fix(log): never rotate a log file another live process is writing; qbzd logs to qbzd.log

open_log_file_at renamed the live file to .prev on every open, so a second
qbz launch (the #749 two-line qbz.log) or a qbzd in the same directory took
the running process's log away. Writers now hold a shared advisory lock on
<name>.lock; an opener that cannot take it exclusively appends instead of
rotating. The daemon gets its own qbzd.log.

cc

crates/qbz-log/src/install.rs

@@ -1,43 +1,52 @@
 //! One-shot logger installation + the on-disk file sink (open / rotate).
 
-use std::fs::File;
+use std::fs::{File, OpenOptions, TryLockError};
 use std::io::BufWriter;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
 
 use crate::tee::TeeLogger;
 
 static INSTALLED: AtomicBool = AtomicBool::new(false);
 
-/// Install the [`TeeLogger`] as the global `log` logger.
+/// The desktop app's log file name.
+pub const DEFAULT_LOG_FILE: &str = "qbz.log";
+
+/// The on-disk sink plus the advisory lock that marks this process as a live
+/// writer of that file (see [`open_log_file_at`]). Dropping it releases both.
+pub(crate) struct FileSink {
+    pub(crate) writer: BufWriter<File>,
+    pub(crate) lock: Option<File>,
+}
+
+/// Install the [`TeeLogger`] as the global `log` logger, writing to
+/// [`DEFAULT_LOG_FILE`].
 ///
 /// Builds the inner `env_logger` logger from `RUST_LOG` (falling back to `default_level`),
 /// opens/rotates the on-disk file, then sets the boxed logger + max level. Idempotent:
 /// a second call is a guarded no-op (it neither rotates the file again nor panics).
 pub fn install(default_level: &str) {
-    install_with_file_sink(default_level, true);
+    install_with_file_sink(default_level, Some(DEFAULT_LOG_FILE));
+}
+
+/// Same as [`install`], but to `file_name` inside the same logs directory.
+/// `qbzd` uses `qbzd.log` so a daemon never shares (or rotates) the desktop
+/// app's file.
+pub fn install_named(default_level: &str, file_name: &str) {
+    install_with_file_sink(default_level, Some(file_name));
 }
 
 /// Same as [`install`], but with the on-disk file sink DISABLED (stderr + ring only).
 ///
-/// For an internal, disposable child process that re-enters the same `main`.
-/// [`open_log_file`] rotates `qbz.log` to `qbz.log.prev` and creates a fresh
-/// file on every call, so a child running it MID-RUN renames the live log out
-/// from under the parent — which keeps writing to the renamed inode — and
-/// leaves `qbz.log` holding nothing but the child's own first lines.
-///
-/// Field-reported in #749: on Linux every launch spawns the presentation /
-/// GPU preflight child, so `~/.local/share/qbz/logs/qbz.log` ended at two
-/// lines on EVERY run while the app itself was healthy and its whole log went
-/// to `qbz.log.prev`. The user's log file, the first thing a bug report
-/// attaches, was structurally useless. A child's output is already captured
-/// by the parent (proof line on stdout, `preflight_output_tail` on failure),
-/// so it loses nothing by staying off the file.
+/// For an internal, disposable child process that re-enters the same `main`
+/// (presentation / GPU preflight, #749). A child's output is already captured
+/// by the parent, so it loses nothing by staying off the file — and it never
+/// has to touch the parent's file at all.
 pub fn install_without_file_sink(default_level: &str) {
-    install_with_file_sink(default_level, false);
+    install_with_file_sink(default_level, None);
 }
 
-fn install_with_file_sink(default_level: &str, file_sink: bool) {
+fn install_with_file_sink(default_level: &str, file_name: Option<&str>) {
     // True one-shot guard: avoid re-rotating the log file or fighting an already-set logger.
     if INSTALLED.swap(true, Ordering::SeqCst) {
         return;
@@ -57,10 +66,10 @@ fn install_with_file_sink(default_level: &str, file_sink: bool) {
     )
     .build();
     let level = inner.filter();
-    let file = if file_sink { open_log_file() } else { None };
+    let sink = file_name.and_then(open_log_file);
 
     // Ignore the Err if a logger was somehow already set elsewhere.
-    if log::set_boxed_logger(Box::new(TeeLogger::new(inner, file))).is_ok() {
+    if log::set_boxed_logger(Box::new(TeeLogger::new(inner, sink))).is_ok() {
         log::set_max_level(level);
     }
 }
@@ -70,35 +79,74 @@ pub fn set_level(level: log::LevelFilter) {
     log::set_max_level(level);
 }
 
-/// Path to the current-run log file (`~/.local/share/qbz/logs/qbz.log`), if a data dir exists.
+/// `~/.local/share/qbz/logs`, if a data dir exists.
+fn logs_dir() -> Option<PathBuf> {
+    Some(dirs::data_dir()?.join("qbz").join("logs"))
+}
+
+/// Path to the desktop app's current-run log file (`~/.local/share/qbz/logs/qbz.log`).
 pub fn log_file_path() -> Option<PathBuf> {
-    Some(dirs::data_dir()?.join("qbz").join("logs").join("qbz.log"))
+    log_file_path_named(DEFAULT_LOG_FILE)
 }
 
-/// Open the log file for this run, rotating any previous one to `qbz.log.prev`.
-/// Returns `None` (file sink disabled, gracefully) on any filesystem error.
-fn open_log_file() -> Option<BufWriter<File>> {
-    open_log_file_at(&log_file_path()?)
+/// Path to `file_name` inside the logs directory (`qbzd.log` for the daemon).
+pub fn log_file_path_named(file_name: &str) -> Option<PathBuf> {
+    Some(logs_dir()?.join(file_name))
 }
 
-/// [`open_log_file`] against an explicit path, so the rotation contract is
-/// testable without a real data dir.
+fn open_log_file(file_name: &str) -> Option<FileSink> {
+    open_log_file_at(&log_file_path_named(file_name)?)
+}
+
+/// Open the run's log file at `path`.
 ///
-/// NOTE the hazard this encodes: every call renames the CURRENT file away and
-/// starts an empty one. That is right once per process run and wrong for
-/// anything else — see [`install_without_file_sink`].
-fn open_log_file_at(path: &std::path::Path) -> Option<BufWriter<File>> {
+/// Rotation (`<name>` → `<name>.prev`, then a fresh file) happens ONLY when no
+/// live process is writing the file. Liveness is an advisory lock on the
+/// sidecar `<name>.lock`: every writer holds it shared for its lifetime; an
+/// opener probes it with a non-blocking exclusive lock and, when that is
+/// refused, APPENDS to the live file instead of renaming it away. That is
+/// what a second `qbz` launch, a `qbzd` sharing the directory, or any future
+/// caller of [`install`] used to do to a running process (#749, and the
+/// 2026-09 static review). The lock lives on the sidecar, never on the log
+/// itself, so it can never interfere with writes (a shared `LockFileEx`
+/// would, on Windows). Returns `None` (file sink disabled, gracefully) on any
+/// filesystem error.
+fn open_log_file_at(path: &Path) -> Option<FileSink> {
     let dir = path.parent()?;
     std::fs::create_dir_all(dir).ok()?;
+    let name = path.file_name()?.to_string_lossy().into_owned();
 
-    if path.exists() {
-        let prev = dir.join("qbz.log.prev");
-        // Best-effort rotation; a failure here must not disable logging.
-        let _ = std::fs::rename(path, &prev);
+    let lock = OpenOptions::new()
+        .create(true)
+        .read(true)
+        .write(true)
+        .open(dir.join(format!("{name}.lock")))
+        .ok();
+    let live_writer = match lock.as_ref().map(File::try_lock) {
+        Some(Err(TryLockError::WouldBlock)) => true,
+        // Granted, unsupported (exotic filesystem) or no sidecar: nobody we
+        // can see is live — keep the historical rotate behaviour.
+        Some(Ok(())) | Some(Err(TryLockError::Error(_))) | None => false,
+    };
+    if let Some(lock) = lock.as_ref() {
+        // Join the live writers (downgrade the probe if we held it).
+        let _ = lock.unlock();
+        let _ = lock.lock_shared();
     }
 
-    let file = File::create(path).ok()?;
-    Some(BufWriter::new(file))
+    let file = if live_writer && path.exists() {
+        OpenOptions::new().append(true).open(path).ok()?
+    } else {
+        if path.exists() {
+            // Best-effort rotation; a failure here must not disable logging.
+            let _ = std::fs::rename(path, dir.join(format!("{name}.prev")));
+        }
+        File::create(path).ok()?
+    };
+    Some(FileSink {
+        writer: BufWriter::new(file),
+        lock,
+    })
 }
 
 #[cfg(test)]
@@ -106,32 +154,68 @@ mod tests {
     use super::*;
     use std::io::Write;
 
-    /// The rotation contract, and the reason a child process must never run it:
-    /// a second open renames the live file to `qbz.log.prev` and leaves an
-    /// empty `qbz.log` behind. This is #749 in three lines.
-    #[test]
-    fn a_second_open_rotates_the_live_file_away() {
-        let dir = std::env::temp_dir().join(format!("qbz-log-rotate-{}", std::process::id()));
+    fn fresh_dir(tag: &str) -> PathBuf {
+        let dir = std::env::temp_dir().join(format!("qbz-log-{tag}-{}", std::process::id()));
         let _ = std::fs::remove_dir_all(&dir);
+        dir
+    }
+
+    /// #749 was a child process renaming the live log away; a second `qbz`
+    /// launch or a `qbzd` in the same directory did the same. A live writer
+    /// holds the sidecar lock, so a second opener APPENDS.
+    #[test]
+    fn a_second_open_while_the_first_writer_is_live_appends() {
+        let dir = fresh_dir("live");
         let path = dir.join("qbz.log");
 
         let mut first = open_log_file_at(&path).expect("first open");
-        writeln!(first, "parent line").unwrap();
-        first.flush().unwrap();
-        assert_eq!(std::fs::read_to_string(&path).unwrap(), "parent line\n");
+        writeln!(first.writer, "parent line").unwrap();
+        first.writer.flush().unwrap();
 
-        // A second process (the preflight child) installing its own logger.
         let mut second = open_log_file_at(&path).expect("second open");
-        writeln!(second, "child line").unwrap();
-        second.flush().unwrap();
+        writeln!(second.writer, "second-instance line").unwrap();
+        second.writer.flush().unwrap();
 
-        assert_eq!(std::fs::read_to_string(&path).unwrap(), "child line\n");
         assert_eq!(
-            std::fs::read_to_string(dir.join("qbz.log.prev")).unwrap(),
-            "parent line\n",
-            "the parent's log survives only under .prev"
+            std::fs::read_to_string(&path).unwrap(),
+            "parent line\nsecond-instance line\n"
         );
-
+        assert!(!dir.join("qbz.log.prev").exists(), "nothing was rotated");
+        drop(second);
+        drop(first);
         let _ = std::fs::remove_dir_all(&dir);
     }
+
+    /// With no live writer the historical contract holds: rotate to
+    /// `<name>.prev` and start fresh. The prev name follows the file name.
+    #[test]
+    fn open_after_the_previous_writer_closed_rotates_to_prev() {
+        let dir = fresh_dir("rotate");
+        let path = dir.join("qbzd.log");
+        {
+            let mut first = open_log_file_at(&path).expect("first open");
+            writeln!(first.writer, "run one").unwrap();
+            first.writer.flush().unwrap();
+        }
+        let mut second = open_log_file_at(&path).expect("second open");
+        writeln!(second.writer, "run two").unwrap();
+        second.writer.flush().unwrap();
+
+        assert_eq!(std::fs::read_to_string(&path).unwrap(), "run two\n");
+        assert_eq!(
+            std::fs::read_to_string(dir.join("qbzd.log.prev")).unwrap(),
+            "run one\n"
+        );
+        drop(second);
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn named_log_paths_share_the_logs_directory() {
+        let Some(default) = log_file_path() else { return };
+        let daemon = log_file_path_named("qbzd.log").unwrap();
+        assert_eq!(default.parent(), daemon.parent());
+        assert_eq!(default.file_name().unwrap(), DEFAULT_LOG_FILE);
+        assert_eq!(daemon.file_name().unwrap(), "qbzd.log");
+    }
 }

crates/qbz-log/src/lib.rs

@@ -4,7 +4,8 @@
 //! `env_logger`'s built `Logger` and fans visible records out to three sinks:
 //!   1. **stderr** (redacted text, same line format as the file sink),
 //!   2. a bounded **in-memory ring** ([`ring`], cap [`ring::RING_CAP`]), and
-//!   3. an **on-disk file** (`~/.local/share/qbz/logs/qbz.log`, prev-rotated at startup).
+//!   3. an **on-disk file** (`~/.local/share/qbz/logs/qbz.log`; `qbzd.log` for the daemon),
+//!      prev-rotated at startup unless another live process is writing it.
 //!
 //! Secret **redaction** ([`redact`]) is applied once at the single write choke point,
 //! so every downstream consumer (stderr, ring, file, clipboard, paste upload) gets clean text.
@@ -26,6 +27,6 @@ pub mod tee;
 
 pub use bundle::{format_diagnostics_bundle, DiagFields};
 pub use fatal::install as install_fatal_signal_reporter;
-pub use install::{install, install_without_file_sink, set_level};
+pub use install::{install, install_named, install_without_file_sink, set_level};
 pub use line::LogLine;
 pub use redact::{redact, register_secret};

crates/qbz-log/src/tee.rs

@@ -7,6 +7,7 @@ use std::time::Instant;
 
 use log::{Log, Metadata, Record};
 
+use crate::install::FileSink;
 use crate::line::LogLine;
 use crate::repeat::ConsecutiveRecords;
 use crate::{redact, ring};
@@ -25,15 +26,23 @@ pub struct TeeLogger {
 
 struct Output {
     file: Option<BufWriter<File>>,
+    /// Held for the process lifetime: marks this process as a live writer of
+    /// the log file (see `install::open_log_file_at`).
+    _lock: Option<File>,
     consecutive: ConsecutiveRecords,
 }
 
 impl TeeLogger {
-    pub(crate) fn new(inner: env_logger::Logger, file: Option<BufWriter<File>>) -> Self {
+    pub(crate) fn new(inner: env_logger::Logger, sink: Option<FileSink>) -> Self {
+        let (file, lock) = match sink {
+            Some(sink) => (Some(sink.writer), sink.lock),
+            None => (None, None),
+        };
         Self {
             inner,
             output: Mutex::new(Output {
                 file,
+                _lock: lock,
                 consecutive: ConsecutiveRecords::default(),
             }),
         }
@@ -140,7 +149,13 @@ mod tests {
         let inner = env_logger::Builder::new()
             .filter_level(log::LevelFilter::Info)
             .build();
-        let logger = TeeLogger::new(inner, Some(BufWriter::new(file)));
+        let logger = TeeLogger::new(
+            inner,
+            Some(FileSink {
+                writer: BufWriter::new(file),
+                lock: None,
+            }),
+        );
         logger.log(
             &Record::builder()
                 .level(Level::Info)