Rust SDK: Document that you need to call flush_blocking when using …

…memory sink (#2630)

See:
* #2627

I added some log warnings too, but they are not sufficient. If there is
no call to `flush_blocking`, data is still silently dropped somehow.
Maybe @teh-cmc has some idea where?

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/2630) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2630)
- [Docs
preview](https://rerun.io/preview/pr%3Aemilk%2Fdocument-flush/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Aemilk%2Fdocument-flush/examples)

crates/re_log_types/src/data_table_batcher.rs

@@ -223,7 +223,11 @@ impl Drop for DataTableBatcherInner {
     fn drop(&mut self) {
         // Drop the receiving end of the table stream first and foremost, so that we don't block
         // even if the output channel is bounded and currently full.
-        drop(self.rx_tables.take());
+        if let Some(rx_tables) = self.rx_tables.take() {
+            if !rx_tables.is_empty() {
+                re_log::warn!("Dropping data");
+            }
+        }
 
         // NOTE: The command channel is private, if we're here, nothing is currently capable of
         // sending data down the pipeline.

crates/re_sdk/src/log_sink.rs

@@ -109,6 +109,14 @@ pub struct MemorySinkStorage {
     pub(crate) rec_stream: Option<crate::RecordingStream>,
 }
 
+impl Drop for MemorySinkStorage {
+    fn drop(&mut self) {
+        if !self.msgs.read().is_empty() {
+            re_log::warn!("Dropping data in MemorySink");
+        }
+    }
+}
+
 impl MemorySinkStorage {
     /// Write access to the inner array of [`LogMsg`].
     #[inline]
@@ -134,9 +142,7 @@ impl MemorySinkStorage {
         }
         std::mem::take(&mut *self.msgs.write())
     }
-}
 
-impl MemorySinkStorage {
     /// Convert the stored messages into an in-memory Rerun log file.
     #[inline]
     pub fn concat_memory_sinks_as_bytes(

crates/re_sdk/src/recording_stream.rs

@@ -153,7 +153,7 @@ impl RecordingStreamBuilder {
     ///
     /// ## Example
     ///
-    /// ```no_run
+    /// ```
     /// let rec_stream = re_sdk::RecordingStreamBuilder::new("my_app").buffered()?;
     /// # Ok::<(), Box<dyn std::error::Error>>(())
     /// ```
@@ -176,8 +176,15 @@ impl RecordingStreamBuilder {
     ///
     /// ## Example
     ///
-    /// ```no_run
+    /// ```
+    /// # fn log_data(_: &re_sdk::RecordingStream) { }
+    ///
     /// let (rec_stream, storage) = re_sdk::RecordingStreamBuilder::new("my_app").memory()?;
+    ///
+    /// log_data(&rec_stream);
+    ///
+    /// let data = storage.take();
+    ///
     /// # Ok::<(), Box<dyn std::error::Error>>(())
     /// ```
     pub fn memory(