syscallcompat: add Readlinkat

We need readlinkat to implement Readlink
symlink-race-free.

1 files changed, 23 insertions, 0 deletions

diff --git a/internal/syscallcompat/sys_common.go b/internal/syscallcompat/sys_common.go
new file mode 100644
index 0000000..440b42f
--- /dev/null
+++ b/internal/syscallcompat/sys_common.go
@@ -0,0 +1,23 @@
+package syscallcompat
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// Readlinkat exists both in Linux and in MacOS 10.10+. We may add an
+// emulated version for users on older MacOS versions if there is
+// demand.
+// Buffer allocation is handled internally, unlike the bare unix.Readlinkat.
+func Readlinkat(dirfd int, path string) (string, error) {
+	// Allocate the buffer exponentially like os.Readlink does.
+	for bufsz := 128; ; bufsz *= 2 {
+		buf := make([]byte, bufsz)
+		n, err := unix.Readlinkat(dirfd, path, buf)
+		if err != nil {
+			return "", err
+		}
+		if n < bufsz {
+			return string(buf[0:n]), nil
+		}
+	}
+}