[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/internal/util/mmap.go

@@ -0,0 +1,97 @@
+//go:build (darwin || linux) && (amd64 || arm64 || riscv64) && !(sqlite3_noshm || sqlite3_nosys)
+
+package util
+
+import (
+	"context"
+	"os"
+	"unsafe"
+
+	"github.com/tetratelabs/wazero/api"
+	"github.com/tetratelabs/wazero/experimental"
+	"golang.org/x/sys/unix"
+)
+
+func withAllocator(ctx context.Context) context.Context {
+	return experimental.WithMemoryAllocator(ctx,
+		experimental.MemoryAllocatorFunc(virtualAlloc))
+}
+
+type mmapState struct {
+	regions []*MappedRegion
+}
+
+func (s *mmapState) new(ctx context.Context, mod api.Module, size int32) *MappedRegion {
+	// Find unused region.
+	for _, r := range s.regions {
+		if !r.used && r.size == size {
+			return r
+		}
+	}
+
+	// Allocate page aligned memmory.
+	alloc := mod.ExportedFunction("aligned_alloc")
+	stack := [2]uint64{
+		uint64(unix.Getpagesize()),
+		uint64(size),
+	}
+	if err := alloc.CallWithStack(ctx, stack[:]); err != nil {
+		panic(err)
+	}
+	if stack[0] == 0 {
+		panic(OOMErr)
+	}
+
+	// Save the newly allocated region.
+	ptr := uint32(stack[0])
+	buf := View(mod, ptr, uint64(size))
+	addr := uintptr(unsafe.Pointer(&buf[0]))
+	s.regions = append(s.regions, &MappedRegion{
+		Ptr:  ptr,
+		addr: addr,
+		size: size,
+	})
+	return s.regions[len(s.regions)-1]
+}
+
+type MappedRegion struct {
+	addr uintptr
+	Ptr  uint32
+	size int32
+	used bool
+}
+
+func MapRegion(ctx context.Context, mod api.Module, f *os.File, offset int64, size int32, prot int) (*MappedRegion, error) {
+	s := ctx.Value(moduleKey{}).(*moduleState)
+	r := s.new(ctx, mod, size)
+	err := r.mmap(f, offset, prot)
+	if err != nil {
+		return nil, err
+	}
+	return r, nil
+}
+
+func (r *MappedRegion) Unmap() error {
+	// We can't munmap the region, otherwise it could be remaped.
+	// Instead, convert it to a protected, private, anonymous mapping.
+	// If successful, it can be reused for a subsequent mmap.
+	_, err := mmap(r.addr, uintptr(r.size),
+		unix.PROT_NONE, unix.MAP_PRIVATE|unix.MAP_ANON|unix.MAP_FIXED,
+		-1, 0)
+	r.used = err != nil
+	return err
+}
+
+func (r *MappedRegion) mmap(f *os.File, offset int64, prot int) error {
+	_, err := mmap(r.addr, uintptr(r.size),
+		prot, unix.MAP_SHARED|unix.MAP_FIXED,
+		int(f.Fd()), offset)
+	r.used = err == nil
+	return err
+}
+
+// We need the low level mmap for MAP_FIXED to work.
+// Bind the syscall version hoping that it is more stable.
+
+//go:linkname mmap syscall.mmap
+func mmap(addr, length uintptr, prot, flag, fd int, pos int64) (*byte, error)