diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 7401213e3..f5c709043 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -306,6 +306,33 @@ hb_buffer_t::clear ()
   scratch_flags = HB_BUFFER_SCRATCH_FLAG_DEFAULT;
 }
 
+void
+hb_buffer_t::enter ()
+{
+  deallocate_var_all ();
+  serial = 0;
+  scratch_flags = HB_BUFFER_SCRATCH_FLAG_DEFAULT;
+  if (likely (!hb_unsigned_mul_overflows (len, HB_BUFFER_MAX_LEN_FACTOR)))
+  {
+    max_len = hb_max (len * HB_BUFFER_MAX_LEN_FACTOR,
+		      (unsigned) HB_BUFFER_MAX_LEN_MIN);
+  }
+  if (likely (!hb_unsigned_mul_overflows (len, HB_BUFFER_MAX_OPS_FACTOR)))
+  {
+    max_ops = hb_max (len * HB_BUFFER_MAX_OPS_FACTOR,
+		      (unsigned) HB_BUFFER_MAX_OPS_MIN);
+  }
+}
+void
+hb_buffer_t::leave ()
+{
+  max_len = HB_BUFFER_MAX_LEN_DEFAULT;
+  max_ops = HB_BUFFER_MAX_OPS_DEFAULT;
+  deallocate_var_all ();
+  serial = 0;
+}
+
+
 void
 hb_buffer_t::add (hb_codepoint_t  codepoint,
 		  unsigned int    cluster)
diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index f2cbc7580..7d3ef34a2 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -125,8 +125,9 @@ struct hb_buffer_t
   hb_codepoint_t context[2][CONTEXT_LENGTH];
   unsigned int context_len[2];
 
+
   /*
-   * Not part of content.
+   * Managed by enter / leave
    */
 
 #ifndef HB_NDEBUG
@@ -138,7 +139,11 @@ struct hb_buffer_t
   int max_ops; /* Maximum allowed operations. */
   /* The bits here reflect current allocations of the bytes in glyph_info_t's var1 and var2. */
 
-  /* Debugging API */
+
+  /*
+   * Messaging callback
+   */
+
 #ifndef HB_NO_BUFFER_MESSAGE
   hb_buffer_message_func_t message_func;
   void *message_data;
@@ -203,6 +208,10 @@ struct hb_buffer_t
   HB_INTERNAL void reset ();
   HB_INTERNAL void clear ();
 
+  /* Called around shape() */
+  HB_INTERNAL void enter ();
+  HB_INTERNAL void leave ();
+
   unsigned int backtrack_len () const { return have_output ? out_len : idx; }
   unsigned int lookahead_len () const { return len - idx; }
   uint8_t next_serial () { return ++serial ? serial : ++serial; }
diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index dc66d2c8c..11fcc11f9 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -1145,19 +1145,7 @@ hb_propagate_flags (hb_buffer_t *buffer)
 static void
 hb_ot_shape_internal (hb_ot_shape_context_t *c)
 {
-  c->buffer->deallocate_var_all ();
-  c->buffer->serial = 0;
-  c->buffer->scratch_flags = HB_BUFFER_SCRATCH_FLAG_DEFAULT;
-  if (likely (!hb_unsigned_mul_overflows (c->buffer->len, HB_BUFFER_MAX_LEN_FACTOR)))
-  {
-    c->buffer->max_len = hb_max (c->buffer->len * HB_BUFFER_MAX_LEN_FACTOR,
-				 (unsigned) HB_BUFFER_MAX_LEN_MIN);
-  }
-  if (likely (!hb_unsigned_mul_overflows (c->buffer->len, HB_BUFFER_MAX_OPS_FACTOR)))
-  {
-    c->buffer->max_ops = hb_max (c->buffer->len * HB_BUFFER_MAX_OPS_FACTOR,
-				 (unsigned) HB_BUFFER_MAX_OPS_MIN);
-  }
+  c->buffer->enter ();
 
   /* Save the original direction, we use it later. */
   c->target_direction = c->buffer->props.direction;
@@ -1189,10 +1177,7 @@ hb_ot_shape_internal (hb_ot_shape_context_t *c)
 
   c->buffer->props.direction = c->target_direction;
 
-  c->buffer->max_len = HB_BUFFER_MAX_LEN_DEFAULT;
-  c->buffer->max_ops = HB_BUFFER_MAX_OPS_DEFAULT;
-  c->buffer->deallocate_var_all ();
-  c->buffer->serial = 0;
+  c->buffer->leave ();
 }