Limit number of image resizes happening at once.

Attempt to ease memory spikes.

Change-Id: I4df7177c063b284ac841335d8f18b1a6da5289d0

TODO

@@ -14,8 +14,6 @@ Offline list:
    instead of using schema.WriteFileFromReader if the thumbnail
    is smaller than blobserver.MaxBlobSize (16MB).
 
--- gate in front of image resizes. limit a few at a time.
-
 -- blobhub.go NotifyBlobReceived should do the sync hooks before firing off
    goroutines to notify that things were good. they might not be good yet
    if the hook fails. also, no need for the slice. can just fire off goroutines

pkg/server/image.go

@@ -36,6 +36,7 @@ import (
 	"camlistore.org/pkg/magic"
 	"camlistore.org/pkg/schema"
 	"camlistore.org/pkg/search"
+	"camlistore.org/pkg/syncutil"
 
 	_ "camlistore.org/third_party/github.com/nf/cr2"
 )
@@ -152,6 +153,20 @@ func (ih *ImageHandler) scaledCached(buf *bytes.Buffer, file blob.Ref) (format s
 	return format
 }
 
+// These gates control the max concurrency of slurping raw images
+// (e.g. JPEG bytes) to RAM, and then decoding and resizing them,
+// respectively.  We allow more concurrency for the former because
+// it's slower and less memory-intensive.  The actual resizing takes
+// much more CPU and RAM.
+
+// TODO: these numbers were just guesses and not based on any
+// data. measure? make these configurable? Automatically tuned
+// somehow? Based on memory usage/availability?
+var (
+	scaleImageGateSlurp  = syncutil.NewGate(5)
+	scaleImageGateResize = syncutil.NewGate(2)
+)
+
 func (ih *ImageHandler) scaleImage(buf *bytes.Buffer, file blob.Ref) (format string, err error) {
 	fr, err := schema.NewFileReader(ih.storageSeekFetcher(), file)
 	if err != nil {
@@ -159,10 +174,16 @@ func (ih *ImageHandler) scaleImage(buf *bytes.Buffer, file blob.Ref) (format str
 	}
 	defer fr.Close()
 
+	scaleImageGateSlurp.Start()
 	_, err = io.Copy(buf, fr)
+	scaleImageGateSlurp.Done()
 	if err != nil {
 		return format, fmt.Errorf("image resize: error reading image %s: %v", file, err)
 	}
+
+	scaleImageGateResize.Start()
+	defer scaleImageGateResize.Done()
+
 	i, imConfig, err := images.Decode(bytes.NewReader(buf.Bytes()),
 		&images.DecodeOpts{MaxWidth: ih.MaxWidth, MaxHeight: ih.MaxHeight})
 	if err != nil {