Fixed capacity virtual memory.

internal/util/alloc_other.go

@@ -0,0 +1,9 @@
+//go:build !(unix || windows) || sqlite3_nosys
+
+package util
+
+import "github.com/tetratelabs/wazero/experimental"
+
+func virtualAlloc(cap, max uint64) experimental.LinearMemory {
+	return sliceAlloc(cap, max)
+}

internal/util/alloc_slice.go

@@ -0,0 +1,25 @@
+//go:build !(darwin || linux) || !(amd64 || arm64 || riscv64) || sqlite3_noshm || sqlite3_nosys
+
+package util
+
+import "github.com/tetratelabs/wazero/experimental"
+
+func sliceAlloc(cap, max uint64) experimental.LinearMemory {
+	return &sliceBuffer{make([]byte, cap), max}
+}
+
+type sliceBuffer struct {
+	buf []byte
+	max uint64
+}
+
+func (b *sliceBuffer) Free() {}
+
+func (b *sliceBuffer) Reallocate(size uint64) []byte {
+	if cap := uint64(cap(b.buf)); size > cap {
+		b.buf = append(b.buf[:cap], make([]byte, size-cap)...)
+	} else {
+		b.buf = b.buf[:size]
+	}
+	return b.buf
+}

internal/util/alloc_unix.go

@@ -9,7 +9,7 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func newAllocator(cap, max uint64) experimental.LinearMemory {
+func virtualAlloc(cap, max uint64) experimental.LinearMemory {
 	// Round up to the page size.
 	rnd := uint64(unix.Getpagesize() - 1)
 	max = (max + rnd) &^ rnd

internal/util/alloc_windows.go

@@ -11,7 +11,7 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func newAllocator(cap, max uint64) experimental.LinearMemory {
+func virtualAlloc(cap, max uint64) experimental.LinearMemory {
 	// Round up to the page size.
 	rnd := uint64(windows.Getpagesize() - 1)
 	max = (max + rnd) &^ rnd

internal/util/mmap.go

@@ -14,7 +14,7 @@ import (
 
 func withAllocator(ctx context.Context) context.Context {
 	return experimental.WithMemoryAllocator(ctx,
-		experimental.MemoryAllocatorFunc(newAllocator))
+		experimental.MemoryAllocatorFunc(virtualAlloc))
 }
 
 type mmapState struct {

internal/util/mmap_other.go

@@ -1,11 +1,21 @@
-//go:build !(darwin || linux || windows) || !(amd64 || arm64 || riscv64) || sqlite3_noshm || sqlite3_nosys
+//go:build !(darwin || linux) || !(amd64 || arm64 || riscv64) || sqlite3_noshm || sqlite3_nosys
 
 package util
 
-import "context"
+import (
+	"context"
+
+	"github.com/tetratelabs/wazero/experimental"
+)
 
 type mmapState struct{}
 
 func withAllocator(ctx context.Context) context.Context {
-	return ctx
+	return experimental.WithMemoryAllocator(ctx,
+		experimental.MemoryAllocatorFunc(func(cap, max uint64) experimental.LinearMemory {
+			if cap == max {
+				return virtualAlloc(cap, max)
+			}
+			return sliceAlloc(cap, max)
+		}))
 }

internal/util/mmap_windows.go

@@ -1,16 +0,0 @@
-//go:build (amd64 || arm64) && !(sqlite3_noshm || sqlite3_nosys)
-
-package util
-
-import (
-	"context"
-
-	"github.com/tetratelabs/wazero/experimental"
-)
-
-type mmapState struct{}
-
-func withAllocator(ctx context.Context) context.Context {
-	return experimental.WithMemoryAllocator(ctx,
-		experimental.MemoryAllocatorFunc(newAllocator))
-}

tests/testcfg/testcfg.go

@@ -9,7 +9,9 @@ import (
 )
 
 func init() {
-	sqlite3.RuntimeConfig = wazero.NewRuntimeConfig().WithMemoryLimitPages(1024)
+	sqlite3.RuntimeConfig = wazero.NewRuntimeConfig().
+		WithMemoryCapacityFromMax(true).
+		WithMemoryLimitPages(1024)
 
 	if os.Getenv("CI") != "" {
 		path := filepath.Join(os.TempDir(), "wazero")