[Bug]: sqlite3 configured to only use mmap, no fallback to regular IO

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I'm the author of mergerfs. As a FUSE filesystem it is possible for individual to disable mmap support. Numerous Nextcloud + mergerfs users have run into an issue where they get IO errors from sqlite because (I'm guessing) nextcloud's usage of sqlite3 sets up mmap and when a query fails doesn't reset it to regular IO. As a result they get unclear messages about IO errors.

Steps to reproduce

1. Setup mergerfs with cache.files=off
2. Point Nextcloud to the mergerfs mount during install.

Expected behavior

Ideally the code managing sqlite3 could confirm the target filesystem supports mmap and falls back to regular IO.

Installation method

None

Nextcloud Server version

28

Operating system

None

PHP engine version

None

Web server

None

Database engine version

SQlite

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

The issue identifies SQLite3 configuration and a mergerfs mount with cache.files=off, but names no source files or tests. Start by reproducing the installation on that mount and tracing the SQLite database setup and resulting I/O errors. Done means mmap-only configuration no longer produces unclear I/O failures when mmap is unavailable.