CR: clean up graphics context code

Signal/src/ViewControllers/CropScaleImageViewController.swift

@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 import Foundation
@@ -490,15 +490,18 @@ import SignalMessaging
         let dstScale: CGFloat = 1.0 // The size is specified in pixels, not in points.
         UIGraphicsBeginImageContextWithOptions(dstSizePixels, !hasAlpha, dstScale)
 
-        let context = UIGraphicsGetCurrentContext()
-        context!.interpolationQuality = .high
+        guard let context = UIGraphicsGetCurrentContext() else {
+            owsFailDebug("could not generate dst image.")
+            return nil
+        }
+        context.interpolationQuality = .high
 
         let imageViewFrame = imageRenderRect(forDstSize: dstSizePixels)
         srcImage.draw(in: imageViewFrame)
 
-        let scaledImage = UIGraphicsGetImageFromCurrentImageContext()
-        if scaledImage == nil {
+        guard let scaledImage = UIGraphicsGetImageFromCurrentImageContext() else {
             owsFailDebug("could not generate dst image.")
+            return nil
         }
         UIGraphicsEndImageContext()
         return scaledImage

SignalMessaging/Views/ImageEditor/ImageEditorModel.swift

@@ -648,7 +648,7 @@ public class ImageEditorModel: NSObject {
         defer { UIGraphicsEndImageContext() }
 
         guard let context = UIGraphicsGetCurrentContext() else {
-            owsFailDebug("Could not create output context.")
+            owsFailDebug("context was unexpectedly nil")
             return nil
         }
         context.interpolationQuality = .high

SignalMessaging/utils/OWSContactAvatarBuilder.m

@@ -193,9 +193,11 @@ NS_ASSUME_NONNULL_BEGIN
     CGFloat paddedheight = baseImage.size.height * paddingFactor;
 
     UIGraphicsBeginImageContextWithOptions(CGSizeMake(paddedWidth, paddedheight), NO, 0.0);
-    CGContextRef context = UIGraphicsGetCurrentContext();
-    UIGraphicsPushContext(context);
-
+    CGContextRef _Nullable context = UIGraphicsGetCurrentContext();
+    if (context == nil) {
+        OWSFailDebug(@"failure: context was unexpectedly nil");
+        return nil;
+    }
     [backgroundColor setFill];
     CGContextFillRect(context, CGRectMake(0, 0, paddedWidth, paddedheight));
 
@@ -203,10 +205,11 @@ NS_ASSUME_NONNULL_BEGIN
                                  (paddedheight - baseImage.size.height) / 2.0f);
     [baseImage drawAtPoint:origin];
 
-
-    // Clean up and get the new image.
-    UIGraphicsPopContext();
     UIImage *paddedImage = UIGraphicsGetImageFromCurrentImageContext();
+    if (paddedImage == nil) {
+        OWSFailDebug(@"failure: paddedImage was unexpectedly nil");
+        return nil;
+    }
     UIGraphicsEndImageContext();
 
     return paddedImage;

SignalMessaging/utils/UIImage+OWS.swift

@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 import Foundation
@@ -11,6 +11,7 @@ extension UIImage {
         defer { UIGraphicsEndImageContext() }
 
         guard let context = UIGraphicsGetCurrentContext() else {
+            owsFailDebug("context was unexpectedly nil")
             return nil
         }