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))