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[weekly-r317] PostingsForLabelMatching: check for waiting writers #760

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[weekly-r317] PostingsForLabelMatching: check for waiting writers #760

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19 changes: 19 additions & 0 deletions tsdb/index/postings.go
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Original file line number Diff line number Diff line change
Expand Up @@ -471,7 +471,17 @@ func (p *MemPostings) PostingsForLabelMatching(ctx context.Context, name string,
its := make([]Postings, 0, len(vals))
p.mtx.RLock()
e := p.m[name]
iter := 0
for _, v := range vals {
iter++
if iter%1024 == 0 && p.writersWaiting() {
// There are writers waiting, so unlock the mutex, and use this opportunity to check the context.
p.mtx.RUnlock()
if ctx.Err() != nil {
return ErrPostings(ctx.Err())
}
p.mtx.RLock()
}
if refs, ok := e[v]; ok {
// Some of the values may have been garbage-collected in the meantime this is fine, we'll just skip them.
// If we didn't let the mutex go, we'd have these postings here, but they would be pointing nowhere
Expand All @@ -486,6 +496,15 @@ func (p *MemPostings) PostingsForLabelMatching(ctx context.Context, name string,
return Merge(ctx, its...)
}

// writersWaiting can be called from an RLock()ed-goroutine to check whether there are other goroutines waiting on Lock().
func (p *MemPostings) writersWaiting() bool {
if !p.mtx.TryRLock() {
return true
}
p.mtx.RUnlock()
return false
}

// ExpandPostings returns the postings expanded as a slice.
func ExpandPostings(p Postings) (res []storage.SeriesRef, err error) {
for p.Next() {
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