crash when type checking file in a read only directory

Crash Report

mypy gives INTERNAL_ERROR when type checking files on read only filesystem (e.g. ro bind mount).
This can happen when running inside docker/checking files that you don't have write permission to.

Traceback

$ mypy hello.py --show-traceback
error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 2.1.0
Traceback (most recent call last):
  File "/home/karlicos/.local/share/uv/tools/mypy/lib/python3.14/site-packages/mypy/__main__.py", line 16, in console_entry
    main()
    ~~~~^^
  File "mypy/main.py", line 154, in main
  File "mypy/main.py", line 244, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "mypy/build.py", line 385, in build
  File "mypy/build.py", line 1916, in create_metastore
    mds: MetadataStore = SqliteMetadataStore(
  File "mypy/metastore.py", line 192, in __init__
  File "<frozen os>", line 226, in makedirs
  File "<frozen os>", line 236, in makedirs
OSError: [Errno 30] Read-only file system: '.mypy_cache'

To Reproduce

mkdir    /tmp/ro
cd       /tmp/ro
echo 'x: int = "a"' > hello.py
chmod -w /tmp/ro

mypy hello.py --show-traceback

Your Environment

• Mypy version used: 2.1.0
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.14
• Operating system and version: Linux 7.0.0-15-generic #15-Ubuntu SMP PREEMPT_DYNAMIC

Debugging attempt

I also reproduced in a clean environment with Claude, so pretty sure it's unaffected by my system conviguration.

• with --no-sqlite-cache the crash doesn't happen
• bisection shows that it started happening when sqlite cache became the default, on 1.20

Reproduce the issue with the read-only /tmp/ro setup, then read mypy/build.py around create_metastore and mypy/metastore.py around SqliteMetadataStore. Compare behavior with --no-sqlite-cache and verify that type checking a read-only directory reports the source error without an INTERNAL_ERROR or attempted cache creation failure.