1 files changed, 193 insertions, 0 deletions
diff --git a/internal/syscallcompat/emulate.go b/internal/syscallcompat/emulate.go
new file mode 100644
index 0000000..6cb0ea0
--- /dev/null
+++ b/internal/syscallcompat/emulate.go
@@ -0,0 +1,193 @@
+package syscallcompat
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+var chdirMutex sync.Mutex
+
+// emulateOpenat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateOpenat(dirfd int, path string, flags int, mode uint32) (int, error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	if !filepath.IsAbs(path) {
+		// Save the old working directory
+		oldWd, err := os.Getwd()
+		if err != nil {
+			return -1, err
+		}
+		// Chdir to target directory
+		err = syscall.Fchdir(dirfd)
+		if err != nil {
+			return -1, err
+		}
+		// Chdir back at the end
+		defer os.Chdir(oldWd)
+	}
+	return syscall.Open(path, flags, mode)
+}
+
+// emulateRenameat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateRenameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	// Unless both paths are absolute we have to save the old working dir and
+	// Chdir(oldWd) back to it in the end. If we error out before the first
+	// chdir, Chdir(oldWd) is unneccassary but does no harm.
+	if !filepath.IsAbs(oldpath) || !filepath.IsAbs(newpath) {
+		oldWd, err := os.Getwd()
+		if err != nil {
+			return err
+		}
+		defer os.Chdir(oldWd)
+	}
+	// Make oldpath absolute
+	oldpath, err := dirfdAbs(olddirfd, oldpath)
+	if err != nil {
+		return err
+	}
+	// Make newpath absolute
+	newpath, err = dirfdAbs(newdirfd, newpath)
+	if err != nil {
+		return err
+	}
+	return syscall.Rename(oldpath, newpath)
+}
+
+// emulateUnlinkat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateUnlinkat(dirfd int, path string, flags int) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(cwd)
+	err = syscall.Fchdir(dirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	if (flags & unix.AT_REMOVEDIR) != 0 {
+		return syscall.Rmdir(path)
+	} else {
+		return syscall.Unlink(path)
+	}
+}
+
+// emulateMknodat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateMknodat(dirfd int, path string, mode uint32, dev int) error {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	if !filepath.IsAbs(path) {
+		oldWd, err := os.Getwd()
+		if err != nil {
+			return err
+		}
+		defer os.Chdir(oldWd)
+	}
+	path, err := dirfdAbs(dirfd, path)
+	if err != nil {
+		return err
+	}
+	return syscall.Mknod(path, mode, dev)
+}
+
+// dirfdAbs transforms the dirfd-relative "path" to an absolute one. If the
+// path is not already absolute, this function will change the working
+// directory. The caller has to chdir back.
+func dirfdAbs(dirfd int, path string) (string, error) {
+	if filepath.IsAbs(path) {
+		return path, nil
+	}
+	err := syscall.Fchdir(dirfd)
+	if err != nil {
+		return "", err
+	}
+	wd, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(wd, path), nil
+}
+
+// emulateFchmodat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateFchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(cwd)
+	err = syscall.Fchdir(dirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	return syscall.Chmod(path, mode)
+}
+
+// emulateFchownat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateFchownat(dirfd int, path string, uid int, gid int, flags int) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(cwd)
+	err = syscall.Fchdir(dirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	return syscall.Lchown(path, uid, gid)
+}
+
+// emulateSymlinkat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateSymlinkat(oldpath string, newdirfd int, newpath string) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(cwd)
+	err = syscall.Fchdir(newdirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	return syscall.Symlink(oldpath, newpath)
+}
+
+// emulateMkdirat emulates the syscall for platforms that don't have it
+// in the kernel (darwin).
+func emulateMkdirat(dirfd int, path string, mode uint32) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(cwd)
+	err = syscall.Fchdir(dirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	return syscall.Mkdir(path, mode)
+}