[experiment] add alternative wasm sqlite3 implementation available via build-tag (#2863)

This allows for building GoToSocial with [SQLite transpiled to WASM](https://github.com/ncruces/go-sqlite3) and accessed through [Wazero](https://wazero.io/).

vendor/github.com/ncruces/go-sqlite3/vfs/lock.go

@@ -0,0 +1,144 @@
+//go:build (linux || darwin || windows || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && !sqlite3_nosys
+
+package vfs
+
+import "github.com/ncruces/go-sqlite3/internal/util"
+
+// SupportsFileLocking is false on platforms that do not support file locking.
+// To open a database file on those platforms,
+// you need to use the [nolock] or [immutable] URI parameters.
+//
+// [nolock]: https://sqlite.org/uri.html#urinolock
+// [immutable]: https://sqlite.org/uri.html#uriimmutable
+const SupportsFileLocking = true
+
+const (
+	_PENDING_BYTE  = 0x40000000
+	_RESERVED_BYTE = (_PENDING_BYTE + 1)
+	_SHARED_FIRST  = (_PENDING_BYTE + 2)
+	_SHARED_SIZE   = 510
+)
+
+func (f *vfsFile) Lock(lock LockLevel) error {
+	// Argument check. SQLite never explicitly requests a pending lock.
+	if lock != LOCK_SHARED && lock != LOCK_RESERVED && lock != LOCK_EXCLUSIVE {
+		panic(util.AssertErr())
+	}
+
+	switch {
+	case f.lock < LOCK_NONE || f.lock > LOCK_EXCLUSIVE:
+		// Connection state check.
+		panic(util.AssertErr())
+	case f.lock == LOCK_NONE && lock > LOCK_SHARED:
+		// We never move from unlocked to anything higher than a shared lock.
+		panic(util.AssertErr())
+	case f.lock != LOCK_SHARED && lock == LOCK_RESERVED:
+		// A shared lock is always held when a reserved lock is requested.
+		panic(util.AssertErr())
+	}
+
+	// If we already have an equal or more restrictive lock, do nothing.
+	if f.lock >= lock {
+		return nil
+	}
+
+	// Do not allow any kind of write-lock on a read-only database.
+	if f.readOnly && lock >= LOCK_RESERVED {
+		return _IOERR_LOCK
+	}
+
+	switch lock {
+	case LOCK_SHARED:
+		// Must be unlocked to get SHARED.
+		if f.lock != LOCK_NONE {
+			panic(util.AssertErr())
+		}
+		if rc := osGetSharedLock(f.File); rc != _OK {
+			return rc
+		}
+		f.lock = LOCK_SHARED
+		return nil
+
+	case LOCK_RESERVED:
+		// Must be SHARED to get RESERVED.
+		if f.lock != LOCK_SHARED {
+			panic(util.AssertErr())
+		}
+		if rc := osGetReservedLock(f.File); rc != _OK {
+			return rc
+		}
+		f.lock = LOCK_RESERVED
+		return nil
+
+	case LOCK_EXCLUSIVE:
+		// Must be SHARED, RESERVED or PENDING to get EXCLUSIVE.
+		if f.lock <= LOCK_NONE || f.lock >= LOCK_EXCLUSIVE {
+			panic(util.AssertErr())
+		}
+		reserved := f.lock == LOCK_RESERVED
+		// A PENDING lock is needed before acquiring an EXCLUSIVE lock.
+		if f.lock < LOCK_PENDING {
+			// If we're already RESERVED, we can block indefinitely,
+			// since only new readers may briefly hold the PENDING lock.
+			if rc := osGetPendingLock(f.File, reserved /* block */); rc != _OK {
+				return rc
+			}
+			f.lock = LOCK_PENDING
+		}
+		// We already have PENDING, so we're just waiting for readers to leave.
+		// If we were RESERVED, we can wait for a little while, before invoking
+		// the busy handler; we will only do this once.
+		if rc := osGetExclusiveLock(f.File, reserved /* wait */); rc != _OK {
+			return rc
+		}
+		f.lock = LOCK_EXCLUSIVE
+		return nil
+
+	default:
+		panic(util.AssertErr())
+	}
+}
+
+func (f *vfsFile) Unlock(lock LockLevel) error {
+	// Argument check.
+	if lock != LOCK_NONE && lock != LOCK_SHARED {
+		panic(util.AssertErr())
+	}
+
+	// Connection state check.
+	if f.lock < LOCK_NONE || f.lock > LOCK_EXCLUSIVE {
+		panic(util.AssertErr())
+	}
+
+	// If we don't have a more restrictive lock, do nothing.
+	if f.lock <= lock {
+		return nil
+	}
+
+	switch lock {
+	case LOCK_SHARED:
+		rc := osDowngradeLock(f.File, f.lock)
+		f.lock = LOCK_SHARED
+		return rc
+
+	case LOCK_NONE:
+		rc := osReleaseLock(f.File, f.lock)
+		f.lock = LOCK_NONE
+		return rc
+
+	default:
+		panic(util.AssertErr())
+	}
+}
+
+func (f *vfsFile) CheckReservedLock() (bool, error) {
+	// Connection state check.
+	if f.lock < LOCK_NONE || f.lock > LOCK_EXCLUSIVE {
+		panic(util.AssertErr())
+	}
+
+	if f.lock >= LOCK_RESERVED {
+		return true, nil
+	}
+	return osCheckReservedLock(f.File)
+}