469: feat: Add support for resetting a Writer back by a clearable buffer

avro/src/lib.rs

@@ -98,8 +98,8 @@ pub use schema::Schema;
 pub use serde::{AvroSchema, AvroSchemaComponent, from_value, to_value};
 pub use uuid::Uuid;
 pub use writer::{
-    GenericSingleObjectWriter, SpecificSingleObjectWriter, Writer, WriterBuilder, to_avro_datum,
-    to_avro_datum_schemata, write_avro_datum_ref,
+    Clearable, GenericSingleObjectWriter, SpecificSingleObjectWriter, Writer, WriterBuilder,
+    to_avro_datum, to_avro_datum_schemata, write_avro_datum_ref,
 };
 
 #[cfg(feature = "derive")]

avro/src/writer.rs

@@ -542,6 +542,60 @@ impl<'a, W: Write> Writer<'a, W> {
     }
 }
 
+/// A buffer that can be cleared.
+pub trait Clearable {
+    /// Clear the buffer, keeping the capacity.
+    fn clear(&mut self);
+}
+
+impl Clearable for Vec<u8> {
+    fn clear(&mut self) {
+        self.clear();
+    }
+}
+
+impl<'a, W: Clearable + Write> Writer<'a, W> {
+    /// Reset the writer.
+    ///
+    /// This will clear the underlying writer and the internal buffer.
+    /// It will also clear any user metadata added.
+    ///
+    /// # Example
+    /// ```
+    /// # use apache_avro::{Writer, Schema, Error};
+    /// # let schema = Schema::Boolean;
+    /// # let values = [true, false];
+    /// # fn send(_: &Vec<u8>) {}
+    /// let mut writer = Writer::new(&schema, Vec::new())?;
+    ///
+    /// // Write some values
+    /// for value in values {
+    ///     writer.append_value(value)?;
+    /// }
+    ///
+    /// // Flush the buffer and only then do something with buffer
+    /// writer.flush()?;
+    /// send(writer.get_ref());
+    ///
+    /// // Reset the writer
+    /// writer.reset();
+    ///
+    /// // Write some values again
+    /// for value in values {
+    ///     writer.append(value)?;
+    /// }
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn reset(&mut self) {
+        self.buffer.clear();
+        self.writer.clear();
+        self.has_header = false;
+        self.num_values = 0;
+        self.user_metadata.clear();
+    }
+}
+
 impl<W: Write> Drop for Writer<'_, W> {
     /// Drop the writer, will try to flush ignoring any errors.
     fn drop(&mut self) {
@@ -1858,4 +1912,31 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn avro_rs_469_reset_writer() -> TestResult {
+        let schema = Schema::Boolean;
+        let values = [true, false, true, false];
+        let mut writer = Writer::new(&schema, Vec::new())?;
+
+        for value in values {
+            writer.append_value(value)?;
+        }
+
+        writer.flush()?;
+        let first_buffer = writer.get_ref().clone();
+
+        writer.reset();
+        assert_eq!(writer.get_ref().len(), 0);
+
+        for value in values {
+            writer.append_value(value)?;
+        }
+
+        writer.flush()?;
+        let second_buffer = writer.get_ref().clone();
+        assert_eq!(first_buffer, second_buffer);
+
+        Ok(())
+    }
 }