fix Go 1.7 pointer reference bug

This commit fixes a bug where page end-of-header pointers were being
converted to byte slices even when the pointer did not point to
allocated memory. This occurs with pages that have a `page.count`
of zero.

Note: This was not an issue in Go 1.6 but the new Go 1.7 SSA backend
handles `nil` checks differently.

See golang/go#16772

Author: benbjohnson

freelist.go

@@ -166,12 +166,16 @@ func (f *freelist) read(p *page) {
 	}
 
 	// Copy the list of page ids from the freelist.
-	ids := ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[idx:count]
-	f.ids = make([]pgid, len(ids))
-	copy(f.ids, ids)
+	if count == 0 {
+		f.ids = nil
+	} else {
+		ids := ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[idx:count]
+		f.ids = make([]pgid, len(ids))
+		copy(f.ids, ids)
 
-	// Make sure they're sorted.
-	sort.Sort(pgids(f.ids))
+		// Make sure they're sorted.
+		sort.Sort(pgids(f.ids))
+	}
 
 	// Rebuild the page cache.
 	f.reindex()
@@ -189,7 +193,9 @@ func (f *freelist) write(p *page) error {
 
 	// The page.count can only hold up to 64k elements so if we overflow that
 	// number then we handle it by putting the size in the first element.
-	if len(ids) < 0xFFFF {
+	if len(ids) == 0 {
+		p.count = uint16(len(ids))
+	} else if len(ids) < 0xFFFF {
 		p.count = uint16(len(ids))
 		copy(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[:], ids)
 	} else {

node.go

@@ -201,6 +201,11 @@ func (n *node) write(p *page) {
 	}
 	p.count = uint16(len(n.inodes))
 
+	// Stop here if there are no items to write.
+	if p.count == 0 {
+		return
+	}
+
 	// Loop over each item and write it to the page.
 	b := (*[maxAllocSize]byte)(unsafe.Pointer(&p.ptr))[n.pageElementSize()*len(n.inodes):]
 	for i, item := range n.inodes {

page.go

@@ -62,6 +62,9 @@ func (p *page) leafPageElement(index uint16) *leafPageElement {
 
 // leafPageElements retrieves a list of leaf nodes.
 func (p *page) leafPageElements() []leafPageElement {
+	if p.count == 0 {
+		return nil
+	}
 	return ((*[0x7FFFFFF]leafPageElement)(unsafe.Pointer(&p.ptr)))[:]
 }
 
@@ -72,6 +75,9 @@ func (p *page) branchPageElement(index uint16) *branchPageElement {
 
 // branchPageElements retrieves a list of branch nodes.
 func (p *page) branchPageElements() []branchPageElement {
+	if p.count == 0 {
+		return nil
+	}
 	return ((*[0x7FFFFFF]branchPageElement)(unsafe.Pointer(&p.ptr)))[:]
 }