tokio-xmpp: Send the initial stream:stream in a single packet

Prosody would let the stream opening timeout if it receives it in more
than one TCP packet, which seems like a bug.

The behaviour was highly non-deterministic, a release build would almost
always (but not always) succeed to connect to a localhost Prosody, a
debug build would almost always (but not always) fail, and
RUST_LOG=trace would make any build fail to connect, as would strace.

My theory is that successful runs rely on the kernel merging all five
TCP packets in a single one, and any tracing would add sufficient delay
in-between to let it send every packet on its own.

tokio-xmpp/ChangeLog

@@ -16,6 +16,8 @@ Version NEXT:
         `tokio_xmpp::connect::DnsConfig`
     * Fixed:
       - Ignore missing "version" stream attribute for 0114 components.
+      - Always send the stream:stream opening in one TCP packet, to workaround
+        issue #1995 in Prosody. (!680)
       - Gate `AsRawFd` behind `ktls` feature to make Windows build work again.
       - Implicitly use the `Client`'s bound JID on empty `from` and `to` in
         tokio-xmpp's `IqResponseTracker`.

tokio-xmpp/src/xmlstream/common.rs

@@ -803,48 +803,38 @@ impl StreamHeader<'_> {
         mut stream: Pin<&mut RawXmlStream<Io>>,
     ) -> io::Result<()> {
         stream
-            .send(Item::XmlDeclaration(rxml::XmlVersion::V1_0))
-            .await?;
-        stream
-            .send(Item::ElementHeadStart(
-                Namespace::from(XML_STREAM_NS),
-                Cow::Borrowed(xml_ncname!("stream")),
-            ))
-            .await?;
+            .as_mut()
+            .start_send(Item::XmlDeclaration(rxml::XmlVersion::V1_0))?;
+        stream.as_mut().start_send(Item::ElementHeadStart(
+            Namespace::from(XML_STREAM_NS),
+            Cow::Borrowed(xml_ncname!("stream")),
+        ))?;
         if let Some(from) = self.from {
-            stream
-                .send(Item::Attribute(
-                    Namespace::NONE,
-                    Cow::Borrowed(xml_ncname!("from")),
-                    from,
-                ))
-                .await?;
+            stream.as_mut().start_send(Item::Attribute(
+                Namespace::NONE,
+                Cow::Borrowed(xml_ncname!("from")),
+                from,
+            ))?;
         }
         if let Some(to) = self.to {
-            stream
-                .send(Item::Attribute(
-                    Namespace::NONE,
-                    Cow::Borrowed(xml_ncname!("to")),
-                    to,
-                ))
-                .await?;
+            stream.as_mut().start_send(Item::Attribute(
+                Namespace::NONE,
+                Cow::Borrowed(xml_ncname!("to")),
+                to,
+            ))?;
         }
         if let Some(id) = self.id {
-            stream
-                .send(Item::Attribute(
-                    Namespace::NONE,
-                    Cow::Borrowed(xml_ncname!("id")),
-                    id,
-                ))
-                .await?;
-        }
-        stream
-            .send(Item::Attribute(
+            stream.as_mut().start_send(Item::Attribute(
                 Namespace::NONE,
-                Cow::Borrowed(xml_ncname!("version")),
-                Cow::Borrowed("1.0"),
-            ))
-            .await?;
+                Cow::Borrowed(xml_ncname!("id")),
+                id,
+            ))?;
+        }
+        stream.as_mut().start_send(Item::Attribute(
+            Namespace::NONE,
+            Cow::Borrowed(xml_ncname!("version")),
+            Cow::Borrowed("1.0"),
+        ))?;
         stream.send(Item::ElementHeadEnd).await?;
         Ok(())
     }