examples/observe/server: fix buf usage

[maribu]

The observe server example uses the same buffer for both the content of
the Content-Format and the Observe option. This does work int he example
by accident, as the used Content-Format is 0 and, hence, is encoded with
a zero-length option value. If modified like this, the issue becomes
apparent:

diff --git a/examples/observe/server/main.go b/examples/observe/server/main.go
index 08664a6..6ab6119 100644
--- a/examples/observe/server/main.go
+++ b/examples/observe/server/main.go
@@ -30,10 +30,10 @@ func sendResponse(cc mux.Client, token []byte, subded time.Time, obs int64) erro
     }
     var opts message.Options
     var buf []byte
-    opts, n, err := opts.SetContentFormat(buf, message.TextPlain)
+    opts, n, err := opts.SetContentFormat(buf, message.AppJSON)
     if err == message.ErrTooSmall {
         buf = append(buf, make([]byte, n)...)
-        opts, _, err = opts.SetContentFormat(buf, message.TextPlain)
+        opts, _, err = opts.SetContentFormat(buf, message.AppJSON)
     }
     if err != nil {
         return fmt.Errorf("cannot set content format to response: %w", err)

As message.AppJSON is a non-zero value, it will actually write content
to the buffer buf. This buffer is subsequently reused for the Observe
option and the value it holds (the 50 for AppJSON) is overwritten by
the Observe Option content, corrupting the outgoing message. (This can
easily verified with Wireshark, which shows the content format to count
up with the observe number after applying the patch above).

This commit uses different buffers for each option. Note that while this
bug is not triggered with message.TextPlain, application developers will
base their code on this example and are bound to be hitting this bug the
moment the use a different content format.

[Danielius1922]

Using two buffers works, though i'd say the idiomatic way is to update the buffer head to be past the bytes written by the last option:

var opts message.Options
var buf []byte
opts, n, err := opts.SetContentFormat(buf, message.TextPlain)
if err == message.ErrTooSmall {
	buf = append(buf, make([]byte, n)...)
	opts, n, err = opts.SetContentFormat(buf, message.TextPlain)
}
if err != nil {
	return fmt.Errorf("cannot set content format to response: %w", err)
}

buf = buf[n:]

if obs >= 0 {
	opts, n, err = opts.SetObserve(buf, uint32(obs))

[maribu]

Updated as suggested. Shall I squash?

[Danielius1922]

Yes, squash and you can merge, good job!

[jkralik]

Thx for your contribution!