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|
// Package wasm provides the wazero runtime for the gonix WASM evaluator.
// It loads libnixexpr.wasm and wires up:
// - WASI snapshot preview 1 (filesystem, clock, etc.)
// - "env" module via wazero's emscripten package (invoke_* + our __cxa_* stubs)
// - "lix_store" module (store I/O dispatch to a Go Store)
package wasm
import (
"context"
"fmt"
"math"
"os"
"strings"
"sync/atomic"
"time"
"github.com/nixos/lix/gonix/codec"
"github.com/nixos/lix/gonix/store"
"github.com/tetratelabs/wazero"
"github.com/tetratelabs/wazero/api"
"github.com/tetratelabs/wazero/imports/wasi_snapshot_preview1"
)
// Runtime wraps the wazero runtime and the instantiated libnixexpr module.
type Runtime struct {
r wazero.Runtime
mod api.Module
store store.Store
// Counters tracks host-function call counts, readable after any call.
Counters Counters
// fatalErr holds a non-EH trap error (abort, OOB, etc.) that occurred
// during WASM execution. Set by callInvoke when it detects a fatal trap.
// Checked by exported methods before calling WASM functions.
fatalErr error
// nixConfig is the nix.conf contents served to the WASM module via
// lix_host_get_config. Empty string means no config (use defaults).
nixConfig string
// hostTableEntries maps __indirect_function_table slot → "env" import name
// for every host-imported function that appears in the function table.
// Built once at init time; used by invoke_* dispatch to short-circuit
// wazero's LookupFunction for host-module entries (which crashes on wazevo).
hostTableEntries map[uint32]string
// envHandlers maps "env" export name → Go handler function.
// Populated during instantiateEnv so dispatchHostTableEntry can call host
// functions directly without going through the module API (which forbids
// ExportedFunction on host modules).
envHandlers map[string]api.GoModuleFunc
// trampolineFns caches the GoModuleFunction values from the "_invoke"
// trampoline module, keyed by invoke_* name. Extracted via
// ExportedFunctionDefinitions at init time (safe for host modules).
// Called directly in the WASM-defined fallback path of invoke_* dispatch.
trampolineFns map[string]api.GoModuleFunction
// Exported function handles (cached after instantiation).
fnNixInit api.Function
fnNixBuiltinNames api.Function
fnNixParseExpr api.Function
fnNixEvalExpr api.Function
fnNixForceValue api.Function
fnNixValueType api.Function
fnNixGetString api.Function
fnNixGetInt api.Function
fnNixGetFloat api.Function
fnNixGetBool api.Function
fnNixGetAttr api.Function
fnNixGetAttrNames api.Function
fnNixListLength api.Function
fnNixListGet api.Function
fnNixFreeValue api.Function
fnNixLastError api.Function
fnNixValuePtr api.Function
fnMalloc api.Function
fnFree api.Function
}
// New creates a Runtime from a WASM binary and a backing Store.
// nixConfig is the contents of nix.conf to pass to the evaluator (may be empty).
// compiled selects the native compiler (wazevo) instead of the interpreter.
// The WASM module is instantiated once; call Close() when done.
func New(ctx context.Context, wasmBytes []byte, s store.Store, nixConfig string, useCompiler bool) (*Runtime, error) {
var rc wazero.RuntimeConfig
if useCompiler {
rc = wazero.NewRuntimeConfig()
} else {
rc = wazero.NewRuntimeConfigInterpreter()
}
rc = rc.WithCoreFeatures(api.CoreFeaturesV2).
WithDebugInfoEnabled(true)
r := wazero.NewRuntimeWithConfig(ctx, rc)
// 1. WASI snapshot preview 1 — handles fd_write, clock_time_get, etc.
if _, err := wasi_snapshot_preview1.Instantiate(ctx, r); err != nil {
r.Close(ctx)
return nil, fmt.Errorf("wasi_snapshot_preview1: %w", err)
}
rt := &Runtime{r: r, store: s, nixConfig: nixConfig}
// 2. Compile the WASM module first so we can inspect its imports.
t0 := time.Now()
compiled, err := r.CompileModule(ctx, wasmBytes)
fmt.Fprintf(os.Stderr, "[timing] wasm compile: %s\n", time.Since(t0).Round(time.Millisecond))
if err != nil {
r.Close(ctx)
return nil, fmt.Errorf("compile libnixexpr: %w", err)
}
// 3. Parse function table to find which slots hold host ("env") imports.
// This is used by our invoke_* handlers to avoid wazero's LookupFunction
// for host-module entries (which crashes in wazevo due to opaque-ptr layout
// differences between host and WASM modules).
hostTableEntries, err := parseHostTableEntries(wasmBytes, compiled)
if err != nil {
r.Close(ctx)
return nil, fmt.Errorf("parse host table entries: %w", err)
}
rt.hostTableEntries = hostTableEntries
// 4. "env" module — our own invoke_* dispatch + __cxa_* + syscall + thread stubs.
// A private "_invoke" trampoline module holds wazero's emscripten invoke_*
// functions for the WASM-defined-function fallback path.
if err := rt.instantiateEnv(ctx, compiled); err != nil {
r.Close(ctx)
return nil, fmt.Errorf("env module: %w", err)
}
// 5. "lix_store" module — store I/O dispatch.
if err := rt.instantiateLixStore(ctx); err != nil {
r.Close(ctx)
return nil, fmt.Errorf("lix_store module: %w", err)
}
// 6. Instantiate the WASM module.
// Use WithStartFunctions() (empty) so wazero doesn't auto-call _initialize.
// We set rt.mod first, then call _initialize manually — this is required
// because _initialize runs C++ global constructors which may call
// __resumeException / __cxa_find_matching_catch, which need rt.mod set
// to reach libnixexpr's exported helper functions (setThrew, tempret_set).
cfg := wazero.NewModuleConfig().
WithName("libnixexpr").
WithStdout(os.Stdout).
WithStderr(os.Stderr).
// Mount the real filesystem so `import ./foo.nix` can read files.
WithFSConfig(wazero.NewFSConfig().WithDirMount("/", "/")).
WithStartFunctions()
t1 := time.Now()
mod, err := r.InstantiateModule(ctx, compiled, cfg)
if err != nil {
r.Close(ctx)
return nil, fmt.Errorf("instantiate libnixexpr: %w", err)
}
// Set rt.mod before _initialize so EH stubs can reach libnixexpr exports.
rt.mod = mod
if initFn := mod.ExportedFunction("_initialize"); initFn != nil {
if _, err := initFn.Call(ctx); err != nil {
r.Close(ctx)
return nil, fmt.Errorf("_initialize: %w", err)
}
}
fmt.Fprintf(os.Stderr, "[timing] wasm instantiate: %s\n", time.Since(t1).Round(time.Millisecond))
// Cache exported functions.
rt.fnNixInit = mod.ExportedFunction("nix_init")
rt.fnNixBuiltinNames = mod.ExportedFunction("nix_builtin_names")
rt.fnNixParseExpr = mod.ExportedFunction("nix_parse_expr")
rt.fnNixEvalExpr = mod.ExportedFunction("nix_eval_expr")
rt.fnNixForceValue = mod.ExportedFunction("nix_force_value")
rt.fnNixValueType = mod.ExportedFunction("nix_value_type")
rt.fnNixGetString = mod.ExportedFunction("nix_get_string")
rt.fnNixGetInt = mod.ExportedFunction("nix_get_int")
rt.fnNixGetFloat = mod.ExportedFunction("nix_get_float")
rt.fnNixGetBool = mod.ExportedFunction("nix_get_bool")
rt.fnNixGetAttr = mod.ExportedFunction("nix_get_attr")
rt.fnNixGetAttrNames = mod.ExportedFunction("nix_get_attr_names")
rt.fnNixListLength = mod.ExportedFunction("nix_list_length")
rt.fnNixListGet = mod.ExportedFunction("nix_list_get")
rt.fnNixFreeValue = mod.ExportedFunction("nix_free_value")
rt.fnNixLastError = mod.ExportedFunction("nix_last_error")
rt.fnNixValuePtr = mod.ExportedFunction("nix_value_ptr")
rt.fnMalloc = mod.ExportedFunction("malloc")
rt.fnFree = mod.ExportedFunction("free")
return rt, nil
}
// fatalCheck returns fatalErr if set, otherwise nil.
func (rt *Runtime) fatalCheck() error {
if rt.fatalErr != nil {
return fmt.Errorf("wasm fatal: %w", rt.fatalErr)
}
return nil
}
// HasFatalError reports whether an unrecoverable trap has been recorded.
// After a fatal error all subsequent WASM calls will fail.
func (rt *Runtime) HasFatalError() bool {
return rt.fatalErr != nil
}
// Init calls nix_init with the given store directory (e.g. "/nix/store").
func (rt *Runtime) Init(ctx context.Context, storeDir string) error {
ptr, err := rt.allocString(ctx, storeDir)
if err != nil {
return err
}
defer rt.freePtr(ctx, ptr)
_, err = rt.fnNixInit.Call(ctx, uint64(ptr), uint64(len(storeDir)))
if err != nil {
return fmt.Errorf("nix_init: %w", err)
}
return nil
}
// ParseExpr parses a Nix expression string and returns the post-finalize AST
// encoded in the lix codec wire format. The caller decodes it with eval.DecodeAST.
//
// basePath is the directory of the file being parsed (e.g. "/testfs/sub/").
// Relative path literals like ./foo.nix are resolved against basePath.
// Pass "/" for the legacy root-relative behaviour.
//
// filePath is the absolute path of the file being parsed (e.g. "/testfs/sub/foo.nix"),
// used only for position information. Pass "" if unknown.
func (rt *Runtime) ParseExpr(ctx context.Context, expr string, basePath string, filePath string) ([]byte, error) {
if err := rt.fatalCheck(); err != nil {
return nil, err
}
if rt.fnNixParseExpr == nil {
return nil, fmt.Errorf("nix_parse_expr not available in this WASM build")
}
if basePath == "" {
basePath = "/"
}
exprPtr, err := rt.allocString(ctx, expr)
if err != nil {
return nil, err
}
defer rt.freePtr(ctx, exprPtr)
basePathPtr, err := rt.allocString(ctx, basePath)
if err != nil {
return nil, err
}
defer rt.freePtr(ctx, basePathPtr)
filePathPtr, err := rt.allocString(ctx, filePath)
if err != nil {
return nil, err
}
defer rt.freePtr(ctx, filePathPtr)
// Allocate a response buffer (32 MiB — sufficient for large nixpkgs files).
const respMax = 32 * 1024 * 1024
respPtr, err := rt.malloc(ctx, respMax)
if err != nil {
return nil, err
}
defer rt.freePtr(ctx, respPtr)
res, err := rt.fnNixParseExpr.Call(ctx,
uint64(exprPtr), uint64(len(expr)),
uint64(basePathPtr), uint64(len(basePath)),
uint64(filePathPtr), uint64(len(filePath)),
uint64(respPtr), uint64(respMax),
)
if err != nil {
// A C++ exception escaped the WASM boundary without going through the
// normal invoke_* EH protocol. Reset EH globals so the runtime is
// usable for subsequent calls.
rt.resetEHState(ctx)
return nil, fmt.Errorf("nix_parse_expr: %w", err)
}
n := int32(res[0])
if n < 0 {
return nil, fmt.Errorf("parse error: %s", rt.lastError(ctx))
}
b, ok := rt.mod.Memory().Read(respPtr, uint32(n))
if !ok {
return nil, fmt.Errorf("nix_parse_expr: memory read failed")
}
// Return a copy so the WASM memory can be freed.
out := make([]byte, len(b))
copy(out, b)
return out, nil
}
// BuiltinNames returns the ordered list of builtin names from the C++ base
// environment. The order matches ExprVar.Displ values resolved by nix_parse_expr,
// so the Go evaluator must register builtins in this exact order.
func (rt *Runtime) BuiltinNames(ctx context.Context) ([]string, error) {
if err := rt.fatalCheck(); err != nil {
return nil, err
}
if rt.fnNixBuiltinNames == nil {
return nil, fmt.Errorf("nix_builtin_names not available in this WASM build")
}
const respMax = 64 * 1024 // 64 KiB — more than enough for builtin names
respPtr, err := rt.malloc(ctx, respMax)
if err != nil {
return nil, err
}
defer rt.freePtr(ctx, respPtr)
res, err := rt.fnNixBuiltinNames.Call(ctx, uint64(respPtr), uint64(respMax))
if err != nil {
return nil, fmt.Errorf("nix_builtin_names: %w", err)
}
n := int32(res[0])
if n < 0 {
return nil, fmt.Errorf("nix_builtin_names error: %s", rt.lastError(ctx))
}
b, ok := rt.mod.Memory().Read(respPtr, uint32(n))
if !ok {
return nil, fmt.Errorf("nix_builtin_names: memory read failed")
}
v, err := codec.Decode(b)
if err != nil {
return nil, fmt.Errorf("nix_builtin_names: decode: %w", err)
}
if v.Kind != codec.KindList {
return nil, fmt.Errorf("nix_builtin_names: expected list, got kind %d", v.Kind)
}
names := make([]string, 0, len(v.Items))
for i, item := range v.Items {
s, err := item.AsString()
if err != nil {
return nil, fmt.Errorf("nix_builtin_names: item[%d]: %w", i, err)
}
names = append(names, s)
}
return names, nil
}
// EvalExpr evaluates a Nix expression string and returns a handle.
func (rt *Runtime) EvalExpr(ctx context.Context, expr string) (int32, error) {
if err := rt.fatalCheck(); err != nil {
return -1, err
}
ptr, err := rt.allocString(ctx, expr)
if err != nil {
return -1, err
}
defer rt.freePtr(ctx, ptr)
res, err := rt.fnNixEvalExpr.Call(ctx, uint64(ptr), uint64(len(expr)))
if err != nil {
rt.resetEHState(ctx)
return -1, fmt.Errorf("nix_eval_expr: %w", err)
}
handle := int32(res[0])
if handle < 0 {
return -1, fmt.Errorf("eval error: %s", rt.lastError(ctx))
}
return handle, nil
}
// ForceValue forces a value to WHNF. Returns the same handle on success.
func (rt *Runtime) ForceValue(ctx context.Context, handle int32) error {
if err := rt.fatalCheck(); err != nil {
return err
}
res, err := rt.fnNixForceValue.Call(ctx, uint64(handle))
if err != nil {
rt.resetEHState(ctx)
return fmt.Errorf("nix_force_value: %w", err)
}
if int32(res[0]) < 0 {
return fmt.Errorf("force error: %s", rt.lastError(ctx))
}
return nil
}
// ValueType returns the ValueType of a forced value.
func (rt *Runtime) ValueType(ctx context.Context, handle int32) (int32, error) {
res, err := rt.fnNixValueType.Call(ctx, uint64(handle))
if err != nil {
return -1, err
}
return int32(res[0]), nil
}
// ValuePtr returns the WASM linear-memory address of the Value* backing the
// handle. Two handles pointing to the same Nix Value object return the same
// address. Used by the printer for cycle/repeat detection (mirrors cppnix's
// pointer-identity ValuesSeen set). Returns 0 on invalid handle.
func (rt *Runtime) ValuePtr(ctx context.Context, handle int32) (uint32, error) {
if rt.fnNixValuePtr == nil {
return 0, nil // older WASM without the export — degrade gracefully
}
res, err := rt.fnNixValuePtr.Call(ctx, uint64(handle))
if err != nil {
return 0, err
}
return uint32(res[0]), nil
}
// GetString reads a string value from a handle.
func (rt *Runtime) GetString(ctx context.Context, handle int32) (string, error) {
return rt.callStringOut(ctx, rt.fnNixGetString, handle)
}
// GetInt reads an integer value.
func (rt *Runtime) GetInt(ctx context.Context, handle int32) (int64, error) {
res, err := rt.fnNixGetInt.Call(ctx, uint64(handle))
if err != nil {
return 0, err
}
return int64(res[0]), nil
}
// GetFloat reads a float value.
func (rt *Runtime) GetFloat(ctx context.Context, handle int32) (float64, error) {
res, err := rt.fnNixGetFloat.Call(ctx, uint64(handle))
if err != nil {
return 0, err
}
return math.Float64frombits(res[0]), nil
}
// GetBool reads a boolean value.
func (rt *Runtime) GetBool(ctx context.Context, handle int32) (bool, error) {
res, err := rt.fnNixGetBool.Call(ctx, uint64(handle))
if err != nil {
return false, err
}
return res[0] != 0, nil
}
// GetAttr looks up an attribute by name and returns a new handle.
func (rt *Runtime) GetAttr(ctx context.Context, handle int32, name string) (int32, error) {
namePtr, err := rt.allocString(ctx, name)
if err != nil {
return -1, err
}
defer rt.freePtr(ctx, namePtr)
res, err := rt.fnNixGetAttr.Call(ctx, uint64(handle), uint64(namePtr), uint64(len(name)))
if err != nil {
return -1, err
}
h := int32(res[0])
if h < 0 {
return -1, fmt.Errorf("attr %q not found: %s", name, rt.lastError(ctx))
}
return h, nil
}
// GetAttrNames returns all attribute names as a newline-separated string.
func (rt *Runtime) GetAttrNames(ctx context.Context, handle int32) (string, error) {
return rt.callStringOut(ctx, rt.fnNixGetAttrNames, handle)
}
// ListLength returns the length of a list value.
func (rt *Runtime) ListLength(ctx context.Context, handle int32) (int32, error) {
res, err := rt.fnNixListLength.Call(ctx, uint64(handle))
if err != nil {
return -1, err
}
return int32(res[0]), nil
}
// ListGet returns the element at index idx as a new handle.
func (rt *Runtime) ListGet(ctx context.Context, handle int32, idx int32) (int32, error) {
res, err := rt.fnNixListGet.Call(ctx, uint64(handle), uint64(idx))
if err != nil {
return -1, err
}
h := int32(res[0])
if h < 0 {
return -1, fmt.Errorf("list index %d out of range", idx)
}
return h, nil
}
// FreeValue releases a handle.
func (rt *Runtime) FreeValue(ctx context.Context, handle int32) {
rt.fnNixFreeValue.Call(ctx, uint64(handle)) //nolint:errcheck
}
// counterFor returns the appropriate aggregate atomic counter for a host function name.
// For invoke_* this returns InvokeTotal; fine-grained InvokeHost/InvokeWasm and
// CxaThrow are incremented separately in callInvoke and dispatchHostTableEntry.
func (rt *Runtime) counterFor(name string) *atomic.Int64 {
switch {
case strings.HasPrefix(name, "invoke_"):
return &rt.Counters.InvokeTotal
case strings.HasPrefix(name, "__cxa_") ||
name == "__resumeException" ||
name == "llvm_eh_typeid_for":
return &rt.Counters.CxaTotal
case strings.HasPrefix(name, "__syscall_"):
return &rt.Counters.Syscall
default:
return &rt.Counters.Other
}
}
// Close shuts down the runtime.
func (rt *Runtime) Close(ctx context.Context) error {
return rt.r.Close(ctx)
}
// ---------------------------------------------------------------------------
// Internal helpers
// ---------------------------------------------------------------------------
const bufSize = 65536 // 64 KiB response buffer
// callStringOut calls a function with (handle, bufPtr, bufMax) -> len
// and returns the string written into the buffer.
func (rt *Runtime) callStringOut(ctx context.Context, fn api.Function, handle int32) (string, error) {
bufPtr, err := rt.malloc(ctx, bufSize)
if err != nil {
return "", err
}
defer rt.freePtr(ctx, bufPtr)
res, err := fn.Call(ctx, uint64(handle), uint64(bufPtr), bufSize)
if err != nil {
return "", err
}
n := int32(res[0])
if n < 0 {
return "", fmt.Errorf("%s", rt.lastError(ctx))
}
b, ok := rt.mod.Memory().Read(bufPtr, uint32(n))
if !ok {
return "", fmt.Errorf("memory read failed")
}
return string(b), nil
}
func (rt *Runtime) lastError(ctx context.Context) string {
bufPtr, err := rt.malloc(ctx, 4096)
if err != nil {
return "(failed to allocate error buffer)"
}
defer rt.freePtr(ctx, bufPtr)
res, err := rt.fnNixLastError.Call(ctx, uint64(bufPtr), 4096)
if err != nil {
return fmt.Sprintf("(nix_last_error failed: %v)", err)
}
if int32(res[0]) <= 0 {
return fmt.Sprintf("(nix_last_error returned %d — g_last_error is empty)", int32(res[0]))
}
n := uint32(res[0])
b, ok := rt.mod.Memory().Read(bufPtr, n)
if !ok {
return "(memory read failed)"
}
return string(b)
}
// allocString writes s into WASM linear memory as a null-terminated C string
// and returns the pointer.
func (rt *Runtime) allocString(ctx context.Context, s string) (uint32, error) {
ptr, err := rt.malloc(ctx, uint32(len(s)+1))
if err != nil {
return 0, err
}
if !rt.mod.Memory().WriteString(ptr, s) {
rt.freePtr(ctx, ptr)
return 0, fmt.Errorf("memory write failed")
}
if !rt.mod.Memory().WriteByte(ptr+uint32(len(s)), 0) {
rt.freePtr(ctx, ptr)
return 0, fmt.Errorf("memory write null terminator failed")
}
return ptr, nil
}
func (rt *Runtime) malloc(ctx context.Context, size uint32) (uint32, error) {
res, err := rt.fnMalloc.Call(ctx, uint64(size))
if err != nil {
return 0, fmt.Errorf("malloc: %w", err)
}
if res[0] == 0 {
return 0, fmt.Errorf("malloc returned null")
}
return uint32(res[0]), nil
}
func (rt *Runtime) freePtr(ctx context.Context, ptr uint32) {
rt.fnFree.Call(ctx, uint64(ptr)) //nolint:errcheck
}
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