Add context

This commit adds and passes context around to (almost) every function.
This allows cancelling (almost) every function call globally.

pkg/cancel/copy.go

@@ -0,0 +1,57 @@
+package cancel
+
+import (
+	"context"
+	"io"
+	"time"
+)
+
+type SetDeadline interface {
+	SetDeadline(t time.Time)
+}
+
+type SetReadDeadline interface {
+	SetReadDeadline(t time.Time) error
+}
+
+type SetWriteDeadline interface {
+	SetWriteDeadline(t time.Time) error
+}
+
+// Copy reads from src, and copies to dst.
+//
+// If the context is closed before src is closed, attempts to close the underlying reader and writer.
+func Copy(ctx context.Context, dst io.Writer, src io.Reader) (written int64, err error) {
+
+	// if the context has a deadline, the propanate that deadline to the underyling file.
+	// this might cause the read call to not block.
+	if deadline, ok := ctx.Deadline(); ok {
+		var zero time.Time
+
+		if file, ok := src.(SetReadDeadline); ok {
+			file.SetReadDeadline(deadline)
+			defer file.SetReadDeadline(zero)
+		} else if file, ok := src.(SetDeadline); ok {
+			file.SetDeadline(deadline)
+			defer file.SetDeadline(zero)
+		}
+
+		if file, ok := dst.(SetWriteDeadline); ok {
+			file.SetWriteDeadline(deadline)
+			defer file.SetWriteDeadline(zero)
+		} else if file, ok := dst.(SetDeadline); ok {
+			file.SetDeadline(deadline)
+			defer file.SetDeadline(zero)
+		}
+	}
+
+	written, err, _ = WithContext2(ctx, func(start func()) (int64, error) {
+		start()
+		return io.Copy(dst, src)
+	}, func() {
+		if closer, ok := src.(io.Closer); ok {
+			closer.Close()
+		}
+	})
+	return written, err
+}