1 files changed, 46 insertions, 0 deletions
diff --git a/common/copy.go b/common/copy.go
new file mode 100644
index 0000000..a896fb4
--- /dev/null
+++ b/common/copy.go
@@ -0,0 +1,46 @@
+package common
+
+/*
+Copied from https://ixday.github.io/post/golang-cancel-copy/
+*/
+
+import (
+	"context"
+	"io"
+)
+
+// here is some syntax sugar inspired by the Tomas Senart's video,
+// it allows me to inline the Reader interface
+type readerFunc func(p []byte) (n int, err error)
+
+func (rf readerFunc) Read(p []byte) (n int, err error) { return rf(p) }
+
+// Copy slightly modified function signature:
+// - context has been added in order to propagate cancellation
+// - (undo by Keuin) I do not return the number of bytes written, has it is not useful in my use case
+// - (added by Keuin) add a isCancelled return value indicating the copy is stopped by cancelling the context
+func Copy(ctx context.Context, out io.Writer, in io.Reader) (written int64, err error, isCancelled bool) {
+	isCancelled = false
+
+	// Copy will call the Reader and Writer interface multiple time, in order
+	// to copy by chunk (avoiding loading the whole file in memory).
+	// I insert the ability to cancel before read time as it is the earliest
+	// possible in the call process.
+	written, err = io.Copy(out, readerFunc(func(p []byte) (int, error) {
+
+		// golang non-blocking channel: https://gobyexample.com/non-blocking-channel-operations
+		select {
+
+		// if context has been canceled
+		case <-ctx.Done():
+			// stop process and propagate "context canceled" error
+			isCancelled = true
+			return 0, ctx.Err()
+		default:
+			// otherwise just run default io.Reader implementation
+			return in.Read(p)
+		}
+	}))
+
+	return
+}