tracing-journald: allow custom journal fields

tracing-journald/src/lib.rs

@@ -40,7 +40,7 @@
 
 #[cfg(unix)]
 use std::os::unix::net::UnixDatagram;
-use std::{fmt, io, io::Write};
+use std::{collections::HashMap, fmt, io, io::Write};
 
 use tracing_core::{
     event::Event,
@@ -85,6 +85,7 @@ pub struct Subscriber {
     socket: UnixDatagram,
     field_prefix: Option<String>,
     syslog_identifier: String,
+    additional_fields: HashMap<String, String>,
 }
 
 #[cfg(unix)]
@@ -109,6 +110,7 @@ impl Subscriber {
                     .map(|n| n.to_string_lossy().into_owned())
                     // If we fail to get the name of the current executable fall back to an empty string.
                     .unwrap_or_else(String::new),
+                additional_fields: HashMap::new(),
             };
             // Check that we can talk to journald, by sending empty payload which journald discards.
             // However if the socket didn't exist or if none listened we'd get an error here.
@@ -150,6 +152,18 @@ impl Subscriber {
         self
     }
 
+    /// Adds a field that will get be passed to journald with every log entry.
+    ///
+    /// This can for example be used to configure the syslog facility.
+    ///
+    /// See [Journal Fields](https://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html)
+    /// and [journalctl](https://www.freedesktop.org/software/systemd/man/journalctl.html)
+    /// for more information.
+    pub fn with_journal_field(mut self, key: String, value: String) -> Self {
+        self.additional_fields.insert(key, value);
+        self
+    }
+
     /// Returns the syslog identifier in use.
     pub fn syslog_identifier(&self) -> &str {
         &self.syslog_identifier
@@ -258,6 +272,10 @@ where
             write!(buf, "{}", self.syslog_identifier).unwrap()
         });
 
+        for (name, value) in &self.additional_fields {
+            put_field_length_encoded(&mut buf, name, |buf| write!(buf, "{}", value).unwrap())
+        }
+
         event.record(&mut EventVisitor::new(
             &mut buf,
             self.field_prefix.as_deref(),

tracing-journald/tests/journal.rs

@@ -237,6 +237,27 @@ fn simple_metadata() {
     });
 }
 
+#[test]
+fn journal_fields() {
+    let sub = Subscriber::new()
+        .unwrap()
+        .with_field_prefix(None)
+        .with_journal_field("SYSLOG_FACILITY".to_string(), "17".to_string())
+        .with_journal_field("DOCUMENTATION".to_string(), "aBc".to_string());
+    with_journald_subscriber(sub, || {
+        info!(test.name = "journal_fields", "Hello World");
+
+        let message = retry_read_one_line_from_journal("journal_fields");
+        assert_eq!(message["MESSAGE"], "Hello World");
+        assert_eq!(message["PRIORITY"], "5");
+        assert_eq!(message["TARGET"], "journal");
+        assert_eq!(message["SYSLOG_FACILITY"], "17");
+        assert_eq!(message["DOCUMENTATION"], "aBc");
+        assert!(message["CODE_FILE"].as_text().is_some());
+        assert!(message["CODE_LINE"].as_text().is_some());
+    });
+}
+
 #[test]
 fn span_metadata() {
     with_journald(|| {