Devolutions · clintcan · Jun 2, 2026
diff --git a/crates/ironrdp-rdpsnd/Cargo.toml b/crates/ironrdp-rdpsnd/Cargo.toml
@@ -14,7 +14,7 @@ categories.workspace = true
 
 [lib]
 doctest = false
-test = false
+# test = false
 
 [features]
 default = []

diff --git a/crates/ironrdp-rdpsnd/src/server.rs b/crates/ironrdp-rdpsnd/src/server.rs
@@ -28,34 +28,77 @@ pub enum RdpsndServerMessage {
     Error(Box<dyn RdpsndError>),
 }
 
+/// A server-offered audio format that the client also advertised support for,
+/// paired with the `wFormatNo` the client expects for it on the wire.
+///
+/// The crate computes the set of these — the intersection of the server's
+/// [`get_formats`] and the client's accepted formats — and hands it to
+/// [`RdpsndServerHandler::choose_format`], which returns the one to stream.
+///
+/// `wformat_no` is intentionally private and there is no public constructor:
+/// a handler can neither build nor mutate a `NegotiatedFormat`, so the index
+/// stamped onto every Wave/Wave2 PDU is always a valid position in the
+/// client's own format list. This makes it impossible to emit an out-of-range
+/// `wFormatNo` (which a compliant client rejects, silently dropping all audio
+/// — the classic footgun of the old index-returning API).
+///
+/// [`get_formats`]: RdpsndServerHandler::get_formats
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct NegotiatedFormat {
+    /// The negotiated audio format (common to server and client).
+    format: pdu::AudioFormat,
+    /// Position of `format` in the client's Client Audio Formats list — the
+    /// `wFormatNo` the client resolves each wave against. Crate-owned.
+    wformat_no: u16,
+}
+
+impl NegotiatedFormat {
+    /// The negotiated audio format — common to both server and client, and the
+    /// one the returned wave data should match.
+    pub fn format(&self) -> &pdu::AudioFormat {
+        &self.format
+    }
+}
+
 /// Handler for the server side of the Audio Output Virtual Channel (`RDPSND`).
 ///
-/// Implementations supply the list of audio formats the server offers, decide
-/// which format to use once the client replies, and produce the audio waves to
-/// stream (via [`RdpsndServer::wave`]).
+/// Implementations supply the list of audio formats the server offers, choose
+/// which negotiated format to use once the client replies, and produce the
+/// audio waves to stream (via [`RdpsndServer::wave`]).
 pub trait RdpsndServerHandler: Send + core::fmt::Debug {
     /// The audio formats the server advertises in the Server Audio Formats and
     /// Version PDU (MS-RDPEA 2.2.2.1).
     fn get_formats(&self) -> &[pdu::AudioFormat];
 
-    /// Called once the client has replied with the formats it accepts
-    /// (`client_format`, the Client Audio Formats and Version PDU). Returns the
-    /// `wFormatNo` to stamp on every subsequent Wave/Wave2 PDU, or [`None`] if
-    /// no offered format is acceptable (no audio is then streamed).
+    /// Select which format to stream, once the client has replied with the
+    /// formats it accepts.
     ///
-    /// **The returned index addresses `client_format.formats` — the formats the
-    /// client just echoed back — NOT the server's own [`get_formats`] list.**
-    /// The client resolves each wave's format as `ClientFormats[wFormatNo]`
-    /// against the list *it* sent, and a compliant client rejects any
-    /// `wFormatNo >= client_format.formats.len()`, silently dropping all audio.
-    /// The client's list is its accepted subset of the server's formats, so the
-    /// two lists generally differ in both length and ordering; an index into
-    /// [`get_formats`] only happens to work when the chosen format sits at the
-    /// same position in both. Pick the format you intend to send, then return
-    /// its position within `client_format.formats`.
+    /// `common` is the set of formats from [`get_formats`] that the client also
+    /// advertised, in the server's preference order; each carries the
+    /// `wFormatNo` the client expects, so the crate — not the handler — owns
+    /// the index arithmetic and the MS-RDPEA rule that `wFormatNo` addresses
+    /// the *client's* list. `common` is never empty: when server and client
+    /// share no format, this method is not called and no audio is streamed.
+    ///
+    /// Return the [`NegotiatedFormat`] to stream (a reference borrowed from
+    /// `common`), or [`None`] to decline. Returning a borrow from `common`
+    /// — rather than an index or a constructed value — makes it impossible to
+    /// pick a format the client did not accept or to produce an invalid
+    /// `wFormatNo`. This is a pure selection step: any encoder/producer setup
+    /// belongs in [`start`], which the crate calls next with the chosen format.
     ///
     /// [`get_formats`]: RdpsndServerHandler::get_formats
-    fn start(&mut self, client_format: &ClientAudioFormatPdu) -> Option<u16>;
+    /// [`start`]: RdpsndServerHandler::start
+    fn choose_format<'a>(&mut self, common: &'a [NegotiatedFormat]) -> Option<&'a NegotiatedFormat>;
+
+    /// Begin streaming with the `format` just selected by [`choose_format`].
+    ///
+    /// Called once per session, immediately after a successful
+    /// [`choose_format`]. This is the lifecycle hook: initialize encoder state,
+    /// spawn the producer, etc. Waves are then emitted via [`RdpsndServer::wave`].
+    ///
+    /// [`choose_format`]: RdpsndServerHandler::choose_format
+    fn start(&mut self, format: &NegotiatedFormat);
 
     /// Called when the audio stream is torn down (e.g. the client closed the
     /// channel or the session ended).
@@ -173,6 +216,43 @@ impl RdpsndServer {
     }
 }
 
+/// Build the set of formats common to the server (`server_formats`, kept in the
+/// server's preference order) and the client (`client_formats`), each tagged
+/// with its `wFormatNo` — its index in the *client's* list, which is what the
+/// client resolves waves against (MS-RDPEA). The result mirrors the server's
+/// ordering so the handler can express preference simply by `get_formats`
+/// order, while the `wFormatNo` always points into the client list.
+fn negotiate_formats(
+    server_formats: &[pdu::AudioFormat],
+    client_formats: &[pdu::AudioFormat],
+) -> Vec<NegotiatedFormat> {
+    server_formats
+        .iter()
+        .filter_map(|server_format| {
+            client_formats
+                .iter()
+                .position(|client_fmt| audio_format_eq(client_fmt, server_format))
+                .and_then(|idx| u16::try_from(idx).ok())
+                .map(|wformat_no| NegotiatedFormat {
+                    format: server_format.clone(),
+                    wformat_no,
+                })
+        })
+        .collect()
+}
+
+/// Compare two audio formats by their WAVEFORMATEX identity — wave format tag,
+/// channel count, sample rate, and bit depth. Derived fields
+/// (`n_avg_bytes_per_sec`, `n_block_align`) and the codec-specific `data` blob
+/// are deliberately ignored: a client echoes back a format it accepts but is
+/// not guaranteed to reproduce those byte-for-byte.
+fn audio_format_eq(a: &pdu::AudioFormat, b: &pdu::AudioFormat) -> bool {
+    a.format == b.format
+        && a.n_channels == b.n_channels
+        && a.n_samples_per_sec == b.n_samples_per_sec
+        && a.bits_per_sample == b.bits_per_sample
+}
+
 impl_as_any!(RdpsndServer);
 
 impl SvcProcessor for RdpsndServer {
@@ -220,8 +300,27 @@ impl SvcProcessor for RdpsndServer {
                     return Ok(vec![]);
                 };
                 let client_format = self.client_format.as_ref().expect("available in this state");
+                // Formats common to server and client, in the server's
+                // preference order, each tagged with its wFormatNo (its
+                // position in the *client's* list). Keeping this in the crate
+                // means the handler never does index arithmetic and can't emit
+                // an out-of-range wFormatNo.
+                let common = negotiate_formats(self.handler.get_formats(), &client_format.formats);
                 self.state = RdpsndState::Ready;
-                self.format_no = self.handler.start(client_format);
+                self.format_no = if common.is_empty() {
+                    debug!("No audio format in common with the client; audio disabled");
+                    None
+                } else if let Some(chosen) = self.handler.choose_format(&common) {
+                    // `chosen` borrows `common` (not `self`), so the encoder
+                    // is read off it and the handler is free to borrow again
+                    // for the `start` lifecycle hook.
+                    let wformat_no = chosen.wformat_no;
+                    self.handler.start(chosen);
+                    Some(wformat_no)
+                } else {
+                    debug!("Handler declined every common audio format; audio disabled");
+                    None
+                };
                 vec![]
             }
             RdpsndState::Ready => {
@@ -260,3 +359,77 @@ impl Drop for RdpsndServer {
 }
 
 impl SvcServerProcessor for RdpsndServer {}
+
+#[cfg(test)]
+mod tests {
+    use super::{audio_format_eq, negotiate_formats};
+    use crate::pdu::{AudioFormat, WaveFormat};
+
+    fn fmt(format: WaveFormat, rate: u32) -> AudioFormat {
+        AudioFormat {
+            format,
+            n_channels: 2,
+            n_samples_per_sec: rate,
+            n_avg_bytes_per_sec: rate * 4,
+            n_block_align: 4,
+            bits_per_sample: 16,
+            data: None,
+        }
+    }
+
+    #[test]
+    fn wformat_no_addresses_the_client_list_not_the_server_list() {
+        // Server prefers AAC over PCM; the client lists them in the opposite
+        // order. wFormatNo must follow the CLIENT's indices.
+        let server = [fmt(WaveFormat::AAC_MS, 44100), fmt(WaveFormat::PCM, 44100)];
+        let client = [fmt(WaveFormat::PCM, 44100), fmt(WaveFormat::AAC_MS, 44100)];
+
+        let common = negotiate_formats(&server, &client);
+
+        // Ordering follows the server's preference (AAC first)...
+        assert_eq!(common.len(), 2);
+        assert_eq!(common[0].format().format, WaveFormat::AAC_MS);
+        assert_eq!(common[1].format().format, WaveFormat::PCM);
+        // ...but each wFormatNo is the position in the CLIENT list.
+        assert_eq!(common[0].wformat_no, 1); // AAC is client index 1
+        assert_eq!(common[1].wformat_no, 0); // PCM is client index 0
+    }
+
+    #[test]
+    fn pcm_only_client_gets_a_valid_client_index() {
+        // Regression for the --enable-aac trap: server advertises [AAC, PCM]
+        // but a PCM-only client must get wFormatNo 0 (its sole index), not
+        // PCM's server-list index of 1 (which the client would reject).
+        let server = [fmt(WaveFormat::AAC_MS, 44100), fmt(WaveFormat::PCM, 44100)];
+        let client = [fmt(WaveFormat::PCM, 44100)];
+
+        let common = negotiate_formats(&server, &client);
+
+        assert_eq!(common.len(), 1);
+        assert_eq!(common[0].format().format, WaveFormat::PCM);
+        assert_eq!(common[0].wformat_no, 0);
+    }
+
+    #[test]
+    fn no_shared_format_yields_empty() {
+        let server = [fmt(WaveFormat::OPUS, 48000)];
+        let client = [fmt(WaveFormat::PCM, 44100)];
+        assert!(negotiate_formats(&server, &client).is_empty());
+    }
+
+    #[test]
+    fn equality_uses_waveformatex_identity_only() {
+        let mut a = fmt(WaveFormat::PCM, 44100);
+        let mut b = fmt(WaveFormat::PCM, 44100);
+        // Differ only in derived/codec fields — still the same format.
+        b.n_avg_bytes_per_sec = 0;
+        b.n_block_align = 99;
+        a.data = Some(vec![1, 2, 3]);
+        b.data = None;
+        assert!(audio_format_eq(&a, &b));
+
+        // A differing identity field (sample rate) is a different format.
+        let c = fmt(WaveFormat::PCM, 48000);
+        assert!(!audio_format_eq(&a, &c));
+    }
+}
diff --git a/crates/ironrdp/examples/server.rs b/crates/ironrdp/examples/server.rs
@@ -11,8 +11,8 @@ use std::sync::{Arc, Mutex};
 use anyhow::Context as _;
 use ironrdp::cliprdr::backend::{CliprdrBackend, CliprdrBackendFactory};
 use ironrdp::connector::DesktopSize;
-use ironrdp::rdpsnd::pdu::{AudioFormat, ClientAudioFormatPdu, WaveFormat};
-use ironrdp::rdpsnd::server::{RdpsndServerHandler, RdpsndServerMessage};
+use ironrdp::rdpsnd::pdu::{AudioFormat, WaveFormat};
+use ironrdp::rdpsnd::server::{NegotiatedFormat, RdpsndServerHandler, RdpsndServerMessage};
 use ironrdp::server::tokio::sync::mpsc::UnboundedSender;
 use ironrdp::server::tokio::time::{self, Duration, sleep};
 use ironrdp::server::{
@@ -255,17 +255,6 @@ struct SndHandler {
     task: Option<tokio::task::JoinHandle<()>>,
 }
 
-impl SndHandler {
-    fn choose_format(&self, client_formats: &[AudioFormat]) -> Option<u16> {
-        for (n, fmt) in client_formats.iter().enumerate() {
-            if self.get_formats().contains(fmt) {
-                return u16::try_from(n).ok();
-            }
-        }
-        None
-    }
-}
-
 impl RdpsndServerHandler for SndHandler {
     fn get_formats(&self) -> &[AudioFormat] {
         &[
@@ -290,30 +279,32 @@ impl RdpsndServerHandler for SndHandler {
         ]
     }
 
-    fn start(&mut self, client_format: &ClientAudioFormatPdu) -> Option<u16> {
-        debug!(?client_format);
+    fn choose_format<'a>(&mut self, common: &'a [NegotiatedFormat]) -> Option<&'a NegotiatedFormat> {
+        debug!(?common);
 
-        let Some(nfmt) = self.choose_format(&client_format.formats) else {
-            return Some(0);
-        };
+        // The crate hands us the formats common to both peers in our preference
+        // order; take the most-preferred one.
+        common.first()
+    }
 
-        let fmt = client_format.formats[usize::from(nfmt)].clone();
+    fn start(&mut self, format: &NegotiatedFormat) {
+        let fmt = format.format().clone();
 
         let mut opus_enc = if fmt.format == WaveFormat::OPUS {
             let n_channels: opus2::Channels = match fmt.n_channels {
                 1 => opus2::Channels::Mono,
                 2 => opus2::Channels::Stereo,
                 n => {
                     warn!("Invalid OPUS channels: {}", n);
-                    return Some(0);
+                    return;
                 }
             };
 
             match opus2::Encoder::new(fmt.n_samples_per_sec, n_channels, opus2::Application::Audio) {
                 Ok(enc) => Some(enc),
                 Err(err) => {
                     warn!("Failed to create OPUS encoder: {}", err);
-                    return Some(0);
+                    return;
                 }
             }
         } else {
@@ -348,8 +339,6 @@ impl RdpsndServerHandler for SndHandler {
                 ts = ts.wrapping_add(100);
             }
         }));
-
-        Some(nfmt)
     }
 
     fn stop(&mut self) {