per design: "back" is retake for first photo

Signal/src/ViewControllers/Photos/SendMediaNavigationController.swift

@@ -259,6 +259,13 @@ extension SendMediaNavigationController: UINavigationControllerDelegate {
                 owsFailDebug("unexpected navigationBar: \(navigationBar)")
             }
         }
+
+        if viewController is PhotoCaptureViewController && !isInBatchSelectMode {
+            // We're either showing the captureView for the first time or the user is navigating "back"
+            // indicating they want to "retake". We should discard any current image.
+            discardCameraDraft()
+        }
+
         self.updateButtons(topViewController: viewController)
     }
 
@@ -305,6 +312,14 @@ extension SendMediaNavigationController: PhotoCaptureViewControllerDelegate {
         let dontAbandonText = NSLocalizedString("SEND_MEDIA_RETURN_TO_CAMERA", comment: "alert action when the user decides not to cancel the media flow after all.")
         didRequestExit(dontAbandonText: dontAbandonText)
     }
+
+    func discardCameraDraft() {
+        assert(attachmentDraftCollection.cameraAttachments.count <= 1)
+        if let lastCameraAttachment = attachmentDraftCollection.cameraAttachments.last {
+            attachmentDraftCollection.remove(attachment: lastCameraAttachment)
+        }
+        assert(attachmentDraftCollection.cameraAttachments.count == 0)
+    }
 }
 
 extension SendMediaNavigationController: ImagePickerGridControllerDelegate {
@@ -457,6 +472,17 @@ private struct AttachmentDraftCollection {
         }
     }
 
+    var cameraAttachments: [SignalAttachment] {
+        return attachmentDrafts.compactMap { attachmentDraft in
+            switch attachmentDraft.source {
+            case .picker:
+                return nil
+            case .camera(let cameraAttachment):
+                return cameraAttachment
+            }
+        }
+    }
+
     mutating func append(_ element: AttachmentDraft) {
         attachmentDrafts.append(element)
     }