+use std::collections::hash_map::{Entry, HashMap};
+use std::rc::Rc;
+
+/// Structure used to link wasm modules/instances together.
+///
+/// This structure is used to assist in instantiating a [`Module`]. A `Linker`
+/// is a way of performing name resolution to make instantiating a module easier
+/// (as opposed to calling [`Instance::new`]). `Linker` is a name-based resolver
+/// where names are dynamically defined and then used to instantiate a
+/// [`Module`]. The goal of a `Linker` is to have a one-argument method,
+/// [`Linker::instantiate`], which takes a [`Module`] and produces an
+/// [`Instance`].  This method will automatically select all the right imports
+/// for the [`Module`] to be instantiated, and will otherwise return an error
+/// if an import isn't satisfied.
+///
+/// ## Name Resolution
+///
+/// As mentioned previously, `Linker` is a form of name resolver. It will be
+/// using the string-based names of imports on a module to attempt to select a
+/// matching item to hook up to it. This name resolution has two-levels of
+/// namespaces, a module level and a name level. Each item is defined within a
+/// module and then has its own name. This basically follows the wasm standard
+/// for modularization.
+///
+/// Names in a `Linker` can be defined twice, but only for different signatures
+/// of items. This means that every item defined in a `Linker` has a unique
+/// name/type pair. For example you can define two functions with the module
+/// name `foo` and item name `bar`, so long as they have different function
+/// signatures. Currently duplicate memories and tables are not allowed, only
+/// one-per-name is allowed.
+pub struct Linker {
+    store: Store,
+    string2idx: HashMap<Rc<str>, usize>,
+    strings: Vec<Rc<str>>,
+    map: HashMap<ImportKey, Extern>,
+}
+
+#[derive(Hash, PartialEq, Eq)]
+struct ImportKey {
+    name: usize,
+    module: usize,
+    kind: ImportKind,
+}
+
+#[derive(Hash, PartialEq, Eq, Debug)]
+enum ImportKind {
+    Func(FuncType),
+    Global(GlobalType),
+    Memory,
+    Table,
+}
+
+impl Linker {
+    /// Creates a new [`Linker`].
+    ///
+    /// This function will create a new [`Linker`] which is ready to start
+    /// linking modules. All items defined in this linker and produced by this
+    /// linker will be connected with `store` and must come from the same
+    /// `store`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use wasmtime::{Linker, Store};
+    ///
+    /// let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// // ...
+    /// ```
+    pub fn new(store: &Store) -> Linker {
+        Linker {
+            store: store.clone(),
+            map: HashMap::new(),
+            string2idx: HashMap::new(),
+            strings: Vec::new(),
+        }
+    }
+
+    /// Defines a new item in this [`Linker`].
+    ///
+    /// This method will add a new definition, by name, to this instance of
+    /// [`Linker`]. The `module` and `name` provided are what to name the
+    /// `item`.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the `module` and `name` already identify an item
+    /// of the same type as the `item` provided. For more information see the
+    /// documentation on [`Linker`].
+    ///
+    /// Also returns an error if `item` comes from a different store than this
+    /// [`Linker`] was created with.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// let ty = GlobalType::new(ValType::I32, Mutability::Const);
+    /// let global = Global::new(&store, ty, Val::I32(0x1234))?;
+    /// linker.define("host", "offset", global);
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "offset" (global i32))
+    ///         (memory 1)
+    ///         (data (global.get 0) "foo")
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn define(
+        &mut self,
+        module: &str,
+        name: &str,
+        item: impl Into<Extern>,
+    ) -> Result<&mut Self> {
+        self._define(module, name, item.into())
+    }
+
+    fn _define(&mut self, module: &str, name: &str, item: Extern) -> Result<&mut Self> {
+        if !item.comes_from_same_store(&self.store) {
+            bail!("all linker items must be from the same store");
+        }
+        self.insert(module, name, &item.ty(), item)?;
+        Ok(self)
+    }
+
+    /// Convenience wrapper to define a function import.
+    ///
+    /// This method is a convenience wrapper around [`Linker::define`] which
+    /// internally delegates to [`Func::wrap`].
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the `module` and `name` already identify a function
+    /// of the same signature as `func`. For more information see the
+    /// documentation on [`Linker`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// linker.func("host", "double", |x: i32| x * 2)?;
+    /// linker.func("host", "log_i32", |x: i32| println!("{}", x))?;
+    /// linker.func("host", "log_str", |caller: Caller, ptr: i32, len: i32| {
+    ///     // ...
+    /// })?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "double" (func (param i32) (result i32)))
+    ///         (import "host" "log_i32" (func (param i32)))
+    ///         (import "host" "log_str" (func (param i32 i32)))
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn func<Params, Args>(
+        &mut self,
+        module: &str,
+        name: &str,
+        func: impl IntoFunc<Params, Args>,
+    ) -> Result<&mut Self> {
+        self._define(module, name, Func::wrap(&self.store, func).into())
+    }
+
+    /// Convenience wrapper to define an entire [`Instance`] in this linker.
+    ///
+    /// This function is a convenience wrapper around [`Linker::define`] which
+    /// will define all exports on `instance` into this linker. The module name
+    /// for each export is `module_name`, and the name for each export is the
+    /// name in the instance itself.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the any item is redefined twice in this linker (for
+    /// example the same `module_name` was already defined), or if `instance`
+    /// comes from a different [`Store`] than this [`Linker`] originally was
+    /// created with.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    ///
+    /// // Instantiate a small instance...
+    /// let wat = r#"(module (func (export "run") ))"#;
+    /// let module = Module::new(&store, wat)?;
+    /// let instance = linker.instantiate(&module)?;
+    ///
+    /// // ... and inform the linker that the name of this instance is
+    /// // `instance1`. This defines the `instance1::run` name for our next
+    /// // module to use.
+    /// linker.instance("instance1", &instance)?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "instance1" "run" (func $instance1_run))
+    ///         (func (export "run")
+    ///             call $instance1_run
+    ///         )
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// let instance = linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn instance(&mut self, module_name: &str, instance: &Instance) -> Result<&mut Self> {
+        if !Store::same(&self.store, instance.store()) {
+            bail!("all linker items must be from the same store");
+        }
+        for (export, item) in instance.module().exports().iter().zip(instance.exports()) {
+            self.insert(module_name, export.name(), export.ty(), item.clone())?;
+        }
+        Ok(self)
+    }
+
+    fn insert(&mut self, module: &str, name: &str, ty: &ExternType, item: Extern) -> Result<()> {
+        let key = self.import_key(module, name, ty);
+        match self.map.entry(key) {
+            Entry::Occupied(o) => bail!(
+                "import of `{}::{}` with as {:?} defined twice",
+                module,
+                name,
+                o.key().kind,
+            ),
+            Entry::Vacant(v) => {
+                v.insert(item);
+            }
+        }
+        Ok(())
+    }
+
+    fn import_key(&mut self, module: &str, name: &str, ty: &ExternType) -> ImportKey {
+        ImportKey {
+            module: self.intern_str(module),
+            name: self.intern_str(name),
+            kind: self.import_kind(ty),
+        }
+    }
+
+    fn import_kind(&self, ty: &ExternType) -> ImportKind {
+        match ty {
+            ExternType::Func(f) => ImportKind::Func(f.clone()),
+            ExternType::Global(f) => ImportKind::Global(f.clone()),
+            ExternType::Memory(_) => ImportKind::Memory,
+            ExternType::Table(_) => ImportKind::Table,
+        }
+    }
+
+    fn intern_str(&mut self, string: &str) -> usize {
+        if let Some(idx) = self.string2idx.get(string) {
+            return *idx;
+        }
+        let string: Rc<str> = string.into();
+        let idx = self.strings.len();
+        self.strings.push(string.clone());
+        self.string2idx.insert(string, idx);
+        idx
+    }
+
+    /// Attempts to instantiate the `module` provided.
+    ///
+    /// This method will attempt to assemble a list of imports that correspond
+    /// to the imports required by the [`Module`] provided. This list
+    /// of imports is then passed to [`Instance::new`] to continue the
+    /// instantiation process.
+    ///
+    /// Each import of `module` will be looked up in this [`Linker`] and must
+    /// have previously been defined. If it was previously defined with an
+    /// incorrect signature or if it was not prevoiusly defined then an error
+    /// will be returned because the import can not be satisfied.
+    ///
+    /// # Errors
+    ///
+    /// This method can fail because an import may not be found, or because
+    /// instantiation itself may fail. For information on instantiation
+    /// failures see [`Instance::new`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// linker.func("host", "double", |x: i32| x * 2)?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "double" (func (param i32) (result i32)))
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn instantiate(&self, module: &Module) -> Result<Instance> {
+        let mut imports = Vec::new();
+        for import in module.imports() {
+            if let Some(item) = self.import_get(import) {
+                imports.push(item.clone());
+                continue;
+            }
+
+            let mut options = String::new();
+            for i in self.map.keys() {
+                if &*self.strings[i.module] != import.module()
+                    || &*self.strings[i.name] != import.name()
+                {
+                    continue;
+                }
+                options.push_str("  * ");
+                options.push_str(&format!("{:?}", i.kind));
+                options.push_str("\n");
+            }
+            if options.len() == 0 {
+                bail!(
+                    "import of `{}::{}` has not been defined",
+                    import.module(),
+                    import.name()
+                )
+            }
+
+            bail!(
+                "failed to find import of `{}::{}` with matching signature\n\
+                 desired signature was: {:?}\n\
+                 signatures available:\n\n{}",
+                import.module(),
+                import.name(),
+                import.ty(),
+                options,
+            )
+        }
+
+        Instance::new(module, &imports)
+    }
+
+    fn import_get(&self, import: &ImportType) -> Option<&Extern> {
+        let key = ImportKey {
+            module: *self.string2idx.get(import.module())?,
+            name: *self.string2idx.get(import.name())?,
+            kind: self.import_kind(import.ty()),
+        };
+        self.map.get(&key)
+    }
+}