utils/streamproducer: add ConsumptionLink

Having an explicit link object makes it much easier to build complex
producers/consumers dynamics graphs.
It provides helper to switch producers and prevent accidentally keeping
broken links around as they are disconnected on Drop.

gstreamer-utils/Cargo.toml

@@ -20,6 +20,7 @@ gst = { package = "gstreamer", path = "../gstreamer", features = ["v1_20"] }
 gst_app = { package = "gstreamer-app", path = "../gstreamer-app", features = ["v1_20"] }
 gst_video = { package = "gstreamer-video", path = "../gstreamer-video", features = ["v1_20"] }
 once_cell = "1"
+thiserror = "1"
 
 [dev-dependencies]
 futures = { version = "0.3", features = ["executor"] }

gstreamer-utils/src/lib.rs

@@ -1,3 +1,3 @@
 mod streamproducer;
 
-pub use crate::streamproducer::StreamProducer;
+pub use crate::streamproducer::{ConsumptionLink, StreamProducer};

gstreamer-utils/src/streamproducer.rs

@@ -3,6 +3,7 @@ use std::mem;
 use std::sync::{atomic, Arc, Mutex};
 
 use once_cell::sync::Lazy;
+use thiserror::Error;
 
 use gst::{glib, prelude::*};
 
@@ -35,6 +36,47 @@ impl PartialEq for StreamProducer {
 
 impl Eq for StreamProducer {}
 
+/// Link between a `StreamProducer` and a consumer, disconnecting the link on `Drop`.
+/// The producer and consumer will stay alive while the link is.
+#[derive(Debug)]
+pub struct ConsumptionLink {
+    consumer: gst_app::AppSrc,
+    producer: Option<StreamProducer>,
+}
+
+impl ConsumptionLink {
+    /// Replace the producer by a new one, keeping the existing consumer.
+    pub fn change_producer(
+        &mut self,
+        new_producer: &StreamProducer,
+    ) -> Result<(), AddConsumerError> {
+        self.disconnect();
+        *self = new_producer.add_consumer(&self.consumer)?;
+        Ok(())
+    }
+
+    /// Disconnect the consumer from the producer
+    pub fn disconnect(&mut self) {
+        if let Some(producer) = self.producer.take() {
+            producer.remove_consumer(&self.consumer);
+        }
+    }
+}
+
+impl Drop for ConsumptionLink {
+    fn drop(&mut self) {
+        self.disconnect();
+    }
+}
+
+#[derive(Debug, Error)]
+/// Error type returned when adding consumers to producers.
+pub enum AddConsumerError {
+    #[error("Consumer already added")]
+    /// Consumer has already been added to this producer.
+    AlreadyAdded,
+}
+
 impl StreamProducer {
     /// Configure a consumer `appsrc` for later use in a `StreamProducer`
     ///
@@ -53,12 +95,17 @@ impl StreamProducer {
         consumer.set_automatic_eos(false);
     }
 
-    /// Add an appsrc to dispatch data to
-    pub fn add_consumer(&self, consumer: &gst_app::AppSrc) {
+    /// Add an appsrc to dispatch data to.
+    ///
+    /// Dropping the returned `ConsumptionLink` will automatically disconnect the consumer from the producer.
+    pub fn add_consumer(
+        &self,
+        consumer: &gst_app::AppSrc,
+    ) -> Result<ConsumptionLink, AddConsumerError> {
         let mut consumers = self.consumers.lock().unwrap();
         if consumers.consumers.contains_key(consumer) {
             gst::error!(CAT, obj: &self.appsink, "Consumer {} ({:?}) already added", consumer.name(), consumer);
-            return;
+            return Err(AddConsumerError::AlreadyAdded);
         }
 
         gst::debug!(CAT, obj: &self.appsink, "Adding consumer {} ({:?})", consumer.name(), consumer);
@@ -88,6 +135,11 @@ impl StreamProducer {
             consumer.clone(),
             StreamConsumer::new(consumer, fku_probe_id),
         );
+
+        Ok(ConsumptionLink {
+            consumer: consumer.clone(),
+            producer: Some(self.clone()),
+        })
     }
 
     /// Remove a consumer appsrc by id
@@ -396,7 +448,7 @@ mod tests {
     use futures::{channel::mpsc::Receiver, SinkExt, StreamExt};
     use gst::prelude::*;
 
-    use crate::StreamProducer;
+    use crate::{ConsumptionLink, StreamProducer};
 
     fn create_producer() -> (
         gst::Pipeline,
@@ -482,13 +534,13 @@ mod tests {
             }
         }
 
-        fn connect(&self, producer: &StreamProducer) {
+        fn connect(&self, producer: &StreamProducer) -> ConsumptionLink {
             {
                 let mut connected = self.connected.lock().unwrap();
                 *connected = true;
             }
 
-            producer.add_consumer(&self.src);
+            producer.add_consumer(&self.src).unwrap()
         }
 
         fn disconnect(&self, producer: &StreamProducer) {
@@ -513,7 +565,7 @@ mod tests {
 
         let mut consumers: Vec<Consumer> = Vec::new();
         let consumer = Consumer::new("consumer1");
-        consumer.connect(&producer);
+        let _link = consumer.connect(&producer);
         consumer
             .pipeline
             .set_state(gst::State::Playing)
@@ -521,7 +573,7 @@ mod tests {
         consumers.push(consumer);
 
         let consumer = Consumer::new("consumer2");
-        consumer.connect(&producer);
+        let _link = consumer.connect(&producer);
         consumer
             .pipeline
             .set_state(gst::State::Playing)