Makes memory limit configurable and a compile error (#419)

This allows users to reduce the memory limit per module below 4 Gi. This
is often needed because Wasm routinely leaves off the max, which implies
spec max (4 Gi). This uses Ki Gi etc in error messages because the spec
chooses to, though we can change to make it less awkward.

This also fixes an issue where we instantiated an engine inside config.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

wasm.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 
 	internalwasm "github.com/tetratelabs/wazero/internal/wasm"
 	"github.com/tetratelabs/wazero/internal/wasm/binary"
@@ -81,8 +82,9 @@ func NewRuntime() Runtime {
 func NewRuntimeWithConfig(config *RuntimeConfig) Runtime {
 	return &runtime{
 		ctx:             config.ctx,
-		store:           internalwasm.NewStore(config.engine, config.enabledFeatures),
+		store:           internalwasm.NewStore(config.newEngine(), config.enabledFeatures),
 		enabledFeatures: config.enabledFeatures,
+		memoryMaxPages:  config.memoryMaxPages,
 	}
 }
 
@@ -91,6 +93,7 @@ type runtime struct {
 	ctx             context.Context
 	store           *internalwasm.Store
 	enabledFeatures internalwasm.Features
+	memoryMaxPages  uint32
 }
 
 // Module implements Runtime.Module
@@ -116,7 +119,13 @@ func (r *runtime) CompileModule(source []byte) (*Module, error) {
 		decoder = text.DecodeModule
 	}
 
-	internal, err := decoder(source, r.enabledFeatures)
+	if r.memoryMaxPages > internalwasm.MemoryMaxPages {
+		return nil, fmt.Errorf("memoryMaxPages %d (%s) > specification max %d (%s)",
+			r.memoryMaxPages, internalwasm.PagesToUnitOfBytes(r.memoryMaxPages),
+			internalwasm.MemoryMaxPages, internalwasm.PagesToUnitOfBytes(internalwasm.MemoryMaxPages))
+	}
+
+	internal, err := decoder(source, r.enabledFeatures, r.memoryMaxPages)
 	if err != nil {
 		return nil, err
 	} else if err = internal.Validate(r.enabledFeatures); err != nil {