pylons__waitress-249

The problem is that `ReadOnlyFileBasedBuffer` in `waitress/buffers.py` doesn't handle `io.IOBase` subclasses that are not seekable correctly. It checks for the existence of `seek` and `tell` methods, but in some `io.IOBase` subclasses, these methods exist but raise `io.UnsupportedOperation` if the stream is not seekable.

I need to:
1.  Locate the `ReadOnlyFileBasedBuffer` class in `waitress/buffers.py`.
2.  Identify where it checks for `seek` and `tell`.
3.  Reproduce the crash using a non-seekable `io.IOBase` subclass.
4.  Fix the check by either checking `.seekable()` or catching `io.UnsupportedOperation`.
5.  Verify the fix.

Let's start by reading `waitress/buffers.py`.

{
  "filepath": "waitress/buffers.py",
  "start_line": 130
}

{
  "status": "ok",
  "filepath": "waitress/buffers.py",
  "content": "        return BytesIO()\n\nclass ReadOnlyFileBasedBuffer(FileBasedBuffer):\n    # used as wsgi.file_wrapper\n\n    def __init__(self, file, block_size=32768):\n        self.file = file\n        self.block_size = block_size # for __iter__\n\n    def prepare(self, size=None):\n        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):\n            start_pos = self.file.tell()\n            self.file.seek(0, 2)\n            end_pos = self.file.tell()\n            self.file.seek(start_pos)\n            fsize = end_pos - start_pos\n            if size is None:\n                self.remain = fsize\n            else:\n                self.remain = min(fsize, size)\n        return self.remain\n\n    def get(self, numbytes=-1, skip=False):\n        # never read more than self.remain (it can be user-specified)\n        if numbytes == -1 or numbytes > self.remain:\n            numbytes = self.remain\n        file = self.file\n        if not skip:\n            read_pos = file.tell()\n        res = file.read(numbytes)\n        if skip:\n            self.remain -= len(res)\n        else:\n            file.seek(read_pos)\n        return res\n\n    def __iter__(self): # called by task if self.filelike has no seek/tell\n        return self\n\n    def next(self):\n        val = self.file.read(self.block_size)\n        if not val:\n            raise StopIteration\n        return val\n\n    __next__ = next # py3\n\n    def append(self, s):\n        raise NotImplementedError\n\nclass OverflowableBuffer(object):\n    \"\"\"\n    This buffer implementation has four stages:\n    - No data\n    - Bytes-based buffer\n    - BytesIO-based buffer\n    - Temporary file storage\n    The first two stages are fastest for simple transfers.\n    \"\"\"\n\n    overflowed = False\n    buf = None\n    strbuf = b'' # Bytes-based buffer.\n\n    def __init__(self, overflow):\n        # overflow is the maximum to be stored in a StringIO buffer.\n        self.overflow = overflow\n\n    def __len__(self):\n        buf = self.buf\n        if buf is not None:\n            # use buf.__len__ rather than len(buf) FBO of not getting\n            # OverflowError on Python 2\n            return buf.__len__()\n        else:\n            return self.strbuf.__len__()\n\n    def __nonzero__(self):\n        # use self.__len__ rather than len(self) FBO of not getting\n        # OverflowError on Python 2\n        return self.__len__() > 0\n\n    __bool__ = __nonzero__ # py3\n\n    def _create_buffer(self):\n        strbuf = self.strbuf\n        if len(strbuf) >= self.overflow:\n            self._set_large_buffer()\n        else:\n            self._set_small_buffer()\n        buf = self.buf\n        if strbuf:\n            buf.append(self.strbuf)\n            self.strbuf = b''\n        return buf\n\n    def _set_small_buffer(self):\n        self.buf = BytesIOBasedBuffer(self.buf)\n        self.overflowed = False\n\n    def _set_large_buffer(self):\n        self.buf = TempfileBasedBuffer(self.buf)\n        self.overflowed = True\n\n    def append(self, s):\n        buf = self.buf\n        if buf is None:\n            strbuf = self.strbuf\n            if len(strbuf) + len(s) < STRBUF_LIMIT:\n                self.strbuf = strbuf + s\n                return\n            buf = self._create_buffer()\n        buf.append(s)\n        # use buf.__len__ rather than len(buf) FBO of not getting\n        # OverflowError on Python 2\n        sz = buf.__len__()\n        if not self.overflowed:\n            if sz >= self.overflow:\n                self._set_large_buffer()\n\n    def get(self, numbytes=-1, skip=False):\n        buf = self.buf\n        if buf is None:\n            strbuf = self.strbuf\n            if not skip:\n                return strbuf\n            buf = self._create_buffer()\n        return buf.get(numbytes, skip)\n\n    def skip(self, numbytes, allow_prune=False):\n        buf = self.buf\n        if buf is None:\n            if allow_prune and numbytes == len(self.strbuf):\n                # We could slice instead of converting to\n                # a buffer, but that would eat up memory in\n                # large transfers.\n                self.strbuf = b''\n                return\n            buf = self._create_buffer()\n        buf.skip(numbytes, allow_prune)\n\n    def prune(self):\n        \"\"\"\n        A potentially expensive operation that removes all data\n        already retrieved from the buffer.\n        \"\"\"\n        buf = self.buf\n        if buf is None:\n            self.strbuf = b''\n            return",
  "start_line": 130,
  "end_line": 279,
  "total_lines": 298,
  "is_truncated": true
}

In `ReadOnlyFileBasedBuffer.prepare`:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            start_pos = self.file.tell()
            self.file.seek(0, 2)
            end_pos = self.file.tell()
            self.file.seek(start_pos)
            fsize = end_pos - start_pos
            if size is None:
                self.remain = fsize
            else:
                self.remain = min(fsize, size)
        return self.remain
```
It checks for `seek` and `tell` attributes. As described, some `io.IOBase` subclasses have these but they raise `io.UnsupportedOperation`.

I should also check `get` method:
```python
    def get(self, numbytes=-1, skip=False):
        # never read more than self.remain (it can be user-specified)
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
        file = self.file
        if not skip:
            read_pos = file.tell()
        res = file.read(numbytes)
        if skip:
            self.remain -= len(res)
        else:
            file.seek(read_pos)
        return res
```
`get` also uses `tell()` and `seek()`. If `prepare` didn't set `self.remain` (because it didn't enter the `if` block), `self.remain` would be undefined if `prepare` is called and the `if` fails. Wait, let's check `__init__`.

```python
    def __init__(self, file, block_size=32768):
        self.file = file
        self.block_size = block_size # for __iter__
```
`self.remain` is not initialized in `__init__`. If `prepare` is called and the `if` condition is false, it will raise `UnboundLocalError` when trying to `return self.remain`.

Actually, looking at `prepare`:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            # ...
            if size is None:
                self.remain = fsize
            else:
                self.remain = min(fsize, size)
        return self.remain
```
If `hasattr` check fails, `self.remain` is indeed not set.

Now let's reproduce this. I'll create a non-seekable `io.IOBase` subclass.

{
  "command": "python3 - <<'PY'\nimport io\nfrom waitress.buffers import ReadOnlyFileBasedBuffer\n\nclass NonSeekableIO(io.IOBase):\n    def read(self, size=-1):\n        return b\"hello world\"\n\n    def tell(self):\n        raise io.UnsupportedOperation(\"operation unsupported\")\n\n    def seek(self, offset, whence=io.SEEK_SET):\n        raise io.UnsupportedOperation(\"operation unsupported\")\n\n# Case 1: prepare() should not crash even if seek/tell exist but raise UnsupportedOperation\nbuf = ReadOnlyFileBasedBuffer(NonSeekableIO())\ntry:\n    buf.prepare()\n    print(\"Prepare succeeded\")\nexcept Exception as e:\n    print(f\"Prepare failed: {type(e).__name__}: {e}\")\n\n# Case 2: get() should be handled too\ntry:\n    buf.get(5)\n    print(\"Get succeeded\")\nexcept Exception as e:\n    print(f\"Get failed: {type(e).__name__}: {e}\")\nPY\n"
}

{
  "status": "ok",
  "stdout": "Prepare failed: UnsupportedOperation: operation unsupported\nGet failed: UnsupportedOperation: operation unsupported\n",
  "stderr": "",
  "exit_code": 0
}

Wait, in my reproduction:
`Prepare failed: UnsupportedOperation: operation unsupported`
`Get failed: UnsupportedOperation: operation unsupported`

It failed at `self.file.tell()` inside `prepare()` because `hasattr(self.file, 'seek')` and `hasattr(self.file, 'tell')` were true.

Now let's see what happens if I use a class that doesn't even have those methods.

{
  "command": "python3 - <<'PY'\nimport io\nfrom waitress.buffers import ReadOnlyFileBasedBuffer\n\nclass NoSeekTellIO(io.IOBase):\n    def read(self, size=-1):\n        return b\"hello world\"\n\nbuf = ReadOnlyFileBasedBuffer(NoSeekTellIO())\ntry:\n    buf.prepare()\n    print(\"Prepare succeeded\")\nexcept Exception as e:\n    print(f\"Prepare failed: {type(e).__name__}: {e}\")\nPY\n"
}

{
  "status": "ok",
  "stdout": "Prepare failed: UnsupportedOperation: seek\n",
  "stderr": "",
  "exit_code": 0
}

Wait, `Prepare failed: UnsupportedOperation: seek`. Why? `NoSeekTellIO` does NOT have `seek` or `tell` methods.

Let's look at `io.IOBase`. `io.IOBase` defines `seek` and `tell` themselves!

```python
# From python source or docs:
class IOBase:
    def seek(self, offset, whence=os.SEEK_SET):
        raise UnsupportedOperation("operation unsupported")
    def tell(self):
        raise UnsupportedOperation("operation unsupported")
```

So `hasattr(self.file, 'seek')` and `hasattr(self.file, 'tell')` are ALWAYS true for any `io.IOBase` subclass.

That's the problem. We should check `.seekable()`.

Let's verify `seekable()` behavior.

{
  "command": "python3 - <<'PY'\nimport io\nclass NoSeekTellIO(io.IOBase):\n    def read(self, size=-1):\n        return b\"hello world\"\n\nbuf = NoSeekTellIO()\nprint(f\"Seekable: {buf.seekable()}\")\nPY\n"
}

{
  "status": "ok",
  "stdout": "Seekable: False\n",
  "stderr": "",
  "exit_code": 0
}

Yes, `seekable()` returns `False` by default.

So the fix should be:
1. In `prepare()`, instead of `hasattr(self.file, 'seek') and hasattr(self.file, 'tell')`, check if it is seekable.
   Wait, some objects might not be `io.IOBase` subclasses but still have `seek` and `tell`.
   If they are not `io.IOBase` subclasses, they might not have a `.seekable()` method.

The problem description says: "In `io.IOBase` subclasses that are not seekable, those two methods exist but raise an `UnsupportedOperation`, in case of io.IOBase instance `.seekable` should probably be checked for `True` or the `UnsupportedOperation` should be trapped."

If the object is not an `io.IOBase` instance, it might not have `.seekable()`.

Let's see how we can robustly check for seekability.

A common pattern is:
```python
def is_seekable(f):
    try:
        return f.seekable()
    except AttributeError:
        return hasattr(f, 'seek') and hasattr(f, 'tell')
```
Wait, if it has `seek` and `tell` but is not `io.IOBase`, it might still be seekable. If it doesn't have `seekable()`, we can't use it.

Actually, if we want to be safe, we can just wrap the `seek`/`tell` calls in a `try...except io.UnsupportedOperation`. Or we can check if `.seekable()` exists and returns `True`.

Let's see how `prepare` is implemented:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            start_pos = self.file.tell()
            self.file.seek(0, 2)
            end_pos = self.file.tell()
            self.file.seek(start_pos)
            fsize = end_pos - start_pos
            if size is None:
                self.remain = fsize
            else:
                self.remain = min(fsize, size)
        return self.remain
```

If `self.file` is not seekable, `prepare` should probably just return some value or leave `self.remain` unset (but it return `self.remain`, so it must be set).

Wait, if it's not seekable, what should `self.remain` be?
If `size` is provided to `prepare(size)`, then `self.remain` should be `size`.
If `size` is `None`, and it's not seekable, we don't know the size.

Let's look at `ReadOnlyFileBasedBuffer.prepare` again.
If the `if` condition is false, it returns `self.remain`. But `self.remain` is not defined in `__init__`. This is a bug regardless of `io.IOBase`.

Let's check if `self.remain` is used elsewhere.
In `get()`:
```python
    def get(self, numbytes=-1, skip=False):
        # never read more than self.remain (it can be user-specified)
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
        # ...
```
If `prepare` was not called or didn't set `self.remain`, `get` will fail.

Usually `prepare` is called before `get`.

If the file is not seekable, we can't determine the size. In that case, `self.remain` should probably be the `size` passed to `prepare` if provided, otherwise... what?

If the file is not seekable, we basically have to read it until EOF.

If we look at `ReadOnlyFileBasedBuffer.prepare`, the goal is to determine `self.remain`.

If `hasattr(self.file, 'seek') and hasattr(self.file, 'tell')` was intended to check if we can determine the size, then for non-seekable files, we simply cannot determine the size.

However, `prepare` must return something.

If `size` is provided, we can set `self.remain = size`.
If `size` is `None` and we can't seek, what should `self.remain` be?

If I look at `get()`:
```python
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
```
If `self.remain` is not set, this will crash.

Wait, let's see if there are other buffers. `FileBasedBuffer` is the base class.

{
  "filepath": "waitress/buffers.py",
  "start_line": 1
}

{
  "status": "ok",
  "filepath": "waitress/buffers.py",
  "content": "##############################################################################\n#\n# Copyright (c) 2001-2004 Zope Foundation and Contributors.\n# All Rights Reserved.\n#\n# This software is subject to the provisions of the Zope Public License,\n# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.\n# THIS SOFTWARE IS PROVIDED \"AS IS\" AND ANY AND ALL EXPRESS OR IMPLIED\n# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED\n# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS\n# FOR A PARTICULAR PURPOSE.\n#\n##############################################################################\n\"\"\"Buffers\n\"\"\"\nfrom io import BytesIO\n\n# copy_bytes controls the size of temp. strings for shuffling data around.\nCOPY_BYTES = 1 << 18 # 256K\n\n# The maximum number of bytes to buffer in a simple string.\nSTRBUF_LIMIT = 8192\n\nclass FileBasedBuffer(object):\n\n    remain = 0\n\n    def __init__(self, file, from_buffer=None):\n        self.file = file\n        if from_buffer is not None:\n            from_file = from_buffer.getfile()\n            read_pos = from_file.tell()\n            from_file.seek(0)\n            while True:\n                data = from_file.read(COPY_BYTES)\n                if not data:\n                    break\n                file.write(data)\n            self.remain = int(file.tell() - read_pos)\n            from_file.seek(read_pos)\n            file.seek(read_pos)\n\n    def __len__(self):\n        return self.remain\n\n    def __nonzero__(self):\n        return True\n\n    __bool__ = __nonzero__ # py3\n\n    def append(self, s):\n        file = self.file\n        read_pos = file.tell()\n        file.seek(0, 2)\n        file.write(s)\n        file.seek(read_pos)\n        self.remain = self.remain + len(s)\n\n    def get(self, numbytes=-1, skip=False):\n        file = self.file\n        if not skip:\n            read_pos = file.tell()\n        if numbytes < 0:\n            # Read all\n            res = file.read()\n        else:\n            res = file.read(numbytes)\n        if skip:\n            self.remain -= len(res)\n        else:\n            file.seek(read_pos)\n        return res\n\n    def skip(self, numbytes, allow_prune=0):\n        if self.remain < numbytes:\n            raise ValueError(\"Can't skip %d bytes in buffer of %d bytes\" % (\n                numbytes, self.remain)\n            )\n        self.file.seek(numbytes, 1)\n        self.remain = self.remain - numbytes\n\n    def newfile(self):\n        raise NotImplementedError()\n\n    def prune(self):\n        file = self.file\n        if self.remain == 0:\n            read_pos = file.tell()\n            file.seek(0, 2)\n            sz = file.tell()\n            file.seek(read_pos)\n            if sz == 0:\n                # Nothing to prune.\n                return\n        nf = self.newfile()\n        while True:\n            data = file.read(COPY_BYTES)\n            if not data:\n                break\n            nf.write(data)\n        self.file = nf\n\n    def getfile(self):\n        return self.file\n\n    def close(self):\n        if hasattr(self.file, 'close'):\n            self.file.close()\n        self.remain = 0\n\nclass TempfileBasedBuffer(FileBasedBuffer):\n\n    def __init__(self, from_buffer=None):\n        FileBasedBuffer.__init__(self, self.newfile(), from_buffer)\n\n    def newfile(self):\n        from tempfile import TemporaryFile\n        return TemporaryFile('w+b')\n\nclass BytesIOBasedBuffer(FileBasedBuffer):\n\n    def __init__(self, from_buffer=None):\n        if from_buffer is not None:\n            FileBasedBuffer.__init__(self, BytesIO(), from_buffer)\n        else:\n            # Shortcut. :-)\n            self.file = BytesIO()\n\n    def newfile(self):\n        return BytesIO()\n\nclass ReadOnlyFileBasedBuffer(FileBasedBuffer):\n    # used as wsgi.file_wrapper\n\n    def __init__(self, file, block_size=32768):\n        self.file = file\n        self.block_size = block_size # for __iter__\n\n    def prepare(self, size=None):\n        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):\n            start_pos = self.file.tell()\n            self.file.seek(0, 2)\n            end_pos = self.file.tell()\n            self.file.seek(start_pos)\n            fsize = end_pos - start_pos\n            if size is None:\n                self.remain = fsize\n            else:\n                self.remain = min(fsize, size)\n        return self.remain",
  "start_line": 1,
  "end_line": 150,
  "total_lines": 298,
  "is_truncated": true
}

`FileBasedBuffer` has `remain = 0` as a class attribute.

```python
class FileBasedBuffer(object):

    remain = 0
```

So `ReadOnlyFileBasedBuffer` (which inherits from `FileBasedBuffer`) will have `self.remain = 0` initially.

Back to `ReadOnlyFileBasedBuffer.prepare`:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            # ...
            if size is None:
                self.remain = fsize
            else:
                self.remain = min(fsize, size)
        return self.remain
```

If `hasattr` check fails, it returns `self.remain` which is 0. This seems wrong if the file actually has content. But if the file is not seekable, we can't know the size. If `size` was provided, we should probably use it.

Wait, if `size` is provided, we should use it even if the file is not seekable.

```python
    def prepare(self, size=None):
        if size is not None:
            self.remain = size
            # But we might want to cap it by the actual file size if seekable
            if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
                try:
                    start_pos = self.file.tell()
                    self.file.seek(0, 2)
                    end_pos = self.file.tell()
                    self.file.seek(start_pos)
                    fsize = end_pos - start_pos
                    self.remain = min(self.remain, fsize)
                except (io.UnsupportedOperation, AttributeError):
                    pass
        elif hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
             # ...
```

Actually, the simplest fix that addresses the reported issue is to wrap the seek/tell logic in a `try...except` block that catches `io.UnsupportedOperation` (and maybe `AttributeError` just in case, though `hasattr` already handles it).

The issue specifically mentions `io.UnsupportedOperation`.

Let's look at how `ReadOnlyFileBasedBuffer.prepare` is written:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            start_pos = self.file.tell()
            self.file.seek(0, 2)
            end_pos = self.file.tell()
            self.file.seek(start_pos)
            fsize = end_pos - start_pos
            if size is None:
                self.remain = fsize
            else:
                self.remain = min(fsize, size)
        return self.remain
```

If we change it to:

```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            try:
                start_pos = self.file.tell()
                self.file.seek(0, 2)
                end_pos = self.file.tell()
                self.file.seek(start_pos)
                fsize = end_pos - start_pos
                if size is None:
                    self.remain = fsize
                else:
                    self.remain = min(fsize, size)
            except (io.UnsupportedOperation, AttributeError):
                if size is not None:
                    self.remain = size
                # else self.remain stays 0
        elif size is not None:
            self.remain = size
        return self.remain
```

Wait, if `size` is provided, it should be used regardless of whether it's seekable.

Correct logic should be:
1. If `size` is provided, `self.remain = size`.
2. If it's seekable, we can potentially reduce `self.remain` to the actual file size.
3. If `size` is NOT provided, and it's seekable, `self.remain` = file size.
4. If `size` is NOT provided and it's NOT seekable, `self.remain` remains 0 (or some default).

Let's re-evaluate `ReadOnlyFileBasedBuffer.prepare`:
Current:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            start_pos = self.file.tell()
            self.file.seek(0, 2)
            end_pos = self.file.tell()
            self.file.seek(start_pos)
            fsize = end_pos - start_pos
            if size is None:
                self.remain = fsize
            else:
                self.remain = min(fsize, size)
        return self.remain
```

If I change it to:
```python
    def prepare(self, size=None):
        if size is not None:
            self.remain = size

        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            try:
                start_pos = self.file.tell()
                self.file.seek(0, 2)
                end_pos = self.file.tell()
                self.file.seek(start_pos)
                fsize = end_pos - start_pos
                if size is None:
                    self.remain = fsize
                else:
                    self.remain = min(self.remain, fsize)
            except (io.UnsupportedOperation, AttributeError):
                pass
        return self.remain
```
Wait, if `size` is `None` and it's not seekable, `self.remain` will be 0. That might be correct for a non-seekable file where size is unknown.

But there is another problem. `get()`:
```python
    def get(self, numbytes=-1, skip=False):
        # never read more than self.remain (it can be user-specified)
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
        file = self.file
        if not skip:
            read_pos = file.tell()
        res = file.read(numbytes)
        # ...
```
If `self.remain` is 0, `get()` will always return 0 bytes! That's definitely wrong for non-seekable files.

Wait, if the file is not seekable, we should probably not be capping `numbytes` by `self.remain`.

Wait, `ReadOnlyFileBasedBuffer` inherits from `FileBasedBuffer`. In `FileBasedBuffer.get()`:
```python
    def get(self, numbytes=-1, skip=False):
        file = self.file
        if not skip:
            read_pos = file.tell()
        if numbytes < 0:
            # Read all
            res = file.read()
        else:
            res = file.read(numbytes)
        if skip:
            self.remain -= len(res)
        else:
            file.seek(read_pos)
        return res
```

But `ReadOnlyFileBasedBuffer` overrides `get()`:
```python
    def get(self, numbytes=-1, skip=False):
        # never read more than self.remain (it can be user-specified)
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
        file = self.file
        if not skip:
            read_pos = file.tell()
        res = file.read(numbytes)
        if skip:
            self.remain -= len(res)
        else:
            file.seek(read_pos)
        return res
```
The override explicitly caps `numbytes` by `self.remain`.
If `self.remain` is 0, it always returns 0 bytes.

This means if the file is not seekable AND `prepare()` was called with `size=None`, `get()` will not work.

Is `ReadOnlyFileBasedBuffer` intended to work with non-seekable files?
The problem statement says: "Currently waitress is just crashing if it receives an `io.IOBase` subclasses which is not seekable as the file to serve."

This suggests that it SHOULD work.

If it's not seekable, we cannot know `self.remain` in advance.
If we can't know `self.remain`, should we just not cap `numbytes`?

If `self.remain` is used to track how much is left to send, we can initialize it to some "unknown" value. But `self.remain` is used as an integer.

Actually, in `ReadOnlyFileBasedBuffer`, the purpose of `self.remain` seems to be to support `Content-Length` when serving a file.

If the file is not seekable, we can't know the length, so we can't send a correct `Content-Length` header unless it's provided by the user via `prepare(size)`.

If `self.remain` is 0, `get()` returns 0 bytes. This is wrong.

Wait, if it's not seekable, `get()` should probably just read from the file regardless of `self.remain`, OR `self.remain` should be initialized to something that doesn't cap the reading.

But `ReadOnlyFileBasedBuffer` is intended to be a `wsgi.file_wrapper`. WSGI servers often use `file_wrapper` to efficiently send file content.

If `self.remain` is not known, we can't use it to cap.

Let's see if `self.remain` is initialized to something else.
In `FileBasedBuffer` (base class): `remain = 0`.

If I change it so that `self.remain` is only used to cap if it was actually set to a positive value? But it's always initialized to 0.

Maybe `self.remain` should be initialized to `None`? No, that would break other things.

Let's look at `get` again.
```python
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
```
If `self.remain` is 0, `numbytes` becomes 0.

If the file is not seekable, maybe we should ignore `self.remain` in `get`?

Wait, if the file is seekable, we use `self.remain` to ensure we don't read past the specified size.

If the file is NOT seekable, we can't use `seek` anyway.

Wait, `get` also uses `tell` and `seek`:
```python
        if not skip:
            read_pos = file.tell()
        res = file.read(numbytes)
        if skip:
            self.remain -= len(res)
        else:
            file.seek(read_pos)
```

If the file is not seekable, `file.tell()` and `file.seek(read_pos)` will both fail with `io.UnsupportedOperation`.

So `ReadOnlyFileBasedBuffer` is fundamentally designed for seekable files.

However, the problem statement says it should work with non-seekable `io.IOBase` subclasses.

If a file is not seekable, then `get()` cannot perform the `read_pos = file.tell()` and `file.seek(read_pos)` dance.

This dance is there because `get()` is allowed to "peek" (if `skip` is False, it reads and then seeks back).

If the file is not seekable, "peeking" is impossible.

So if the file is not seekable, `get` should just read and not try to seek back.

And `prepare` should not try to determine the size.

Let's see if we can just wrap everything in `try...except`.

In `prepare`:
```python
    def prepare(self, size=None):
        if size is not None:
            self.remain = size
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            try:
                start_pos = self.file.tell()
                self.file.seek(0, 2)
                end_pos = self.file.tell()
                self.file.seek(start_pos)
                fsize = end_pos - start_pos
                if size is None:
                    self.remain = fsize
                else:
                    self.remain = min(self.remain, fsize)
            except (io.UnsupportedOperation, AttributeError):
                pass
        return self.remain
```
Wait, if `size` is None and it's not seekable, `self.remain` will be 0.
Then `get()`:
```python
    def get(self, numbytes=-1, skip=False):
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
        # ...
```
If `self.remain` is 0, `numbytes` becomes 0.

If we want to support non-seekable files, we should probably make `self.remain` sentinel if it's not known. But `FileBasedBuffer` uses `remain = 0`.

Maybe we should change `remain` to be something like `float('inf')` if it's unknown? But that might break other things.

Wait, maybe `self.remain` should only cap if it's greater than 0? No, because a file of size 0 is possible.

Actually, if the file is not seekable, `self.remain` cannot be determined. In that case, `numbytes = self.remain` in `get()` is wrong because it'll be 0.

If the file is not seekable, we should probably ignore `self.remain` capping in `get()` if `self.remain` was not explicitly set by `prepare(size)`.

This is getting complicated. Let's see if there is a simpler way.

If we just catch `io.UnsupportedOperation` everywhere it's raised, would that work?

In `prepare`:
```python
    def prepare(self, size=None):
        if hasattr(self.file, 'seek') and hasattr(self.file, 'tell'):
            try:
                start_pos = self.file.tell()
                self.file.seek(0, 2)
                end_pos = self.file.tell()
                self.file.seek(start_pos)
                fsize = end_pos - start_pos
                if size is None:
                    self.remain = fsize
                else:
                    self.remain = min(fsize, size)
            except (io.UnsupportedOperation, AttributeError):
                if size is not None:
                    self.remain = size
        elif size is not None:
            self.remain = size
        return self.remain
```
If `size` is `None` and it's not seekable, `self.remain` is 0.

If we then call `get(10)`:
`numbytes` becomes `min(10, 0) = 0`.

This is still a problem.

Wait, what if `ReadOnlyFileBasedBuffer.prepare` is not called?
Then `self.remain` is 0 (inherited from `FileBasedBuffer`).
`get(10)` will return 0 bytes.

Does `ReadOnlyFileBasedBuffer` ALWAYS have `prepare` called?
In Waitress, `file_wrapper` is used to send the response. The server probably calls `prepare` to set the `Content-Length` header.

If `ReadOnlyFileBasedBuffer` is used, it's almost certainly called via `prepare` first.

If the file is not seekable, we can't know the length. So we can't send `Content-Length`. We'll have to use chunked encoding.

If we use chunked encoding, we probably don't care about `self.remain` capping in `get()`, as long as we read until EOF.

So if `self.remain` is 0, it should not cap `get()` if the file is not seekable?

But how do we know if it's not seekable in `get()`? We could check `hasattr(self.file, 'seekable') and not self.file.seekable()`.

Actually, if we look at `FileBasedBuffer.get()` (the base class):
```python
    def get(self, numbytes=-1, skip=False):
        # ...
        if numbytes < 0:
            # Read all
            res = file.read()
        else:
            res = file.read(numbytes)
        # ...
```
It does NOT cap by `self.remain`. It only updates `self.remain` if `skip` is True.

The capping is specifically in `ReadOnlyFileBasedBuffer.get()`.

```python
    def get(self, numbytes=-1, skip=False):
        # never read more than self.remain (it can be user-specified)
        if numbytes == -1 or numbytes > self.remain:
            numbytes = self.remain
        # ...
```

If we want to support non-seekable files, we should only do this capping if `self.remain > 0`.
But what if the file is empty? Then `self.remain` is 0, and `numbytes` becomes 0, which is correct.

The problem is when `self.remain` is 0 BECAUSE we couldn't determine it, NOT because the file is empty.

Wait, if the file is not seekable, and `size` was not provided, we don't know if it's empty or not.

If we can't determine the size, we should probably not cap.

So, if the file is not seekable, `self

{
  "command": "python3 - <<'PY'\nimport io\nfrom waitress.buffers import ReadOnlyFileBasedBuffer\n\nclass NonSeekableIO(io.IOBase):\n    def read(self, size=-1):\n        return b\"hello world\"\n\n    def tell(self):\n        raise io.UnsupportedOperation(\"operation unsupported\")\n\n    def seek(self, offset, whence=io.SEEK_SET):\n        raise io.UnsupportedOperation(\"operation unsupported\")\n\nbuf = ReadOnlyFileBasedBuffer(NonSeekableIO())\ntry:\n    buf.prepare()\n    print(f\"Prepare succeeded, remain: {buf.remain}\")\nexcept Exception as e:\n    print(f\"Prepare failed: {type(e).__name__}: {e}\")\n\ntry:\n    res = buf.get(5)\n    print(f\"Get succeeded, res: {res!r}\")\nexcept Exception as e:\n    print(f\"Get failed: {type(e).__name__}: {e}\")\nPY\n"
}

{
  "status": "ok",
  "stdout": "Prepare failed: UnsupportedOperation: operation unsupported\nGet failed: UnsupportedOperation: operation unsupported\n",
  "stderr": "",
  "exit_code": 0
}