gotosocial/vendor/github.com/ncruces/go-sqlite3/vfs/README.md

# Go SQLite VFS API

This package implements the SQLite [OS Interface](https://sqlite.org/vfs.html) (aka VFS).

It replaces the default SQLite VFS with a **pure Go** implementation,
and exposes [interfaces](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#VFS)
that should allow you to implement your own [custom VFSes](#custom-vfses).

Since it is a from scratch reimplementation,
there are naturally some ways it deviates from the original.

The main differences are [file locking](#file-locking) and [WAL mode](#write-ahead-logging) support.

### File Locking

POSIX advisory locks, which SQLite uses on Unix, are
[broken by design](https://github.com/sqlite/sqlite/blob/b74eb0/src/os_unix.c#L1073-L1161).

On Linux and macOS, this package uses
[OFD locks](https://www.gnu.org/software/libc/manual/html_node/Open-File-Description-Locks.html)
to synchronize access to database files.
OFD locks are fully compatible with POSIX advisory locks.

This package can also use
[BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2),
albeit with reduced concurrency (`BEGIN IMMEDIATE` behaves like `BEGIN EXCLUSIVE`).
On BSD, macOS, and illumos, BSD locks are fully compatible with POSIX advisory locks;
on Linux and z/OS, they are fully functional, but incompatible;
elsewhere, they are very likely broken.
BSD locks are the default on BSD and illumos,
but you can opt into them with the `sqlite3_flock` build tag.

On Windows, this package uses `LockFileEx` and `UnlockFileEx`,
like SQLite.

Otherwise, file locking is not supported, and you must use
[`nolock=1`](https://sqlite.org/uri.html#urinolock)
(or [`immutable=1`](https://sqlite.org/uri.html#uriimmutable))
to open database files.
To use the [`database/sql`](https://pkg.go.dev/database/sql) driver
with `nolock=1` you must disable connection pooling by calling
[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).

You can use [`vfs.SupportsFileLocking`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsFileLocking)
to check if your build supports file locking.

### Write-Ahead Logging

On little-endian Unix, this package uses `mmap` to implement
[shared-memory for the WAL-index](https://sqlite.org/wal.html#implementation_of_shared_memory_for_the_wal_index),
like SQLite.

With [BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2)
a WAL database can only be accessed by a single proccess.
Other processes that attempt to access a database locked with BSD locks,
will fail with the [`SQLITE_PROTOCOL`](https://sqlite.org/rescode.html#protocol) error code.

Otherwise, [WAL support is limited](https://sqlite.org/wal.html#noshm),
and `EXCLUSIVE` locking mode must be set to create, read, and write WAL databases.
To use `EXCLUSIVE` locking mode with the
[`database/sql`](https://pkg.go.dev/database/sql) driver
you must disable connection pooling by calling
[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).

You can use [`vfs.SupportsSharedMemory`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsSharedMemory)
to check if your build supports shared memory.

### Batch-Atomic Write

On 64-bit Linux, this package supports
[batch-atomic writes](https://sqlite.org/cgi/src/technote/714)
on the F2FS filesystem.

### Checksums

This package can be [configured](https://pkg.go.dev/github.com/ncruces/go-sqlite3#Conn.EnableChecksums)
to add an 8-byte checksum to the end of every page in an SQLite database.
The checksum is added as each page is written
and verified as each page is read.\
The checksum is intended to help detect database corruption
caused by random bit-flips in the mass storage device.

The implementation is compatible with SQLite's
[Checksum VFS Shim](https://sqlite.org/cksumvfs.html).

### Build Tags

The VFS can be customized with a few build tags:
- `sqlite3_flock` forces the use of BSD locks; it can be used on z/OS to enable locking,
  and elsewhere to test BSD locks.
- `sqlite3_nosys` prevents importing [`x/sys`](https://pkg.go.dev/golang.org/x/sys);
  disables locking _and_ shared memory on all platforms.
- `sqlite3_noshm` disables shared memory on all platforms.

> [!IMPORTANT]
> The default configuration of this package is compatible with the standard
> [Unix and Windows SQLite VFSes](https://sqlite.org/vfs.html#multiple_vfses);
> `sqlite3_flock` builds are compatible with the
> [`unix-flock` VFS](https://sqlite.org/compile.html#enable_locking_style).
> If incompatible file locking is used, accessing databases concurrently with
> _other_ SQLite libraries will eventually corrupt data.

### Custom VFSes

- [`github.com/ncruces/go-sqlite3/vfs/adiantum`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/adiantum)
  wraps a VFS to offer encryption at rest.
- [`github.com/ncruces/go-sqlite3/vfs/memdb`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/memdb)
  implements an in-memory VFS.
- [`github.com/ncruces/go-sqlite3/vfs/readervfs`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/readervfs)
  implements a VFS for immutable databases.
- [`github.com/ncruces/go-sqlite3/vfs/xts`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/xts)
  wraps a VFS to offer encryption at rest.