Try to use reflect.SliceHeader correctly this time

freelist.go

@@ -267,18 +267,23 @@ func (f *freelist) read(p *page) {
 	}
 	// If the page.count is at the max uint16 value (64k) then it's considered
 	// an overflow and the size of the freelist is stored as the first element.
-	var idx, count uintptr = 0, uintptr(p.count)
+	var idx, count = 0, int(p.count)
 	if count == 0xFFFF {
 		idx = 1
-		count = uintptr(*(*pgid)(unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p))))
+		c := *(*pgid)(unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p)))
+		count = int(c)
+		if count < 0 {
+			panic(fmt.Sprintf("leading element count %d overflows int", c))
+		}
 	}
 
 	// Copy the list of page ids from the freelist.
 	if count == 0 {
 		f.ids = nil
 	} else {
-		ids := (*[maxAllocSize]pgid)(unsafeAdd(
-			unsafe.Pointer(p), unsafe.Sizeof(*p)))[idx : idx+count : idx+count]
+		var ids []pgid
+		data := unsafeIndex(unsafe.Pointer(p), unsafe.Sizeof(*p), unsafe.Sizeof(ids[0]), idx)
+		unsafeSlice(unsafe.Pointer(&ids), data, count)
 
 		// copy the ids, so we don't modify on the freelist page directly
 		idsCopy := make([]pgid, count)
@@ -316,11 +321,15 @@ func (f *freelist) write(p *page) error {
 		p.count = uint16(l)
 	} else if l < 0xFFFF {
 		p.count = uint16(l)
-		ids := (*[maxAllocSize]pgid)(unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p)))[:l:l]
+		var ids []pgid
+		data := unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p))
+		unsafeSlice(unsafe.Pointer(&ids), data, l)
 		f.copyall(ids)
 	} else {
 		p.count = 0xFFFF
-		ids := (*[maxAllocSize]pgid)(unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p)))[: l+1 : l+1]
+		var ids []pgid
+		data := unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p))
+		unsafeSlice(unsafe.Pointer(&ids), data, l+1)
 		ids[0] = pgid(l)
 		f.copyall(ids[1:])
 	}

page.go

@@ -64,9 +64,10 @@ func (p *page) leafPageElements() []leafPageElement {
 	if p.count == 0 {
 		return nil
 	}
-	const maxArraySize = maxAllocSize / unsafe.Sizeof(leafPageElement{})
-	return (*[maxArraySize]leafPageElement)(unsafeIndex(unsafe.Pointer(p), unsafe.Sizeof(*p),
-		unsafe.Sizeof(leafPageElement{}), 0))[:p.count:p.count]
+	var elems []leafPageElement
+	data := unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p))
+	unsafeSlice(unsafe.Pointer(&elems), data, int(p.count))
+	return elems
 }
 
 // branchPageElement retrieves the branch node by index
@@ -80,9 +81,10 @@ func (p *page) branchPageElements() []branchPageElement {
 	if p.count == 0 {
 		return nil
 	}
-	const maxArraySize = maxAllocSize / unsafe.Sizeof(branchPageElement{})
-	return (*[maxArraySize]branchPageElement)(unsafeIndex(unsafe.Pointer(p), unsafe.Sizeof(*p),
-		unsafe.Sizeof(branchPageElement{}), 0))[:p.count:p.count]
+	var elems []branchPageElement
+	data := unsafeAdd(unsafe.Pointer(p), unsafe.Sizeof(*p))
+	unsafeSlice(unsafe.Pointer(&elems), data, int(p.count))
+	return elems
 }
 
 // dump writes n bytes of the page to STDERR as hex output.

unsafe.go

@@ -1,6 +1,9 @@
 package bbolt
 
-import "unsafe"
+import (
+	"reflect"
+	"unsafe"
+)
 
 func unsafeAdd(base unsafe.Pointer, offset uintptr) unsafe.Pointer {
 	return unsafe.Pointer(uintptr(base) + offset)
@@ -23,3 +26,14 @@ func unsafeByteSlice(base unsafe.Pointer, offset uintptr, i, j int) []byte {
 	// all), so this is believed to be correct.
 	return (*[maxAllocSize]byte)(unsafeAdd(base, offset))[i:j:j]
 }
+
+// unsafeSlice modifies the data, len, and cap of a slice variable pointed to by
+// the slice parameter.  This helper should be used over other direct
+// manipulation of reflect.SliceHeader to prevent misuse, namely, converting
+// from reflect.SliceHeader to a Go slice type.
+func unsafeSlice(slice, data unsafe.Pointer, len int) {
+	s := (*reflect.SliceHeader)(slice)
+	s.Data = uintptr(data)
+	s.Cap = len
+	s.Len = len
+}