allow setting hostname. hostname is implicitly skippable

src/formatting_layer.rs

@@ -45,7 +45,7 @@ fn to_bunyan_level(level: &Level) -> u16 {
 pub struct BunyanFormattingLayer<W: for<'a> MakeWriter<'a> + 'static> {
     make_writer: W,
     pid: u32,
-    hostname: String,
+    hostname: Option<String>,
     bunyan_version: u8,
     name: String,
     default_fields: HashMap<String, Value>,
@@ -119,13 +119,18 @@ impl<W: for<'a> MakeWriter<'a> + 'static> BunyanFormattingLayer<W> {
             make_writer,
             name,
             pid: std::process::id(),
-            hostname: gethostname::gethostname().to_string_lossy().into_owned(),
+            hostname: Some(gethostname::gethostname().to_string_lossy().into_owned()),
             bunyan_version: 0,
             default_fields,
             skip_fields: HashSet::new(),
         }
     }
 
+    pub fn with_hostname(mut self, hostname: Option<String>) -> Self {
+        self.hostname = hostname;
+        self
+    }
+
     /// Add fields to skip when formatting with this layer.
     ///
     /// It returns an error if you try to skip a required core Bunyan field (e.g. `name`).
@@ -165,7 +170,9 @@ impl<W: for<'a> MakeWriter<'a> + 'static> BunyanFormattingLayer<W> {
         map_serializer.serialize_entry(NAME, &self.name)?;
         map_serializer.serialize_entry(MESSAGE, &message)?;
         map_serializer.serialize_entry(LEVEL, &to_bunyan_level(level))?;
-        map_serializer.serialize_entry(HOSTNAME, &self.hostname)?;
+        if let Some(hostname) = &self.hostname {
+            map_serializer.serialize_entry(HOSTNAME, hostname)?;
+        }
         map_serializer.serialize_entry(PID, &self.pid)?;
         if let Ok(time) = &time::OffsetDateTime::now_utc().format(&Rfc3339) {
             map_serializer.serialize_entry(TIME, time)?;

tests/e2e.rs

@@ -171,6 +171,67 @@ fn skip_fields() {
     }
 }
 
+#[test]
+fn set_hostname() {
+    let tracing_output = run_and_get_output(test_action);
+    let record = tracing_output.first().unwrap();
+    let default_hostname = record.get("hostname").unwrap().to_string();
+
+    let expected_hostname = "banana".to_string();
+
+    let buffer = Arc::new(Mutex::new(vec![]));
+    let buffer_clone = buffer.clone();
+
+    let formatting_layer =
+        BunyanFormattingLayer::new("test".into(), move || MockWriter::new(buffer_clone.clone()))
+            .with_hostname(Some(expected_hostname.clone()));
+    let subscriber = Registry::default()
+        .with(JsonStorageLayer)
+        .with(formatting_layer);
+    tracing::subscriber::with_default(subscriber, test_action);
+
+    let buffer_guard = buffer.lock().unwrap();
+    let output = buffer_guard.to_vec();
+    let output = String::from_utf8(output)
+        .unwrap()
+        .lines()
+        .next()
+        .map(ToString::to_string)
+        .unwrap();
+    let output = output.parse::<Value>().unwrap();
+    let new_hostname = output["hostname"].as_str().unwrap();
+
+    assert_ne!(default_hostname, new_hostname);
+    assert_eq!(new_hostname, expected_hostname);
+}
+
+#[test]
+fn skip_hostname() {
+    let buffer = Arc::new(Mutex::new(vec![]));
+    let buffer_clone = buffer.clone();
+
+    let formatting_layer =
+        BunyanFormattingLayer::new("test".into(), move || MockWriter::new(buffer_clone.clone()))
+            .with_hostname(None);
+
+    let subscriber = Registry::default()
+        .with(JsonStorageLayer)
+        .with(formatting_layer);
+    tracing::subscriber::with_default(subscriber, test_action);
+
+    let buffer_guard = buffer.lock().unwrap();
+    let output = buffer_guard.to_vec();
+    let output = String::from_utf8(output)
+        .unwrap()
+        .lines()
+        .next()
+        .map(ToString::to_string)
+        .unwrap();
+    let output = output.parse::<Value>().unwrap();
+
+    assert!(output.get("hostname").is_none());
+}
+
 #[test]
 fn skipping_core_fields_is_not_allowed() {
     let skipped_fields = vec!["level"];