Respond to CR.

// FREEBIE

Signal/src/ViewControllers/GifPicker/GifPickerViewController.swift

@@ -147,6 +147,9 @@ class GifPickerViewController: OWSViewController, UISearchBarDelegate, UICollect
     private func createViews() {
 
         view.backgroundColor = UIColor.white
+
+        // Block UIKit from adjust insets of collection view which screws up
+        // min/max scroll positions.
         self.automaticallyAdjustsScrollViewInsets = false
 
         // Search

Signal/src/network/GiphyDownloader.swift

@@ -23,14 +23,21 @@ class GiphyAssetSegment {
     public let index: UInt
     public let segmentStart: UInt
     public let segmentLength: UInt
+    // The amount of the segment that is overlap.  
+    // The overlap lies in the _first_ n bytes of the segment data.
     public let redundantLength: UInt
 
+    // This state should only be accessed on the main thread.
     public var state: GiphyAssetSegmentState = .waiting {
         didSet {
             AssertIsOnMainThread()
         }
     }
-    private var datas = [Data]()
+    // This state is accessed off the main thread.
+    //
+    // * During downloads it will be accessed on the task delegate queue.
+    // * After downloads it will be accessed on a worker queue. 
+    private var segmentData = NSMutableData()
 
     init(index: UInt,
          segmentStart: UInt,
@@ -43,11 +50,7 @@ class GiphyAssetSegment {
     }
 
     public func totalDataSize() -> UInt {
-        var result: UInt = 0
+        return UInt(segmentData.length)
-        for data in datas {
-            result += UInt(data.count)
-        }
-        return result
     }
 
     public func append(data: Data) {
@@ -56,37 +59,31 @@ class GiphyAssetSegment {
             return
         }
 
-        datas.append(data)
+        segmentData.append(data)
     }
 
-    public func mergeData(assetData: NSMutableData) {
+    public func mergeData(assetData: NSMutableData) -> Bool {
         guard state == .complete else {
             owsFail("\(TAG) merging data in invalid state: \(state)")
-            return
+            return false
+        }
+        guard UInt(segmentData.length) == segmentLength else {
+            owsFail("\(TAG) segment data length: \(segmentData.length) doesn't match expected length: \(segmentLength)")
+            return false
         }
 
         // In some cases the last two segments will overlap.
         // In that case, we only want to append the non-overlapping
-        // tail of the last segment.
+        // tail of the segment data.
-        var bytesToIgnore = Int(redundantLength)
+        let bytesToIgnore = Int(redundantLength)
-        var totalDataBytes: UInt = 0
+        if bytesToIgnore > 0 {
-        for data in datas {
+            let range = NSMakeRange(bytesToIgnore, segmentData.length - bytesToIgnore)
-            totalDataBytes += UInt(data.count)
+            let subdata = (segmentData as NSData).subdata(with: range)
-            if data.count <= bytesToIgnore {
-                bytesToIgnore -= data.count
-            } else if bytesToIgnore > 0 {
-                let range = NSMakeRange(bytesToIgnore, data.count - bytesToIgnore)
-                let subdata = (data as NSData).subdata(with: range)
             assetData.append(subdata)
-                bytesToIgnore = 0
         } else {
-                assetData.append(data)
+            assetData.append(segmentData as Data)
-            }
-        }
-        guard totalDataBytes == segmentLength else {
-            owsFail("\(TAG) segment data length: \(totalDataBytes) doesn't match expected length: \(segmentLength)")
-            return
         }
+        return true
     }
 }
 
@@ -122,6 +119,7 @@ enum GiphyAssetRequestState: UInt {
     // This property is an internal implementation detail of the download process.
     var assetFilePath: String?
 
+    // This state should only be accessed on the main thread.
     private var segments = [GiphyAssetSegment]()
     public var state: GiphyAssetRequestState = .waiting
     public var contentLength: Int = 0 {
@@ -260,7 +258,10 @@ enum GiphyAssetRequestState: UInt {
                 owsFail("\(TAG) could not merge empty segment.")
                 return nil
             }
-            segment.mergeData(assetData: assetData)
+            guard segment.mergeData(assetData: assetData) else {
+                owsFail("\(TAG) failed to merge segment data.")
+                return nil
+            }
         }
 
         guard assetData.length == contentLength else {
@@ -496,7 +497,13 @@ extension URLSessionTask {
                                              success:success,
                                              failure:failure)
         assetRequestQueue.append(assetRequest)
-        processRequestQueue()
+        // Process the queue (which may start this request)
+        // asynchronously so that the caller has time to store
+        // a reference to the asset request returned by this 
+        // method before its success/failure handler is called.
+        DispatchQueue.main.async {
+            self.processRequestQueue()
+        }
         return assetRequest
     }
 
@@ -575,11 +582,12 @@ extension URLSessionTask {
         processRequestQueue()
     }
 
-    // Start a request if necessary, complete asset requests if possible.
+    // * Start a segment request or content length request if possible.
+    // * Complete/cancel asset requests if possible.
+    //
     private func processRequestQueue() {
         AssertIsOnMainThread()
 
-        DispatchQueue.main.async {
         guard let assetRequest = self.popNextAssetRequest() else {
             return
         }
@@ -639,7 +647,6 @@ extension URLSessionTask {
             }
             assetSegment.state = .downloading
 
-
             var request = URLRequest(url: assetRequest.rendition.url as URL)
             let rangeHeaderValue = "bytes=\(assetSegment.segmentStart)-\(assetSegment.segmentStart + assetSegment.segmentLength - 1)"
             request.addValue(rangeHeaderValue, forHTTPHeaderField: "Range")
@@ -652,7 +659,6 @@ extension URLSessionTask {
         // Recurse; we may be able to start multiple downloads.
         self.processRequestQueue()
     }
-    }
 
     private func handleAssetSizeResponse(assetRequest: GiphyAssetRequest, response: URLResponse?, error: Error?) {
         guard error == nil else {