journald: make level mappings configurable (#2824)

This allows to manually map tracing levels to journald levels.

It seems that @little-dude, who started the original PR, doesn't have
time to finish this, so I picked it up.  Reapplied the changes to the
newest master branch and fixed the latest comments/issues.

This will also fix/close:

Closes #2649
Closes #2661
Closes #2347 (the original pr)

tracing-journald/src/lib.rs

@@ -60,14 +60,16 @@ mod socket;
 /// names by translating `.`s into `_`s, stripping leading `_`s and non-ascii-alphanumeric
 /// characters other than `_`, and upcasing.
 ///
-/// Levels are mapped losslessly to journald `PRIORITY` values as follows:
+/// By default, levels are mapped losslessly to journald `PRIORITY` values as follows:
 ///
 /// - `ERROR` => Error (3)
 /// - `WARN` => Warning (4)
 /// - `INFO` => Notice (5)
 /// - `DEBUG` => Informational (6)
 /// - `TRACE` => Debug (7)
 ///
+/// These mappings can be changed with [`Subscriber::with_priority_mappings`].
+///
 /// The standard journald `CODE_LINE` and `CODE_FILE` fields are automatically emitted. A `TARGET`
 /// field is emitted containing the event's target.
 ///
@@ -84,6 +86,7 @@ pub struct Layer {
     field_prefix: Option<String>,
     syslog_identifier: String,
     additional_fields: Vec<u8>,
+    priority_mappings: PriorityMappings,
 }
 
 #[cfg(unix)]
@@ -109,6 +112,7 @@ impl Layer {
                     // If we fail to get the name of the current executable fall back to an empty string.
                     .unwrap_or_default(),
                 additional_fields: Vec::new(),
+                priority_mappings: PriorityMappings::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.
@@ -129,6 +133,41 @@ impl Layer {
         self
     }
 
+    /// Sets how [`tracing_core::Level`]s are mapped to [journald priorities](Priority).
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use tracing_journald::{Priority, PriorityMappings};
+    /// use tracing_subscriber::prelude::*;
+    /// use tracing::error;
+    ///
+    /// let registry = tracing_subscriber::registry();
+    /// match tracing_journald::subscriber() {
+    ///     Ok(subscriber) => {
+    ///         registry.with(
+    ///             subscriber
+    ///                 // We can tweak the mappings between the trace level and
+    ///                 // the journal priorities.
+    ///                 .with_priority_mappings(PriorityMappings {
+    ///                     info: Priority::Informational,
+    ///                     ..PriorityMappings::new()
+    ///                 }),
+    ///         );
+    ///     }
+    ///     // journald is typically available on Linux systems, but nowhere else. Portable software
+    ///     // should handle its absence gracefully.
+    ///     Err(e) => {
+    ///         registry.init();
+    ///         error!("couldn't connect to journald: {}", e);
+    ///     }
+    /// }
+    /// ```
+    pub fn with_priority_mappings(mut self, mappings: PriorityMappings) -> Self {
+        self.priority_mappings = mappings;
+        self
+    }
+
     /// Sets the syslog identifier for this logger.
     ///
     /// The syslog identifier comes from the classic syslog interface (`openlog()`
@@ -232,6 +271,20 @@ impl Layer {
         memfd::seal_fully(mem.as_raw_fd())?;
         socket::send_one_fd_to(&self.socket, mem.as_raw_fd(), JOURNALD_PATH)
     }
+
+    fn put_priority(&self, buf: &mut Vec<u8>, meta: &Metadata) {
+        put_field_wellformed(
+            buf,
+            "PRIORITY",
+            &[match *meta.level() {
+                Level::ERROR => self.priority_mappings.error as u8,
+                Level::WARN => self.priority_mappings.warn as u8,
+                Level::INFO => self.priority_mappings.info as u8,
+                Level::DEBUG => self.priority_mappings.debug as u8,
+                Level::TRACE => self.priority_mappings.trace as u8,
+            }],
+        );
+    }
 }
 
 /// Construct a journald layer
@@ -286,7 +339,7 @@ where
         }
 
         // Record event fields
-        put_priority(&mut buf, event.metadata());
+        self.put_priority(&mut buf, event.metadata());
         put_metadata(&mut buf, event.metadata(), None);
         put_field_length_encoded(&mut buf, "SYSLOG_IDENTIFIER", |buf| {
             write!(buf, "{}", self.syslog_identifier).unwrap()
@@ -374,18 +427,114 @@ impl Visit for EventVisitor<'_> {
     }
 }
 
-fn put_priority(buf: &mut Vec<u8>, meta: &Metadata) {
-    put_field_wellformed(
-        buf,
-        "PRIORITY",
-        match *meta.level() {
-            Level::ERROR => b"3",
-            Level::WARN => b"4",
-            Level::INFO => b"5",
-            Level::DEBUG => b"6",
-            Level::TRACE => b"7",
-        },
-    );
+/// A priority (called "severity code" by syslog) is used to mark the
+/// importance of a message.
+///
+/// Descriptions and examples are taken from the [Arch Linux wiki].
+/// Priorities are also documented in the
+/// [section 6.2.1 of the Syslog protocol RFC][syslog].
+///
+/// [Arch Linux wiki]: https://wiki.archlinux.org/title/Systemd/Journal#Priority_level
+/// [syslog]: https://www.rfc-editor.org/rfc/rfc5424#section-6.2.1
+#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
+#[repr(u8)]
+pub enum Priority {
+    /// System is unusable.
+    ///
+    /// Examples:
+    ///
+    /// - severe Kernel BUG
+    /// - systemd dumped core
+    ///
+    /// This level should not be used by applications.
+    Emergency = b'0',
+    /// Should be corrected immediately.
+    ///
+    /// Examples:
+    ///
+    /// - Vital subsystem goes out of work, data loss:
+    /// - `kernel: BUG: unable to handle kernel paging request at ffffc90403238ffc`
+    Alert = b'1',
+    /// Critical conditions
+    ///
+    /// Examples:
+    ///
+    /// - Crashe, coredumps
+    /// - `systemd-coredump[25319]: Process 25310 (plugin-container) of user 1000 dumped core`
+    Critical = b'2',
+    /// Error conditions
+    ///
+    /// Examples:
+    ///
+    /// - Not severe error reported
+    /// - `kernel: usb 1-3: 3:1: cannot get freq at ep 0x84, systemd[1]: Failed unmounting /var`
+    /// - `libvirtd[1720]: internal error: Failed to initialize a valid firewall backend`
+    Error = b'3',
+    /// May indicate that an error will occur if action is not taken.
+    ///
+    /// Examples:
+    ///
+    /// - a non-root file system has only 1GB free
+    /// - `org.freedesktop. Notifications[1860]: (process:5999): Gtk-WARNING **: Locale not supported by C library. Using the fallback 'C' locale`
+    Warning = b'4',
+    /// Events that are unusual, but not error conditions.
+    ///
+    /// Examples:
+    ///
+    /// - `systemd[1]: var.mount: Directory /var to mount over is not empty, mounting anyway`
+    /// - `gcr-prompter[4997]: Gtk: GtkDialog mapped without a transient parent. This is discouraged`
+    Notice = b'5',
+    /// Normal operational messages that require no action.
+    ///
+    /// Example: `lvm[585]: 7 logical volume(s) in volume group "archvg" now active`
+    Informational = b'6',
+    /// Information useful to developers for debugging the
+    /// application.
+    ///
+    /// Example: `kdeinit5[1900]: powerdevil: Scheduling inhibition from ":1.14" "firefox" with cookie 13 and reason "screen"`
+    Debug = b'7',
+}
+
+/// Mappings from tracing [`Level`]s to journald [priorities].
+///
+/// [priorities]: Priority
+#[derive(Debug, Clone)]
+pub struct PriorityMappings {
+    /// Priority mapped to the `ERROR` level
+    pub error: Priority,
+    /// Priority mapped to the `WARN` level
+    pub warn: Priority,
+    /// Priority mapped to the `INFO` level
+    pub info: Priority,
+    /// Priority mapped to the `DEBUG` level
+    pub debug: Priority,
+    /// Priority mapped to the `TRACE` level
+    pub trace: Priority,
+}
+
+impl PriorityMappings {
+    /// Returns the default priority mappings:
+    ///
+    /// - [`tracing::Level::ERROR`]: [`Priority::Error`] (3)
+    /// - [`tracing::Level::WARN`]: [`Priority::Warning`] (4)
+    /// - [`tracing::Level::INFO`]: [`Priority::Notice`] (5)
+    /// - [`tracing::Level::DEBUG`]: [`Priority::Informational`] (6)
+    /// - [`tracing::Level::TRACE`]: [`Priority::Debug`] (7)
+    pub fn new() -> PriorityMappings {
+        Self {
+            error: Priority::Error,
+            warn: Priority::Warning,
+            info: Priority::Notice,
+            debug: Priority::Informational,
+            trace: Priority::Debug,
+        }
+    }
+}
+
+impl Default for PriorityMappings {
+    fn default() -> Self {
+        Self::new()
+    }
 }
 
 fn put_metadata(buf: &mut Vec<u8>, meta: &Metadata, prefix: Option<&str>) {

tracing-journald/tests/journal.rs

@@ -6,8 +6,8 @@ use std::time::Duration;
 
 use serde::Deserialize;
 
-use tracing::{debug, error, info, info_span, warn};
-use tracing_journald::Layer;
+use tracing::{debug, error, info, info_span, trace, warn};
+use tracing_journald::{Layer, Priority, PriorityMappings};
 use tracing_subscriber::layer::SubscriberExt;
 use tracing_subscriber::Registry;
 
@@ -17,7 +17,16 @@ fn journalctl_version() -> std::io::Result<String> {
 }
 
 fn with_journald(f: impl FnOnce()) {
-    with_journald_layer(Layer::new().unwrap().with_field_prefix(None), f)
+    with_journald_layer(
+        Layer::new()
+            .unwrap()
+            .with_field_prefix(None)
+            .with_priority_mappings(PriorityMappings {
+                trace: Priority::Informational,
+                ..PriorityMappings::new()
+            }),
+        f,
+    )
 }
 
 fn with_journald_layer(layer: Layer, f: impl FnOnce()) {
@@ -168,6 +177,41 @@ fn simple_message() {
     });
 }
 
+#[test]
+fn custom_priorities() {
+    fn check_message(level: &str, priority: &str) {
+        let entry = retry_read_one_line_from_journal(&format!("custom_priority.{}", level));
+        assert_eq!(entry["MESSAGE"], format!("hello {}", level).as_str());
+        assert_eq!(entry["PRIORITY"], priority);
+    }
+
+    let priorities = PriorityMappings {
+        error: Priority::Critical,
+        warn: Priority::Error,
+        info: Priority::Warning,
+        debug: Priority::Notice,
+        trace: Priority::Informational,
+    };
+    let layer = Layer::new()
+        .unwrap()
+        .with_field_prefix(None)
+        .with_priority_mappings(priorities);
+    let test = || {
+        trace!(test.name = "custom_priority.trace", "hello trace");
+        check_message("trace", "6");
+        debug!(test.name = "custom_priority.debug", "hello debug");
+        check_message("debug", "5");
+        info!(test.name = "custom_priority.info", "hello info");
+        check_message("info", "4");
+        warn!(test.name = "custom_priority.warn", "hello warn");
+        check_message("warn", "3");
+        error!(test.name = "custom_priority.error", "hello error");
+        check_message("error", "2");
+    };
+
+    with_journald_layer(layer, test);
+}
+
 #[test]
 fn multiline_message() {
     with_journald(|| {