Skip() return true if the end of the stream is reached (#3242)

* return true if the end of the stream is reached

* update the comment with protocolbuffers/protobuf@ab25c3e

---------

Co-authored-by: zhaohanqing <zhaohanqing@kuaishou.com>

Author: x1wan

src/butil/iobuf.cpp

@@ -1849,10 +1849,14 @@ void IOBufAsZeroCopyInputStream::BackUp(int count) {
     }
 }
 
-// Skips a number of bytes.  Returns false if the end of the stream is
-// reached or some input error occurred.  In the end-of-stream case, the
-// stream is advanced to the end of the stream (so ByteCount() will return
-// the total size of the stream).
+// Skips `count` number of bytes.
+// Returns true on success, or false if some input error occurred, or `count`
+// exceeds the end of the stream. This function may skip up to `count - 1`
+// bytes in case of failure.
+//
+// Preconditions:
+// * `count` is non-negative.
+//
 bool IOBufAsZeroCopyInputStream::Skip(int count) {
     const IOBuf::BlockRef* cur_ref = _buf->_pref_at(_ref_index);
     while (cur_ref) {
@@ -1867,7 +1871,7 @@ bool IOBufAsZeroCopyInputStream::Skip(int count) {
         _byte_count += left_bytes;
         cur_ref = _buf->_pref_at(++_ref_index);
     }
-    return false;
+    return (0 == count);
 }
 
 int64_t IOBufAsZeroCopyInputStream::ByteCount() const {
@@ -2102,4 +2106,4 @@ bool IOBufBytesIterator::forward_one_block(const void** data, size_t* size) {
 
 void* fast_memcpy(void *__restrict dest, const void *__restrict src, size_t n) {
     return butil::iobuf::cp(dest, src, n);
-} // namespace butil
+} // namespace butil