fix: gzip crash-safety + size rotation always bounds live file (logsetup-7/8/9)

_gzip_file now writes to a .tmp sibling and atomically os.replaces it onto
the final .gz path, so a crash/OOM mid-write can never leave a truncated
.gz where retention trusts it; retier's plain/gz dedupe additionally
verifies a .gz decompresses cleanly before removing its plain twin, so a
corrupt .gz is never preferred over the last intact copy.

rotate="size" now always forces the handler's backupCount to fire
regardless of backup_count, so backup_count=0 means "keep zero rolled
files" (each roll deletes itself immediately) instead of silently
disabling rotation and growing the live file unbounded.

Also mirrors retier's mtime+roll-counter sort key into prune() so a
same-second burst with tied mtimes prunes the oldest files instead of
arbitrary listdir order (logsetup-9, adjacent one-line fix).

Bumps to v0.5.1.

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -13,12 +13,12 @@ and emit; their records flow into the handlers `log_setup` wired.
 ## Install
 
 ```
-log_setup @ git+ssh://git@git.rethinkstudios.io/rethink-public/log_setup.git@v0.5.0
+log_setup @ git+ssh://git@git.rethinkstudios.io/rethink-public/log_setup.git@v0.5.1
 ```
 
 No dependencies — stdlib only.
 
-Drop the `@v0.5.0` suffix from the line above to install the latest unpinned.
+Drop the `@v0.5.1` suffix from the line above to install the latest unpinned.
 
 ## Quick start
 
@@ -44,7 +44,10 @@ emits; the records land in the configured root.
   `getLogger` name each module used, so you see which lib/module logged.
 - **Rotation** (`rotate=`):
   - `"daily"` (default) — rolls at midnight into `log_dir`, keeps `backup_count` days.
-  - `"size"` — rolls at `max_bytes` into `log_dir`, keeps `backup_count`.
+  - `"size"` — rolls at `max_bytes` into `log_dir`, keeps `backup_count`. `backup_count=0`
+    means **keep no rolled history**: the live file still rolls at `max_bytes` (size is
+    always bounded), each rolled file is deleted immediately after landing — it does not
+    disable rotation (see **Retention** below).
   - `"on_start"` — on startup, moves an existing live file into `log_dir` and starts fresh;
     prunes to `backup_count`.
   - `None` — single file, no rotation.
@@ -52,7 +55,8 @@ emits; the records land in the configured root.
   `<project>.<timestamp>.log[.gz]`; the live file keeps its own name.
 - **compress=True** (default) gzips each rolled file.
 - **Retention** = `backup_count` (default 14) for every mode — unless tiered retention is
-  enabled (below).
+  enabled (below). For `rotate="size"`, `backup_count=0` is "keep none" (not "disable
+  rotation") — see the `size` bullet above and the note at the bottom of this section.
 - **console=True** (off by default) also logs to stdout in the same format — opt in when
   you want live terminal output alongside the file.
 
@@ -228,6 +232,21 @@ setup_logging(name="run", queue=True)
   duplicate lines) and leaves handlers your app added itself alone.
 - **Never crashes the app over logging:** if `log_dir` isn't writable, it falls back to
   console-only with a warning instead of raising.
+- **`rotate="size"` always bounds the live file (v0.5.1+).** Previously, `backup_count=0`
+  with `rotate="size"` silently disabled rotation entirely (the live file grew forever,
+  ignoring `max_bytes`). As of v0.5.1, the live file always rolls at `max_bytes`
+  regardless of `backup_count`; `backup_count=0` means "keep zero rolled files" (each roll
+  is deleted right after it lands) rather than "never roll." `backup_count>=1` behaves as
+  documented (keeps that many rolled files). This does not change `"daily"`/`"on_start"`,
+  where `backup_count=0` still means "roll, but don't prune the rolled files" (unbounded
+  `log_dir` growth) — that is a separate, pre-existing knob, not this fix's scope.
+- **Gzip writes are crash-safe (v0.5.1+).** `_gzip_file` now writes to a `.tmp` sibling and
+  atomically `os.replace`s it onto the final `.gz` path, so a crash/OOM/power-loss mid-write
+  can never leave a truncated `.gz` at the path retention logic trusts. Tiered retention's
+  plain/gz dedupe additionally verifies a `.gz` decompresses cleanly before deleting its
+  plain twin — a corrupt `.gz` (from before this fix, or an external cause) is never
+  preferred over an intact plain copy; the plain is kept and the `.gz` gets rewritten
+  cleanly on the next retier pass instead of being deleted.
 
 ## Scope — what this is NOT
 

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "log_setup"
-version = "0.5.0"
+version = "0.5.1"
 description = "stdlib app-entry-point logging setup: live run.log, rotation, gzip, retention, consistent format"
 requires-python = ">=3.10"
 dependencies = []

src/log_setup/rotation.py

@@ -4,6 +4,13 @@ the stdlib rotating handlers roll a file next to the live file; these helpers
 override the namer/rotator so rolled files land in `log_dir` and are gzipped when
 asked, keep the live file at its stable path, and handle the on-start and prune
 paths the handlers don't manage themselves.
+
+gzip writes are crash-safe: `_gzip_file` writes to a `.tmp` sibling and atomically
+`os.replace`s it onto the final `.gz` path, so a crash/OOM/power-loss mid-write never
+leaves a truncated `.gz` where retention logic would trust it. `retier`'s plain/gz
+dedupe additionally verifies a `.gz` decompresses cleanly (`_gz_intact`) before removing
+its plain twin, so a corrupt `.gz` (pre-existing or externally caused) is never preferred
+over an intact plain copy.
 """
 
 import gzip
@@ -53,13 +60,28 @@ def _free_dest(dest: str) -> str:
 def _gzip_file(source: str, dest: str) -> None:
     """gzip source into dest then remove source (the rolled-file compression idiom)
 
+    writes to `dest + ".tmp"` first and `os.replace`s it onto `dest` once the gzip
+    write is complete, so a crash/OOM/power-loss mid-write never leaves a truncated
+    `.gz` at `dest` — the partial write stays quarantined in the `.tmp` name and the
+    source is untouched (safe to retry). os.replace is atomic on the same filesystem,
+    which the `.tmp` sibling always is.
+
     the source mtime is carried onto dest so a file keeps its position when it crosses
     the plain->gz tier boundary — retier ranks by mtime, and a fresh write would
     otherwise make a just-compressed file look like the newest one and reshuffle tiers.
     """
     mtime = _safe_mtime(source)
-    with open(source, "rb") as src, gzip.open(dest, "wb") as dst:
+    tmp_dest = dest + ".tmp"
+    try:
+        with open(source, "rb") as src, gzip.open(tmp_dest, "wb") as dst:
             shutil.copyfileobj(src, dst)
+    except BaseException:
+        try:
+            os.remove(tmp_dest)
+        except OSError:
+            pass
+        raise
+    os.replace(tmp_dest, dest)
     os.remove(source)
     try:
         os.utime(dest, (mtime, mtime))
@@ -67,6 +89,24 @@ def _gzip_file(source: str, dest: str) -> None:
         pass
 
 
+def _gz_intact(path: str) -> bool:
+    """return True if the gzip file at path decompresses cleanly end to end
+
+    belt-and-suspenders check before a dedupe site removes a plain twin in favor of
+    its .gz — a truncated/corrupt .gz (partial write, bad copy, disk error) must never
+    be trusted over an intact plain copy. reads the whole stream since gzip.open only
+    validates the end-of-stream marker on a full read; any failure (BadGzipFile, EOFError,
+    OSError, zlib error) is treated as "not intact" so the caller keeps the plain source.
+    """
+    try:
+        with gzip.open(path, "rb") as handle:
+            while handle.read(1 << 20):
+                pass
+        return True
+    except Exception:
+        return False
+
+
 def make_namer(log_dir: str, compress: bool) -> Callable[[str], str]:
     """namer: redirect a rolled filename into log_dir, adding .gz when compressing
 
@@ -120,7 +160,10 @@ def make_rotator(
     `backup_count` newest rolled files. the stdlib handler's own retention
     (`getFilesToDelete`) only scans the live file's directory, so it never sees the
     rolled files we redirect into `log_dir` — pruning here is what bounds retention for
-    the daily and size rolling modes.
+    the daily and size rolling modes. `backup_count <= 0` means "keep no rolled history":
+    `prune()` itself no-ops at `backup_count <= 0` (it means "don't touch history"), so a
+    zero-retention roll is deleted by the rotator directly right after landing, rather than
+    relying on prune to do it.
 
     tiered mode (when `keep_uncompressed`/`keep_compressed` are given): land the rolled
     file PLAIN and re-tier `log_dir` — newest `keep_uncompressed` stay uncompressed, the
@@ -147,7 +190,15 @@ def make_rotator(
             _gzip_file(source, dest)
         else:
             _move(source, dest)
-        if log_dir is not None and prune_stem is not None:
+        if backup_count <= 0:
+            # "keep no history": prune() no-ops at backup_count <= 0 (that's its "leave
+            # history alone" sentinel, not "delete everything"), so a zero-retention roll
+            # deletes its own just-landed file directly instead of relying on prune.
+            try:
+                os.remove(dest)
+            except OSError:
+                pass
+        elif log_dir is not None and prune_stem is not None:
             prune(log_dir, prune_stem, backup_count)
     return rotator
 
@@ -231,16 +282,24 @@ def retier(log_dir: str, stem: str, keep_uncompressed: int, keep_compressed: int
     entries = [os.path.join(log_dir, name) for name in names]
     # dedupe plain/gz twins FIRST (a crash between _gzip_file's write and its os.remove can
     # leave <x>.log beside <x>.log.gz). drop the redundant plain copy up front so the
-    # phantom twin never occupies a retention slot and evicts a distinct older roll.
+    # phantom twin never occupies a retention slot and evicts a distinct older roll — but
+    # ONLY once the .gz is verified to decompress cleanly. _gzip_file now writes atomically
+    # (temp+os.replace) so a fresh truncated twin can't occur, but a pre-existing corrupt
+    # .gz (older data, bad copy, disk error) must never win over an intact plain copy: that
+    # would delete the only good copy and keep garbage, strictly worse than doing nothing.
     present = set(entries)
     kept = []
     for p in entries:
         if not p.endswith(".gz") and (p + ".gz") in present:
+            if _gz_intact(p + ".gz"):
                 try:
                     os.remove(p)
                 except OSError:
                     kept.append(p)  # couldn't remove — keep it in the accounting
                 continue
+            # .gz twin is corrupt/truncated — keep the intact plain, don't touch either
+            # file here (the second dedupe pass below or a future retier will retry the
+            # compress once the plain is re-gzipped cleanly)
         kept.append(p)
     files = [(p, _safe_mtime(p), _roll_counter(p)) for p in kept if os.path.isfile(p)]
     # newest-first: higher mtime first, and within a tied second the higher roll counter
@@ -260,12 +319,17 @@ def retier(log_dir: str, stem: str, keep_uncompressed: int, keep_compressed: int
                 # a crash/power-loss between _gzip_file's write and its os.remove can leave
                 # a plain source beside a fresh .gz. don't keep both (they'd double-count
                 # toward retention and evict a distinct older roll) — drop the redundant
-                # plain twin, keeping the compressed copy.
+                # plain twin, but ONLY once the .gz is verified intact (see _gz_intact):
+                # a corrupt/truncated .gz here must never win over the last good copy.
+                if _gz_intact(dest):
                     try:
                         os.remove(path)
                     except OSError:
                         pass
                     continue
+                # .gz is corrupt — fall through and re-gzip the plain over the bad dest
+                # (_gzip_file writes atomically, so the corrupt dest is only replaced once
+                # a fully valid archive exists)
             try:
                 _gzip_file(path, dest)
             except OSError:
@@ -294,8 +358,9 @@ def _roll_counter(path: str) -> int:
 def prune(log_dir: str, stem: str, backup_count: int) -> None:
     """keep only the newest `backup_count` rolled files for a given stem in log_dir
 
-    matches files beginning with `<stem>.` (e.g. run.*), sorted by mtime, deleting the
-    oldest beyond the count. used for on_start, which the handlers don't auto-prune.
+    matches files beginning with `<stem>.` (e.g. run.*), sorted by mtime (then by the
+    roll counter parsed from the name, mirroring retier — see _roll_counter), deleting
+    the oldest beyond the count. used for on_start, which the handlers don't auto-prune.
 
     `stem` is reduced to its basename so a `name` containing a directory (e.g. "sub/run")
     still matches the basenamed rolled files in log_dir (else nothing matches and old
@@ -312,9 +377,11 @@ def prune(log_dir: str, stem: str, backup_count: int) -> None:
         ]
     except OSError:
         return
-    files = [(p, _safe_mtime(p)) for p in entries if os.path.isfile(p)]
-    files.sort(key=lambda pair: pair[1], reverse=True)
-    for path, _ in files[backup_count:]:
+    files = [(p, _safe_mtime(p), _roll_counter(p)) for p in entries if os.path.isfile(p)]
+    # newest-first: higher mtime first, and within a tied second the higher roll counter
+    # (a later same-second roll) is newer — mirrors retier's ordering (line ~289)
+    files.sort(key=lambda t: (t[1], t[2]), reverse=True)
+    for path, _, _ in files[backup_count:]:
         try:
             os.remove(path)
         except OSError:

src/log_setup/setup.py

@@ -6,6 +6,10 @@ of rolled files, retention, console output, and a consistent format. it is calle
 the APPLICATION, not by reusable libraries (those stay emit-only). it is idempotent
 (no duplicate handlers on repeat calls), never crashes the app over logging, and can
 route through a background queue so an async event loop doesn't block on file I/O.
+
+`rotate="size"` always bounds the live file: the roll fires at `max_bytes` regardless
+of `backup_count`, including `backup_count=0` (which means "keep zero rolled files",
+not "never roll" — each roll is deleted right after landing).
 """
 
 import atexit
@@ -148,9 +152,14 @@ def _file_handler(
     if rotate == "size":
         # stdlib doRollover is a no-op when backupCount == 0, and its numbered .1/.2 shift
         # can't manage files redirected into log_dir. force a nonzero backupCount so the
-        # roll always fires, and let attach_rolling's history namer name + retier/prune
-        # bound retention (keyed to history_stem).
-        size_backup = backup_count if not tiered else max(backup_count, 1)
+        # roll always fires regardless of mode — tiered AND legacy — and let
+        # attach_rolling's history namer name + retier/prune bound retention (keyed to
+        # history_stem). the ORIGINAL backup_count (which may be 0) still flows into
+        # attach_rolling below: make_rotator treats backup_count <= 0 there as "keep no
+        # rolled history" and deletes each roll right after it lands, rather than passing
+        # 0 to prune() (whose own backup_count <= 0 means "leave history alone", a no-op —
+        # that mismatch is exactly what silently disabled rotation before).
+        size_backup = max(backup_count, 1)
         handler = logging.handlers.RotatingFileHandler(
             live_path, maxBytes=max_bytes, backupCount=size_backup, encoding="utf-8",
         )
@@ -233,7 +242,11 @@ def setup_logging(
     applies, so naming a parent ("aiohttp") quiets its whole subtree (aiohttp.client,
     aiohttp.access, ...). each entry accepts a str or int level; a bad value for one
     entry is skipped with a warning and never aborts the others or the setup.
-    `rotate` is "daily" (default), "size", "on_start", or None. `console=True` adds a
+    `rotate` is "daily" (default), "size", "on_start", or None. for `rotate="size"`, the
+    live file always rolls at `max_bytes` regardless of `backup_count`: `backup_count=0`
+    means "keep zero rolled files" (each roll lands then is deleted immediately), NOT
+    "disable rotation" — the live file is always bounded. `backup_count>=1` keeps that
+    many rolled files as before. `console=True` adds a
     stdout handler (off by default — the file is the output). `queue=True` routes records
     through a background QueueListener so file I/O never blocks the caller (the listener
     is stopped at exit). `output` is "text" (default, human `time | module | level |