Enforce 128 MiB minimum free space on uploads

- Add cista/util/diskspace.py with MIN_FREE_BYTES limit and cached
  check_free_space() helper.
- Check available space in File.write() before ftruncate/write.
- Catch ENOSPC in File.write and re-raise as InsufficientStorageError.
- upload_file_chunk catches both proactive and ENOSPC errors and
  returns HTTP 507 Insufficient Storage.
- Add tests for low-disk rejection and ENOSPC handling.

cista/fileio.py

@@ -1,9 +1,11 @@
+import errno
 import os
 import threading
 from pathlib import Path
 
 from cista import config
 from cista.util import filename
+from cista.util.diskspace import InsufficientStorageError, check_free_space
 from cista.util.lrucache import LRUCache
 
 
@@ -34,13 +36,24 @@ class File:
             self.open_rw()
         if self.fd is None:
             raise RuntimeError("file descriptor is not available for write")
+        check_free_space(self.path)
         if file_size is not None:
             if pos + len(buffer) > file_size:
                 raise ValueError("write exceeds declared file size")
-            os.ftruncate(self.fd, file_size)
+            try:
+                os.ftruncate(self.fd, file_size)
+            except OSError as e:
+                if e.errno == errno.ENOSPC:
+                    raise InsufficientStorageError("No space left on device") from e
+                raise
         if buffer:
             os.lseek(self.fd, pos, os.SEEK_SET)
-            os.write(self.fd, buffer)
+            try:
+                os.write(self.fd, buffer)
+            except OSError as e:
+                if e.errno == errno.ENOSPC:
+                    raise InsufficientStorageError("No space left on device") from e
+                raise
 
     def __getitem__(self, slc):
         if self.fd is None:

cista/fileserver.py

@@ -1,5 +1,6 @@
 import asyncio
 import contextlib
+import errno
 import mimetypes
 import os
 import re
@@ -12,11 +13,12 @@ from urllib.parse import unquote, urlparse
 from wsgiref.handlers import format_date_time
 
 from sanic import Blueprint, HTTPResponse, empty, json
-from sanic.exceptions import BadRequest, NotFound
+from sanic.exceptions import BadRequest, NotFound, SanicException
 
 from cista import auth, config, sharefs, watching
 from cista.api import fileserver
 from cista.util import filename
+from cista.util.diskspace import InsufficientStorageError
 
 bp = Blueprint("fileserver", url_prefix="/files")
 
@@ -52,13 +54,22 @@ async def upload_file_chunk(request, name):
 
     rel, path = _safe_relpath(name, request=request)
     rel_name = rel.as_posix()
-    upload_info = await asyncio.to_thread(
-        fileserver.upload_info,
-        rel_name,
-        start,
-        body,
-        total,
-    )
+    try:
+        upload_info = await asyncio.to_thread(
+            fileserver.upload_info,
+            rel_name,
+            start,
+            body,
+            total,
+        )
+    except InsufficientStorageError as e:
+        raise SanicException(str(e), status_code=507, quiet=True) from e
+    except OSError as e:
+        if e.errno == errno.ENOSPC:
+            raise SanicException(
+                "No space left on device", status_code=507, quiet=True
+            ) from e
+        raise
     extras = []
     chunk_len = end - start
     whole_file = start == 0 and end == total

cista/util/diskspace.py

@@ -0,0 +1,49 @@
+import shutil
+import threading
+import time
+from pathlib import Path
+
+MIN_FREE_BYTES = 128 * 1024 * 1024
+_CHECK_CACHE_TTL = 1.0
+
+
+class InsufficientStorageError(Exception):
+    """Raised when there is not enough disk space for an operation."""
+
+
+_cache: dict[Path, tuple[float, int]] = {}
+_lock = threading.Lock()
+
+
+def check_free_space(path: Path) -> None:
+    """Raise InsufficientStorageError if free space on the filesystem containing *path*
+
+    is below MIN_FREE_BYTES. Results are cached per directory for 1 second.
+    """
+    check_path = path.parent if path.parent.exists() else path
+    check_path = check_path.resolve()
+
+    now = time.monotonic()
+    with _lock:
+        ts, free = _cache.get(check_path, (0, 0))
+        if now - ts < _CHECK_CACHE_TTL:
+            if free < MIN_FREE_BYTES:
+                raise InsufficientStorageError(
+                    f"Insufficient storage: {free} bytes free, "
+                    f"need at least {MIN_FREE_BYTES} bytes"
+                )
+            return
+
+    try:
+        free = shutil.disk_usage(check_path).free
+    except OSError as e:
+        raise InsufficientStorageError(f"Cannot check disk usage: {e}") from e
+
+    with _lock:
+        _cache[check_path] = (now, free)
+
+    if free < MIN_FREE_BYTES:
+        raise InsufficientStorageError(
+            f"Insufficient storage: {free} bytes free, "
+            f"need at least {MIN_FREE_BYTES} bytes"
+        )