logging: split journal stream checks from protocol upgrading

This introduces two new helpers to separately check whether stdout
or stderr are connected to a journal stream.
More importantly, it changes `connected_to_journal()` behavior to
only check stderr.

These changes are meant to logically split the `$JOURNAL_STREAM`
helpers from protocol upgrading. The former concern low-level
settings which can be set on stdout and stderr separately. The
latter instead is an high-level concept which assumes that all the
logging happens on stderr.

src/logging.rs

@@ -308,32 +308,65 @@ impl JournalStream {
     }
 }
 
-/// Whether this process is directly connected to the journal.
+/// Whether this process stdout is directly streamed to journal.
 ///
 /// Inspects the `$JOURNAL_STREAM` environment variable and compares the device and inode
-/// numbers in this variable against the stdout and stderr file descriptors.
+/// numbers in this variable against the stdout file descriptor.
 ///
-/// Return `true` if either stream matches the device and inode numbers in `$JOURNAL_STREAM`,
-/// and `false` otherwise (or in case of any IO error).
+/// Return `true` if they match, and `false` otherwise (or in case of any IO error).
 ///
-/// Systemd sets `$JOURNAL_STREAM` to the device and inode numbers of the standard output
-/// or standard error streams of the current process if either of these streams is connected
-/// to the systemd journal.
+/// Systemd sets `$JOURNAL_STREAM` if any of stdout or stderr of the current process is
+/// connected to systemd journal.
 ///
-/// Systemd explicitly recommends that services check this variable to upgrade their logging
-/// to the native systemd journal protocol.
+/// See section “Environment Variables Set or Propagated by the Service Manager” in
+/// [systemd.exec(5)][1] for more information.
+///
+/// [1]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Environment%20Variables%20Set%20or%20Propagated%20by%20the%20Service%20Manager
+pub fn stdout_is_journal_stream() -> bool {
+    JournalStream::from_env_default().map_or(false, |env_stream| {
+        JournalStream::from_fd(io::stdout()).map_or(false, |o| o == env_stream)
+    })
+}
+
+/// Whether this process stderr is directly streamed to journal.
+///
+/// Inspects the `$JOURNAL_STREAM` environment variable and compares the device and inode
+/// numbers in this variable against the stderr file descriptor.
+///
+/// Return `true` if they match, and `false` otherwise (or in case of any IO error).
+///
+/// Systemd sets `$JOURNAL_STREAM` if any of stdout or stderr of the current process is
+/// connected to systemd journal.
 ///
 /// See section “Environment Variables Set or Propagated by the Service Manager” in
 /// [systemd.exec(5)][1] for more information.
 ///
 /// [1]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Environment%20Variables%20Set%20or%20Propagated%20by%20the%20Service%20Manager
-pub fn connected_to_journal() -> bool {
+pub fn stderr_is_journal_stream() -> bool {
     JournalStream::from_env_default().map_or(false, |env_stream| {
-        JournalStream::from_fd(io::stderr()).map_or(false, |e| e == env_stream)
-            || JournalStream::from_fd(io::stdout()).map_or(false, |o| o == env_stream)
+        JournalStream::from_fd(io::stderr()).map_or(false, |o| o == env_stream)
     })
 }
 
+/// Whether this process can be automatically upgraded to native journal logging.
+///
+/// Inspects the `$JOURNAL_STREAM` environment variable and compares the device and inode
+/// numbers in this variable against the stderr file descriptor.
+///
+/// Return `true` if they match, and `false` otherwise (or in case of any IO error).
+/// This is effectively equivalent to [stderr_is_journal_stream].
+///
+/// For services normally logging to stderr but also supporting systemd-style structured
+/// logging, it is recommended to perform this check and then possibly upgrade to the
+/// native systemd journal protocol.
+///
+/// See section “Automatic Protocol Upgrading” in [systemd documentation][1] for more information.
+///
+/// [1]: https://systemd.io/JOURNAL_NATIVE_PROTOCOL/#automatic-protocol-upgrading
+pub fn connected_to_journal() -> bool {
+    stderr_is_journal_stream()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;