Dedupe Reload/NoSyncReload, prefer empty instead of nil init

Reload and NoSyncReload have duplicated code, this unifies both
for later refactoring.

This PR is split from etcd-io#786, where the tests found differences on reloading
and nil/empty initializations. Added some more clarifications in godocs
for certain panic behavior and expected returns on the interface.

Signed-off-by: Thomas Jungblut <[email protected]>

internal/freelist/freelist.go

@@ -11,7 +11,7 @@ type ReadWriter interface {
 	// Write writes the freelist into the given page.
 	Write(page *common.Page)
 
-	// EstimatedWritePageSize returns the size of the freelist after serialization in Write.
+	// EstimatedWritePageSize returns the size in bytes of the freelist after serialization in Write.
 	// This should never underestimate the size.
 	EstimatedWritePageSize() int
 }
@@ -46,13 +46,14 @@ type Interface interface {
 	ReleasePendingPages()
 
 	// Free releases a page and its overflow for a given transaction id.
-	// If the page is already free then a panic will occur.
+	// If the page is already free or is one of the meta pages, then a panic will occur.
 	Free(txId common.Txid, p *common.Page)
 
 	// Freed returns whether a given page is in the free list.
 	Freed(pgId common.Pgid) bool
 
 	// Rollback removes the pages from a given pending tx.
+	// Should always be followed by Reload or NoSyncReload from last freelist state.
 	Rollback(txId common.Txid)
 
 	// Copyall copies a list of all free ids and all pending ids in one sorted list.
@@ -65,7 +66,7 @@ type Interface interface {
 	// NoSyncReload reads the freelist from Pgids and filters out pending items.
 	NoSyncReload(pgIds common.Pgids)
 
-	// freePageIds returns the IDs of all free pages.
+	// freePageIds returns the IDs of all free pages. No free pages returns an empty slice.
 	freePageIds() common.Pgids
 
 	// pendingPageIds returns all pending pages by transaction id.

internal/freelist/shared.go

@@ -208,25 +208,7 @@ func (t *shared) Copyall(dst []common.Pgid) {
 
 func (t *shared) Reload(p *common.Page) {
 	t.Read(p)
-
-	// Build a cache of only pending pages.
-	pcache := make(map[common.Pgid]bool)
-	for _, txp := range t.pending {
-		for _, pendingID := range txp.ids {
-			pcache[pendingID] = true
-		}
-	}
-
-	// Check each page in the freelist and build a new available freelist
-	// with any pages not in the pending lists.
-	var a []common.Pgid
-	for _, id := range t.freePageIds() {
-		if !pcache[id] {
-			a = append(a, id)
-		}
-	}
-
-	t.Init(a)
+	t.NoSyncReload(t.freePageIds())
 }
 
 func (t *shared) NoSyncReload(pgIds common.Pgids) {
@@ -240,7 +222,7 @@ func (t *shared) NoSyncReload(pgIds common.Pgids) {
 
 	// Check each page in the freelist and build a new available freelist
 	// with any pages not in the pending lists.
-	var a []common.Pgid
+	a := []common.Pgid{}
 	for _, id := range pgIds {
 		if !pcache[id] {
 			a = append(a, id)
@@ -274,7 +256,7 @@ func (t *shared) Read(p *common.Page) {
 
 	// Copy the list of page ids from the freelist.
 	if len(ids) == 0 {
-		t.Init(nil)
+		t.Init([]common.Pgid{})
 	} else {
 		// copy the ids, so we don't modify on the freelist page directly
 		idsCopy := make([]common.Pgid, len(ids))