tx: load freelist on Check() #49
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Otherwise, nil dereference on ReadOnly DB Fixes boltdb#45
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## master #49 +/- ##
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added a little extra synchronization machinery to handle this, but it shouldn't impact the common path |
| @@ -282,6 +278,26 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) { | |||
| return db, nil | |||
| } | |||
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| // readFreelist is called on Open and tx.Check. It assumes there are no | |||
| // concurrenct accesses are being made to the freelist. | |||
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"concurrent accesses being made to the freelist."
| db.stats.FreePageN = len(db.freelist.ids) | ||
| close(db.freelistReady) | ||
| }) | ||
| <-db.freelistReady |
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Conveniently, it looks like we don't need the freelistReady channel. Since "...no call to Do returns until the one call to f returns" (go sync.Once docs), we don't need to wait on the freelistReady channel, it is guaranteed to always be closed.
Go playground sanity check: https://play.golang.org/p/ZJEX1V-IbY
| @@ -282,6 +278,26 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) { | |||
| return db, nil | |||
| } | |||
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| // readFreelist is called on Open and tx.Check. It assumes there are no | |||
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Should we rename this to loadFreelist? That might more strongly suggest what side effects it has.
Might also be worth summarizing purpose of the function in a bit more detail, e.g:
"loadFreelist reads the freelist if it is synced, or reconstructs it by scanning the DB if it is not synced. This must be called before any operations that require on the freelist are executed...."
| @@ -282,6 +278,26 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) { | |||
| return db, nil | |||
| } | |||
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| // readFreelist is called on Open and tx.Check. It assumes there are no | |||
| // concurrenct accesses are being made to the freelist. | |||
| func (db *DB) readFreelist() bool { | |||
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I found the return value of this function a bit tricky to reason about.
It might be cleaner to not return bool and fully separate the concerns of loading the freelist in readFreelist from the concerns of flushing the freelist in db.Open. E.g.:
db.Open:
...
if db.readOnly {
return db, nil
}
hasFreeList := db.meta().freelist != pgidNoFreelist
db.readFreelist()
if !db.NoFreelistSync && !hasFreeList {
...
db.readFreelist:
func (db *DB) readFreelist() {
b.freelistLoad.Do(func() {
hasFreeList := db.meta().freelist != pgidNoFreelist
db.freelist = newFreelist()
if !hasFreeList {
...
And we could even add a db.hasSyncedFreelist() function to encapsulate the db.meta().freelist != pgidNoFreelist check and clean up the code a bit more.
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We will be completing this PR via #52. |
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closing in favor of #49 |
Fixes #45