[RPC][BUGFIX][BACKPORT-0.6] Fix bug in rpc ring buffer shrink (#5516) (…

…#5537)

src/common/ring_buffer.h

@@ -6,9 +6,9 @@
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
- * 
+ *
  *   http://www.apache.org/licenses/LICENSE-2.0
- * 
+ *
  * Unless required by applicable law or agreed to in writing,
  * software distributed under the License is distributed on an
  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
@@ -49,8 +49,12 @@ class RingBuffer {
     return ring_.size();
   }
   /*!
-   * Reserve capacity to be at least n.
-   * Will only increase capacity if n is bigger than current capacity.
+   *  Reserve capacity to be at least n.
+   *  Will only increase capacity if n is bigger than current capacity.
+   *
+   *  The effect of Reserve only lasts before the next call to Reserve.
+   *  Other functions in the ring buffer can also call into the reserve.
+   *
    * \param n The size of capacity.
    */
   void Reserve(size_t n) {
@@ -63,19 +67,27 @@ class RingBuffer {
           size_t ncopy = head_ptr_ + bytes_available_ - old_size;
           memcpy(&ring_[0] + old_size, &ring_[0], ncopy);
         }
-    } else if (ring_.size() > n * 8 && ring_.size() > kInitCapacity) {
-        // shrink too large temporary buffer to avoid out of memory on some embedded devices
+    } else if (ring_.size() > n * 8 &&
+               ring_.size() > kInitCapacity) {
+      // shrink too large temporary buffer to
+      // avoid out of memory on some embedded devices
+      if (bytes_available_ != 0) {
+        // move existing bytes to the head.
         size_t old_bytes = bytes_available_;
-
         std::vector<char> tmp(old_bytes);
-
         Read(&tmp[0], old_bytes);
-        ring_.resize(kInitCapacity);
-        ring_.shrink_to_fit();
 
         memcpy(&ring_[0], &tmp[0], old_bytes);
-        head_ptr_ = 0;
         bytes_available_ = old_bytes;
+      }
+      // shrink the ring.
+      size_t new_size  = kInitCapacity;
+      new_size = std::max(new_size, n);
+      new_size = std::max(new_size, bytes_available_);
+
+      ring_.resize(new_size);
+      ring_.shrink_to_fit();
+      head_ptr_ = 0;
     }
   }