Support scan-keys command to scan the key space starting a specific revision

Signed-off-by: Benjamin Wang <benjamin.ahrtr@gmail.com>

tools/etcd-dump-db/utils.go

@@ -14,9 +14,34 @@
 
 package main
 
-import "os"
+import (
+	"os"
+	"unsafe"
+)
 
 func existFileOrDir(name string) bool {
 	_, err := os.Stat(name)
 	return err == nil
 }
+
+func unsafeAdd(base unsafe.Pointer, offset uintptr) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(base) + offset)
+}
+
+func unsafeIndex(base unsafe.Pointer, offset uintptr, elemsz uintptr, n int) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(base) + offset + uintptr(n)*elemsz)
+}
+
+func unsafeByteSlice(base unsafe.Pointer, offset uintptr, i, j int) []byte {
+	// See: https://github.com/golang/go/wiki/cgo#turning-c-arrays-into-go-slices
+	//
+	// This memory is not allocated from C, but it is unmanaged by Go's
+	// garbage collector and should behave similarly, and the compiler
+	// should produce similar code.  Note that this conversion allows a
+	// subslice to begin after the base address, with an optional offset,
+	// while the URL above does not cover this case and only slices from
+	// index 0.  However, the wiki never says that the address must be to
+	// the beginning of a C allocation (or even that malloc was used at
+	// all), so this is believed to be correct.
+	return (*[pageMaxAllocSize]byte)(unsafeAdd(base, offset))[i:j:j]
+}