Add PID to log file names if log file already exists

Signed-off-by: Daniel Carl Jones <djonesoa@amazon.com>

doc/LOGGING.md

@@ -20,6 +20,11 @@ You can direct logs to a file instead of syslog by providing a destination direc
 
 The directory will be created if it doesn't exist.
 A new log file will be created for each execution of `mount-s3`.
+
+Log file names are not considered stable and may change in the future.
+If multiple Mountpoint processes launched simultaneously try to create the same log file,
+all but the first will include the process ID in the log file name.
+
 Both the directory and log files are created with read/write access for the process owner and read access for the process owner's group.
 Log files are not automatically rotated or cleaned up.
 

mountpoint-s3/CHANGELOG.md

@@ -1,5 +1,12 @@
 ## Unreleased
 
+### Breaking changes
+
+* If two or more Mountpoint processes try to create the same log file, the first will succeed.
+  The others will now include the process ID in the file name.
+  Previously, multiple Mountpoint processes would write to the same log file causing log entries to be interleaved.
+  ([#1041](https://github.com/awslabs/mountpoint-s3/pull/1041))
+
 ## v1.9.1 (September 19, 2024)
 
 ### Other changes

mountpoint-s3/src/logging.rs

@@ -93,22 +93,35 @@ fn init_tracing_subscriber(config: LoggingConfig) -> anyhow::Result<()> {
     RustLogAdapter::try_init().context("failed to initialize CRT logger")?;
 
     let file_layer = if let Some(path) = &config.log_directory {
-        const LOG_FILE_NAME_FORMAT: &[FormatItem<'static>] =
-            macros::format_description!("mountpoint-s3-[year]-[month]-[day]T[hour]-[minute]-[second]Z.log");
-        let filename = OffsetDateTime::now_utc()
-            .format(LOG_FILE_NAME_FORMAT)
-            .context("couldn't format log file name")?;
-
-        // log directories and files created by Mountpoint should not be accessible by other users
+        // log directories and files created by Mountpoint should not be writable by other users
         let mut dir_builder = DirBuilder::new();
         dir_builder.recursive(true).mode(0o750);
         let mut file_options = OpenOptions::new();
-        file_options.mode(0o640).append(true).create(true);
+        file_options.mode(0o640).append(true).create_new(true);
 
         dir_builder.create(path).context("failed to create log folder")?;
-        let file = file_options
-            .open(path.join(filename))
-            .context("failed to create log file")?;
+
+        let timestamp = {
+            const TIMESTAMP_FORMAT: &[FormatItem<'static>] =
+            macros::format_description!("[year]-[month]-[day]T[hour]-[minute]-[second]Z");
+            OffsetDateTime::now_utc()
+                .format(TIMESTAMP_FORMAT)
+                .context("couldn't format timestamp for log file name")?
+        };
+
+        let filename = format!("mountpoint-s3-{timestamp}.log");
+        let file = match file_options.open(path.join(&filename)) {
+            Ok(file) => file,
+            Err(e) if e.kind() == std::io::ErrorKind::AlreadyExists => {
+                // If multiple Mountpoint processes start within the same second, stuff in the process ID.
+                // This will prevent multiple Mountpoint processes interleaving unrelated logs.
+                let process_id = std::process::id();
+                let filename = format!("mountpoint-s3-{timestamp}.pid{process_id}.log");
+                let path = path.join(&filename);
+                file_options.open(path).context("failed to create log file")?
+            }
+            err @ Err(_) => err.context("failed to create log file")?
+        };
 
         let file_layer = tracing_subscriber::fmt::layer()
             .with_ansi(false)