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Ensure that flush on the mfs root flushes its directory #4509

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merged 1 commit into from
Jan 2, 2018
Merged

Ensure that flush on the mfs root flushes its directory #4509

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3 changes: 2 additions & 1 deletion core/coreunix/add.go
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Original file line number Diff line number Diff line change
Expand Up @@ -443,9 +443,10 @@ func (adder *Adder) addFile(file files.File) error {
if err != nil {
return err
}
if err := mr.Flush(); err != nil {
if err := mr.FlushMemFree(adder.ctx); err != nil {
return err
}

adder.liveNodes = 0
}
adder.liveNodes++
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30 changes: 30 additions & 0 deletions mfs/system.go
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Expand Up @@ -123,6 +123,36 @@ func (kr *Root) Flush() error {
return nil
}

// FlushMemFree flushes the root directory and then uncaches all of its links.
// This has the effect of clearing out potentially stale references and allows
// them to be garbage collected.
// CAUTION: Take care not to ever call this while holding a reference to any
// child directories. Those directories will be bad references and using them
// may have unintended racy side effects.
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@Stebalien Stebalien Dec 20, 2017 •
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What exactly do you mean here? How will removing these from the map lead to race conditions?

I think what's throwing me is the term "cache". Is it a write cache? A read cache?

Answered offline. Basically, we could have open file descriptors and we'll lose updates to these files if we remove the references to them from the cache.

// A better implemented mfs system (one that does smarter internal caching and
// refcounting) shouldnt need this method.
func (kr *Root) FlushMemFree(ctx context.Context) error {
dir, ok := kr.GetValue().(*Directory)
if !ok {
return fmt.Errorf("invalid mfs structure, root should be a directory")
}

if err := dir.Flush(); err != nil {
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What happens if we concurrently modify this directory? Can we end up uncaching non-flushed entries? Is that an issue?

Should we have a single FlushAndUncache() method?

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Thats basically the purpose of this method. In my comment i make it explicit that you shouldnt use this method concurrently with any other operations since there are deeper issues that could potentially happen.

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Fair enough. I'm working on improving this situation, FYI.

return err
}

dir.lock.Lock()
defer dir.lock.Unlock()
for name := range dir.files {
delete(dir.files, name)
}
for name := range dir.childDirs {
delete(dir.childDirs, name)
}

return nil
}

// closeChild implements the childCloser interface, and signals to the publisher that
// there are changes ready to be published
func (kr *Root) closeChild(name string, nd node.Node, sync bool) error {
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