github.com/slspeek/camlistore_namedsearch@v0.0.0-20140519202248-ed6f70f7721a/pkg/blobserver/mergedenum.go (about)

     1  /*
     2  Copyright 2011 Google Inc.
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8       http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  package blobserver
    18  
    19  import (
    20  	"camlistore.org/pkg/blob"
    21  	"camlistore.org/pkg/context"
    22  )
    23  
    24  const buffered = 8
    25  
    26  // TODO: it'd be nice to make sources be []BlobEnumerator, but that
    27  // makes callers more complex since assignable interfaces' slice forms
    28  // aren't assignable.
    29  func MergedEnumerate(ctx *context.Context, dest chan<- blob.SizedRef, sources []Storage, after string, limit int) error {
    30  	defer close(dest)
    31  
    32  	startEnum := func(source Storage) (*blob.ChanPeeker, <-chan error) {
    33  		ch := make(chan blob.SizedRef, buffered)
    34  		errch := make(chan error, 1)
    35  		go func() {
    36  			errch <- source.EnumerateBlobs(ctx, ch, after, limit)
    37  		}()
    38  		return &blob.ChanPeeker{Ch: ch}, errch
    39  	}
    40  
    41  	peekers := make([]*blob.ChanPeeker, 0, len(sources))
    42  	errs := make([]<-chan error, 0, len(sources))
    43  	for _, source := range sources {
    44  		peeker, errch := startEnum(source)
    45  		peekers = append(peekers, peeker)
    46  		errs = append(errs, errch)
    47  	}
    48  
    49  	nSent := 0
    50  	lastSent := ""
    51  	for nSent < limit {
    52  		lowestIdx := -1
    53  		var lowest blob.SizedRef
    54  		for idx, peeker := range peekers {
    55  			for !peeker.Closed() && peeker.MustPeek().Ref.String() <= lastSent {
    56  				peeker.Take()
    57  			}
    58  			if peeker.Closed() {
    59  				continue
    60  			}
    61  			sb := peeker.MustPeek()                                       // can't be nil if not Closed
    62  			if lowestIdx == -1 || sb.Ref.String() < lowest.Ref.String() { // TODO: add cheaper Ref comparison function, avoiding String
    63  				lowestIdx = idx
    64  				lowest = sb
    65  			}
    66  		}
    67  		if lowestIdx == -1 {
    68  			// all closed
    69  			break
    70  		}
    71  
    72  		dest <- lowest
    73  		nSent++
    74  		lastSent = lowest.Ref.String()
    75  	}
    76  
    77  	// Once we've gotten enough, ignore the rest of whatever's
    78  	// coming in.
    79  	for _, peeker := range peekers {
    80  		go peeker.ConsumeAll()
    81  	}
    82  
    83  	// If any part returns an error, we return an error.
    84  	var retErr error
    85  	for _, errch := range errs {
    86  		if err := <-errch; err != nil {
    87  			retErr = err
    88  		}
    89  	}
    90  	return retErr
    91  }