feat!: add ability to configure size of the Extism var store (#61)

See extism/extism#692

- Breaking: changes the `Memory` field of `Manifest` to a pointer in
order to better detect when the memory settings weren't provided

extism.go

@@ -90,6 +90,7 @@ type Plugin struct {
 	AllowedPaths         map[string]string
 	LastStatusCode       int
 	MaxHttpResponseBytes int64
+	MaxVarBytes          int64
 	log                  func(LogLevel, string)
 	logLevel             LogLevel
 	guestRuntime         guestRuntime
@@ -216,14 +217,17 @@ func (u WasmUrl) ToWasmData(ctx context.Context) (WasmData, error) {
 	}, nil
 }
 
+type ManifestMemory struct {
+	MaxPages             uint32 `json:"max_pages,omitempty"`
+	MaxHttpResponseBytes int64  `json:"max_http_response_bytes,omitempty"`
+	MaxVarBytes          int64  `json:"max_var_bytes,omitempty"`
+}
+
 // Manifest represents the plugin's manifest, including Wasm modules and configuration.
 // See https://extism.org/docs/concepts/manifest for schema.
 type Manifest struct {
-	Wasm   []Wasm `json:"wasm"`
-	Memory struct {
-		MaxPages             uint32 `json:"max_pages,omitempty"`
-		MaxHttpResponseBytes uint64 `json:"max_http_response_bytes,omitempty"`
-	} `json:"memory,omitempty"`
+	Wasm         []Wasm            `json:"wasm"`
+	Memory       *ManifestMemory   `json:"memory,omitempty"`
 	Config       map[string]string `json:"config,omitempty"`
 	AllowedHosts []string          `json:"allowed_hosts,omitempty"`
 	AllowedPaths map[string]string `json:"allowed_paths,omitempty"`
@@ -232,9 +236,10 @@ type Manifest struct {
 
 type concreteManifest struct {
 	Wasm   []concreteWasm `json:"wasm"`
-	Memory struct {
+	Memory *struct {
 		MaxPages             uint32 `json:"max_pages,omitempty"`
-		MaxHttpResponseBytes uint64 `json:"max_http_response_bytes,omitempty"`
+		MaxHttpResponseBytes *int64 `json:"max_http_response_bytes,omitempty"`
+		MaxVarBytes          *int64 `json:"max_var_bytes,omitempty"`
 	} `json:"memory,omitempty"`
 	Config       map[string]string `json:"config,omitempty"`
 	AllowedHosts []string          `json:"allowed_hosts,omitempty"`
@@ -249,7 +254,25 @@ func (m *Manifest) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
-	m.Memory = tmp.Memory
+	m.Memory = &ManifestMemory{}
+	if tmp.Memory != nil {
+		m.Memory.MaxPages = tmp.Memory.MaxPages
+		if tmp.Memory.MaxHttpResponseBytes != nil {
+			m.Memory.MaxHttpResponseBytes = *tmp.Memory.MaxHttpResponseBytes
+		} else {
+			m.Memory.MaxHttpResponseBytes = -1
+		}
+
+		if tmp.Memory.MaxVarBytes != nil {
+			m.Memory.MaxVarBytes = *tmp.Memory.MaxVarBytes
+		} else {
+			m.Memory.MaxVarBytes = -1
+		}
+	} else {
+		m.Memory.MaxPages = 0
+		m.Memory.MaxHttpResponseBytes = -1
+		m.Memory.MaxVarBytes = -1
+	}
 	m.Config = tmp.Config
 	m.AllowedHosts = tmp.AllowedHosts
 	m.AllowedPaths = tmp.AllowedPaths
@@ -301,8 +324,10 @@ func NewPlugin(
 		rconfig = rconfig.WithCloseOnContextDone(true)
 	}
 
-	if manifest.Memory.MaxPages > 0 {
-		rconfig = rconfig.WithMemoryLimitPages(manifest.Memory.MaxPages)
+	if manifest.Memory != nil {
+		if manifest.Memory.MaxPages > 0 {
+			rconfig = rconfig.WithMemoryLimitPages(manifest.Memory.MaxPages)
+		}
 	}
 
 	rt := wazero.NewRuntimeWithConfig(ctx, rconfig)
@@ -419,9 +444,14 @@ func NewPlugin(
 
 	i := 0
 	httpMax := int64(1024 * 1024 * 50)
-	if manifest.Memory.MaxHttpResponseBytes != 0 {
+	if manifest.Memory != nil && manifest.Memory.MaxHttpResponseBytes >= 0 {
 		httpMax = int64(manifest.Memory.MaxHttpResponseBytes)
 	}
+
+	varMax := int64(1024 * 1024)
+	if manifest.Memory != nil && manifest.Memory.MaxVarBytes >= 0 {
+		varMax = int64(manifest.Memory.MaxVarBytes)
+	}
 	for _, m := range modules {
 		if m.Name() == "main" {
 			p := &Plugin{
@@ -435,6 +465,7 @@ func NewPlugin(
 				LastStatusCode:       0,
 				Timeout:              time.Duration(manifest.Timeout) * time.Millisecond,
 				MaxHttpResponseBytes: httpMax,
+				MaxVarBytes:          varMax,
 				log:                  logStd,
 				logLevel:             logLevel,
 			}

extism_test.go

@@ -573,6 +573,24 @@ func TestVar(t *testing.T) {
 
 }
 
+func TestNoVars(t *testing.T) {
+	manifest := manifest("var.wasm")
+	manifest.Memory = &ManifestMemory{MaxVarBytes: 0}
+
+	if plugin, ok := plugin(t, manifest); ok {
+		defer plugin.Close()
+
+		plugin.Var["a"] = uintToLEBytes(10)
+
+		_, _, err := plugin.Call("run_test", []byte{})
+
+		if err == nil {
+			t.Fail()
+		}
+	}
+
+}
+
 func TestFS(t *testing.T) {
 	manifest := manifest("fs.wasm")
 	manifest.AllowedPaths = map[string]string{

host.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"unsafe"
 
 	// TODO: is there a better package for this?
 	"github.com/gobwas/glob"
@@ -414,34 +415,41 @@ func varSet(ctx context.Context, m api.Module, nameOffset uint64, valueOffset ui
 		panic("Invalid context, `plugin` key not found")
 	}
 
+	if plugin.MaxVarBytes == 0 {
+		panic("Vars are disabled by this host")
+	}
+
 	cp := plugin.currentPlugin()
 
 	name, err := cp.ReadString(nameOffset)
 	if err != nil {
 		panic(fmt.Errorf("Failed to read var name from memory: %v", err))
 	}
 
-	size := 0
-	for _, v := range plugin.Var {
-		size += len(v)
+	// Remove if the value offset is 0
+	if valueOffset == 0 {
+		delete(plugin.Var, name)
+		return
 	}
 
-	// If the store is larger than 100MB then stop adding things
-	if size > 1024*1024*100 && valueOffset != 0 {
-		panic("Variable store is full")
+	value, err := cp.ReadBytes(valueOffset)
+	if err != nil {
+		panic(fmt.Errorf("Failed to read var value from memory: %v", err))
 	}
 
-	// Remove if the value offset is 0
-	if valueOffset == 0 {
-		delete(plugin.Var, name)
-	} else {
-		value, err := cp.ReadBytes(valueOffset)
-		if err != nil {
-			panic(fmt.Errorf("Failed to read var value from memory: %v", err))
-		}
+	// Calculate size including current key/value
+	size := int(unsafe.Sizeof([]byte{})+unsafe.Sizeof("")) + len(name) + len(value)
+	for k, v := range plugin.Var {
+		size += len(k)
+		size += len(v)
+		size += int(unsafe.Sizeof([]byte{}) + unsafe.Sizeof(""))
+	}
 
-		plugin.Var[name] = value
+	if size >= int(plugin.MaxVarBytes) && valueOffset != 0 {
+		panic("Variable store is full")
 	}
+
+	plugin.Var[name] = value
 }
 
 func httpRequest(ctx context.Context, m api.Module, requestOffset uint64, bodyOffset uint64) uint64 {