{snapshot,backup}: Keep track of errors when writing report

pkg/countwriter/writer.go

@@ -0,0 +1,62 @@
+package countwriter
+
+import (
+	"io"
+)
+
+// CountWriter wraps an io.Writer, see [NewCountWriter].
+//
+// It is intended to be used to count different writes to an underlying writer.
+// Once an error occurs, no more writes are passed through, and the underlying error is returned instead.
+// This means that in practice, calls to write can be continued and are ignored silently.
+//
+// The underlying sum of bytes written and error can be seen using [Sum].
+type CountWriter struct {
+	w io.Writer
+
+	n   int
+	err error
+}
+
+// NewCountWriter creates a new [CountWriter] that delegates to w.
+func NewCountWriter(w io.Writer) *CountWriter {
+	return &CountWriter{w: w}
+}
+
+// write performs the write operation w on this writer.
+func (cw *CountWriter) write(w func() (int, error)) (int, error) {
+	// if there was an error, return it and don't do a write
+	if cw.err != nil {
+		return 0, cw.err
+	}
+
+	// call the writer
+	n, err := w()
+
+	// update the underling state
+	cw.n += n
+	cw.err = err
+
+	// and return
+	return n, err
+}
+
+// Write implements [io.Writer]
+func (cw *CountWriter) Write(p []byte) (int, error) {
+	return cw.write(func() (int, error) {
+		return cw.w.Write(p)
+	})
+}
+
+// WriteString implements [io.WriteString].
+// See [Write].
+func (cw *CountWriter) WriteString(s string) (int, error) {
+	return cw.write(func() (int, error) {
+		return io.WriteString(cw.w, s)
+	})
+}
+
+// Sum returns the state, that is the total number of bytes written and any error
+func (cw *CountWriter) Sum() (int, error) {
+	return cw.n, cw.err
+}