wazero/internal/wasm/call_context.go

package wasm

import (
	"context"
	"fmt"
	"sync/atomic"

	"github.com/tetratelabs/wazero/api"
	internalsys "github.com/tetratelabs/wazero/internal/sys"
	"github.com/tetratelabs/wazero/sys"
)

// compile time check to ensure CallContext implements api.Module
var _ api.Module = &CallContext{}

func NewCallContext(ns *Namespace, instance *ModuleInstance, sys *internalsys.Context) *CallContext {
	zero := uint64(0)
	return &CallContext{memory: instance.Memory, module: instance, ns: ns, Sys: sys, closed: &zero}
}

// CallContext is a function call context bound to a module. This is important as one module's functions can call
// imported functions, but all need to effect the same memory.
//
// Note: This does not include the context.Context because doing so risks caching the wrong context which can break
// functionality like trace propagation.
// Note: this also implements api.Module in order to simplify usage as a host function parameter.
type CallContext struct {
	// TODO: We've never found a great name for this. It is only used for function calls, hence CallContext, but it
	// moves on a different axis than, for example, the context.Context. context.Context is the same root for the whole
	// call stack, where the CallContext can change depending on where memory is defined and who defines the calling
	// function. When we rename this again, we should try to capture as many key points possible on the docs.

	module *ModuleInstance
	// memory is returned by Memory and overridden WithMemory
	memory api.Memory
	ns     *Namespace

	// Sys is exposed for use in special imports such as WASI, assemblyscript
	// and gojs.
	//
	// # Notes
	//
	//   - This is a part of CallContext so that scope and Close is coherent.
	//   - This is not exposed outside this repository (as a host function
	//	  parameter) because we haven't thought through capabilities based
	//	  security implications.
	Sys *internalsys.Context

	// closed is the pointer used both to guard moduleEngine.CloseWithExitCode and to store the exit code.
	//
	// The update value is 1 + exitCode << 32. This ensures an exit code of zero isn't mistaken for never closed.
	//
	// Note: Exclusively reading and updating this with atomics guarantees cross-goroutine observations.
	// See /RATIONALE.md
	closed *uint64

	// CodeCloser is non-nil when the code should be closed after this module.
	CodeCloser api.Closer
}

// FailIfClosed returns a sys.ExitError if CloseWithExitCode was called.
func (m *CallContext) FailIfClosed() error {
	if closed := atomic.LoadUint64(m.closed); closed != 0 {
		return sys.NewExitError(m.module.Name, uint32(closed>>32)) // Unpack the high order bits as the exit code.
	}
	return nil
}

// Name implements the same method as documented on api.Module
func (m *CallContext) Name() string {
	return m.module.Name
}

// WithMemory allows overriding memory without re-allocation when the result would be the same.
func (m *CallContext) WithMemory(memory *MemoryInstance) *CallContext {
	if memory != nil && memory != m.memory { // only re-allocate if it will change the effective memory
		return &CallContext{module: m.module, memory: memory, Sys: m.Sys, closed: m.closed}
	}
	return m
}

// String implements the same method as documented on api.Module
func (m *CallContext) String() string {
	return fmt.Sprintf("Module[%s]", m.Name())
}

// Close implements the same method as documented on api.Module.
func (m *CallContext) Close(ctx context.Context) (err error) {
	return m.CloseWithExitCode(ctx, 0)
}

// CloseWithExitCode implements the same method as documented on api.Module.
func (m *CallContext) CloseWithExitCode(ctx context.Context, exitCode uint32) error {
	closed, err := m.close(ctx, exitCode)
	if !closed {
		return nil
	}
	m.ns.deleteModule(m.Name())
	if m.CodeCloser == nil {
		return err
	}
	if e := m.CodeCloser.Close(ctx); e != nil && err == nil {
		err = e
	}
	return err
}

// close marks this CallContext as closed and releases underlying system resources.
//
// Note: The caller is responsible for removing the module from the Namespace.
func (m *CallContext) close(ctx context.Context, exitCode uint32) (c bool, err error) {
	closed := uint64(1) + uint64(exitCode)<<32 // Store exitCode as high-order bits.
	if !atomic.CompareAndSwapUint64(m.closed, 0, closed) {
		return false, nil
	}
	c = true
	if sysCtx := m.Sys; sysCtx != nil { // nil if from HostModuleBuilder
		err = sysCtx.FS(ctx).Close(ctx)
	}
	return
}

// Memory implements the same method as documented on api.Module.
func (m *CallContext) Memory() api.Memory {
	return m.module.Memory
}

// ExportedMemory implements the same method as documented on api.Module.
func (m *CallContext) ExportedMemory(name string) api.Memory {
	exp, err := m.module.getExport(name, ExternTypeMemory)
	if err != nil {
		return nil
	}
	return exp.Memory
}

// ExportedFunction implements the same method as documented on api.Module.
func (m *CallContext) ExportedFunction(name string) api.Function {
	exp, err := m.module.getExport(name, ExternTypeFunc)
	if err != nil {
		return nil
	}

	fi := exp.Function
	ce, err := exp.Function.Module.Engine.NewCallEngine(m, fi)
	if err != nil {
		return nil
	}

	if exp.Function.Module == m.module {
		return &function{fi: fi, ce: ce}
	} else {
		return &importedFn{importingModule: m, importedFn: fi, ce: ce}
	}
}

// function implements api.Function. This couples FunctionInstance with CallEngine so that
// it can be used to make function calls originating from the FunctionInstance.
type function struct {
	fi *FunctionInstance
	ce CallEngine
}

// Definition implements the same method as documented on api.FunctionDefinition.
func (f *function) Definition() api.FunctionDefinition {
	return f.fi.Definition
}

// Call implements the same method as documented on api.Function.
func (f *function) Call(ctx context.Context, params ...uint64) (ret []uint64, err error) {
	return f.ce.Call(ctx, f.fi.Module.CallCtx, params)
}

// importedFn implements api.Function and ensures the call context of an imported function is the importing module.
type importedFn struct {
	ce              CallEngine
	importingModule *CallContext
	importedFn      *FunctionInstance
}

// Definition implements the same method as documented on api.Function.
func (f *importedFn) Definition() api.FunctionDefinition {
	return f.importedFn.Definition
}

// Call implements the same method as documented on api.Function.
func (f *importedFn) Call(ctx context.Context, params ...uint64) (ret []uint64, err error) {
	if f.importedFn.IsHostFunction {
		return nil, fmt.Errorf("directly calling host function is not supported")
	}
	mod := f.importingModule
	return f.ce.Call(ctx, mod, params)
}

// GlobalVal is an internal hack to get the lower 64 bits of a global.
func (m *CallContext) GlobalVal(idx Index) uint64 {
	return m.module.Globals[idx].Val
}

// ExportedGlobal implements the same method as documented on api.Module.
func (m *CallContext) ExportedGlobal(name string) api.Global {
	exp, err := m.module.getExport(name, ExternTypeGlobal)
	if err != nil {
		return nil
	}
	if exp.Global.Type.Mutable {
		return &mutableGlobal{exp.Global}
	}
	valType := exp.Global.Type.ValType
	switch valType {
	case ValueTypeI32:
		return globalI32(exp.Global.Val)
	case ValueTypeI64:
		return globalI64(exp.Global.Val)
	case ValueTypeF32:
		return globalF32(exp.Global.Val)
	case ValueTypeF64:
		return globalF64(exp.Global.Val)
	default:
		panic(fmt.Errorf("BUG: unknown value type %X", valType))
	}
}