rbuffer: Enhance rbuffer_reset to work with filled RBuffers

src/nvim/rbuffer.c

@@ -24,11 +24,13 @@ RBuffer *rbuffer_new(size_t capacity)
   rv->size = 0;
   rv->write_ptr = rv->read_ptr = rv->start_ptr;
   rv->end_ptr = rv->start_ptr + capacity;
+  rv->temp = NULL;
   return rv;
 }
 
 void rbuffer_free(RBuffer *buf)
 {
+  xfree(buf->temp);
   xfree(buf);
 }
 
@@ -69,12 +71,20 @@ char *rbuffer_write_ptr(RBuffer *buf, size_t *write_count) FUNC_ATTR_NONNULL_ALL
   return buf->write_ptr;
 }
 
-// Set read and write pointer for an empty RBuffer to the beginning of the
+// Reset an RBuffer so read_ptr is at the beginning of the memory. If
-// buffer.
+// necessary, this moves existing data by allocating temporary memory.
 void rbuffer_reset(RBuffer *buf) FUNC_ATTR_NONNULL_ALL
 {
-  if (buf->size == 0) {
+  size_t temp_size;
-    buf->write_ptr = buf->read_ptr = buf->start_ptr;
+  if ((temp_size = rbuffer_size(buf))) {
+    if (buf->temp == NULL) {
+      buf->temp = xmalloc(rbuffer_capacity(buf));
+    }
+    rbuffer_read(buf, buf->temp, buf->size);
+  }
+  buf->read_ptr = buf->write_ptr = buf->start_ptr;
+  if (temp_size) {
+    rbuffer_write(buf, buf->temp, temp_size);
   }
 }
 

src/nvim/rbuffer.h

@@ -72,6 +72,8 @@ struct rbuffer {
   rbuffer_callback full_cb, nonfull_cb;
   void *data;
   size_t size;
+  // helper memory used to by rbuffer_reset if required
+  char *temp;
   char *end_ptr, *read_ptr, *write_ptr;
   char start_ptr[];
 };