fix(build): bundle sysshim into build_assets for distribution mode (#4)

When wasmforge runs as a standalone binary with no source tree on disk,
injectSysshimReplace and injectPuregoSysshimReplace failed with
"cannot find wasmforge module root for sysshim injection" because all
four findModuleRoot() fallback paths returned "".

Three-part fix:
- cmd/gen-build-assets: add addSysshimDir (recursive walk) and include
  the internal/sysshim tree under the "sysshim/" prefix in the bundle.
  File count rises from ~399 to 442.
- internal/build/asset_cache.go: new file providing
  ensureAssetsExtracted(), a process-lifetime sync.Once cache that
  extracts build_assets.tar.gz to a temp dir on first call.
- internal/build/compiler.go: add sysshimSourceBase() helper that
  prefers the on-disk module root (dev mode) and falls back to the
  extracted asset cache (distribution mode); rewire both injection
  functions to use it instead of calling findModuleRoot() directly.

cmd/gen-build-assets/main.go

@@ -112,6 +112,15 @@ func generateArchive(moduleRoot, wazeroDir, outputPath string) (int, error) {
 		totalFiles += n
 	}
 
+	// Add sysshim tree (recursive — has subdirs and per-subdir go.mod files).
+	// Needed at runtime when a guest depends on golang.org/x/sys and the
+	// wasmforge module source isn't otherwise locatable on disk.
+	n, err = addSysshimDir(tw, filepath.Join(moduleRoot, "internal", "sysshim"))
+	if err != nil {
+		return 0, fmt.Errorf("adding sysshim: %w", err)
+	}
+	totalFiles += n
+
 	// Add go.sum.
 	gosumPath := filepath.Join(moduleRoot, "go.sum")
 	if err := addFile(tw, gosumPath, "go.sum"); err != nil {
@@ -122,6 +131,38 @@ func generateArchive(moduleRoot, wazeroDir, outputPath string) (int, error) {
 	return totalFiles, nil
 }
 
+// addSysshimDir adds the internal/sysshim tree under the "sysshim/" prefix.
+// Walks recursively because sysshim has nested subdirs (unix, purego,
+// windows, windows/svc, etc.) and per-subdir go.mod files that the
+// injection step needs at runtime. Includes *.go, *.s, and go.mod; excludes
+// _test.go.
+func addSysshimDir(tw *tar.Writer, sysshimDir string) (int, error) {
+	count := 0
+	err := filepath.Walk(sysshimDir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if info.IsDir() {
+			return nil
+		}
+		name := info.Name()
+		if name != "go.mod" && !isSourceFile(name) {
+			return nil
+		}
+		rel, err := filepath.Rel(sysshimDir, path)
+		if err != nil {
+			return err
+		}
+		archivePath := filepath.ToSlash(filepath.Join("sysshim", rel))
+		if err := addFile(tw, path, archivePath); err != nil {
+			return err
+		}
+		count++
+		return nil
+	})
+	return count, err
+}
+
 // addWazeroDir adds the wazero fork to the archive under the "wazero/" prefix.
 // It applies the wazero-specific skip list and only includes *.go, *.s, go.mod, go.sum files.
 func addWazeroDir(tw *tar.Writer, wazeroDir string) (int, error) {

internal/build/asset_cache.go

@@ -0,0 +1,42 @@
+package build
+
+import (
+	"fmt"
+	"os"
+	"sync"
+)
+
+var (
+	cachedAssetDirOnce sync.Once
+	cachedAssetDir     string
+	cachedAssetDirErr  error
+)
+
+// ensureAssetsExtracted returns a process-lifetime temp directory containing
+// the extracted contents of the embedded build_assets bundle. The first call
+// performs the extraction; subsequent calls return the cached path.
+//
+// The directory is not cleaned up — it lives under the OS temp directory
+// (e.g. /tmp on Linux, /var/folders/... on macOS) where the OS reclaims it
+// eventually. This avoids re-extracting on every sysshim injection and lets
+// the directory survive across multiple build pipeline stages within a
+// single wasmforge invocation.
+//
+// Callers should access well-known subtrees via their bundled prefixes:
+// "hostmod", "runtime", "names", "wazero", "sysshim", and "go.sum".
+func ensureAssetsExtracted() (string, error) {
+	cachedAssetDirOnce.Do(func() {
+		dir, err := os.MkdirTemp("", "wasmforge-assets-*")
+		if err != nil {
+			cachedAssetDirErr = fmt.Errorf("creating asset cache dir: %w", err)
+			return
+		}
+		if err := extractBuildAssets(dir); err != nil {
+			os.RemoveAll(dir)
+			cachedAssetDirErr = fmt.Errorf("extracting build assets: %w", err)
+			return
+		}
+		cachedAssetDir = dir
+	})
+	return cachedAssetDir, cachedAssetDirErr
+}

internal/build/compiler.go

@@ -254,6 +254,19 @@ func CompileWASM(patchedGOROOT, pkg, tmpDir string, verbose, win32APIs bool, tar
 	return wasmOut, nil
 }
 
+// sysshimSourceBase returns a directory B such that filepath.Join(B,
+// "sysshim", <pkg>) is a valid wasmforge sysshim package source tree.
+// In development mode (wasmforge module source on disk), B is
+// <moduleRoot>/internal. In distribution mode (standalone binary), B is
+// the cached temp directory populated from the embedded build_assets
+// bundle, where the sysshim tree was packed under the "sysshim/" prefix.
+func sysshimSourceBase() (string, error) {
+	if root := findModuleRoot(); root != "" {
+		return filepath.Join(root, "internal"), nil
+	}
+	return ensureAssetsExtracted()
+}
+
 // injectSysshimReplace checks whether the guest module at pkgDir depends on
 // golang.org/x/sys. If it does, it creates a temporary copy of the module
 // directory with the sysshim replacing the original x/sys code. For vendored
@@ -274,15 +287,16 @@ func injectSysshimReplace(pkgDir, tmpDir string, verbose, win32APIs bool) (strin
 		return "", nil
 	}
 
-	// Locate the sysshim directory (relative to the wasmforge module root).
+	// Locate the sysshim directory (relative to the wasmforge module root, or
-	moduleRoot := findModuleRoot()
+	// from the embedded build assets when running as a standalone binary).
-	if moduleRoot == "" {
+	base, sysErr := sysshimSourceBase()
+	if sysErr != nil {
 		if win32APIs {
-			return "", fmt.Errorf("cannot find wasmforge module root for sysshim injection")
+			return "", fmt.Errorf("locating wasmforge sysshim source: %w", sysErr)
 		}
 		return "", nil
 	}
-	sysshimDir := filepath.Join(moduleRoot, "internal", "sysshim")
+	sysshimDir := filepath.Join(base, "sysshim")
 	if _, err := os.Stat(sysshimDir); err != nil {
 		if win32APIs {
 			return "", fmt.Errorf("sysshim directory not found at %s", sysshimDir)
@@ -567,11 +581,11 @@ func injectPuregoSysshimReplace(pkgDir, tmpDir string, verbose bool) (string, er
 		return "", nil
 	}
 
-	moduleRoot := findModuleRoot()
+	base, sysErr := sysshimSourceBase()
-	if moduleRoot == "" {
+	if sysErr != nil {
-		return "", fmt.Errorf("cannot find wasmforge module root for purego sysshim injection")
+		return "", fmt.Errorf("locating wasmforge purego sysshim source: %w", sysErr)
 	}
-	puregoSysshimDir := filepath.Join(moduleRoot, "internal", "sysshim", "purego")
+	puregoSysshimDir := filepath.Join(base, "sysshim", "purego")
 	if _, err := os.Stat(puregoSysshimDir); err != nil {
 		return "", fmt.Errorf("purego sysshim directory not found at %s", puregoSysshimDir)
 	}