s2: add GetBufferCapacity() method (#832)

Add GetBufferCapacity() method. We are reusing readers with sync.Pool
and we'd like to avoid allocating memory for the default block size
since most of the inputs are smaller. To have a better estimate of how
big the lazy buffer should be, we are thinking about keeping in mind a
running average of the internal buffer capacities. This method would
allow us to implement that.

s2/reader.go

@@ -147,6 +147,13 @@ type Reader struct {
 	ignoreCRC      bool
 }
 
+// GetBufferCapacity returns the capacity of the internal buffer.
+// This might be useful to know when reusing the same reader in combination
+// with the lazy buffer option.
+func (r *Reader) GetBufferCapacity() int {
+	return cap(r.buf)
+}
+
 // ensureBufferSize will ensure that the buffer can take at least n bytes.
 // If false is returned the buffer exceeds maximum allowed size.
 func (r *Reader) ensureBufferSize(n int) bool {