Add asserts around attachment crash.

SignalServiceKit/src/Messages/Attachments/TSAttachmentPointer.m

@@ -8,6 +8,7 @@
 #import <SignalServiceKit/MimeTypeUtil.h>
 #import <SignalServiceKit/SignalServiceKit-Swift.h>
 #import <YapDatabase/YapDatabase.h>
+#import <YapDatabase/YapDatabaseTransaction.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -209,6 +210,22 @@ NS_ASSUME_NONNULL_BEGIN
                              }];
 }
 
+- (void)saveWithTransaction:(YapDatabaseReadWriteTransaction *)transaction
+{
+#ifdef DEBUG
+    if (self.uniqueId.length > 0) {
+        id _Nullable oldObject = [transaction objectForKey:self.uniqueId inCollection:TSAttachment.collection];
+        if ([oldObject isKindOfClass:[TSAttachmentStream class]]) {
+            OWSFailDebug(@"We should never overwrite a TSAttachmentStream with a TSAttachmentPointer.");
+        }
+    } else {
+        OWSFailDebug(@"Missing uniqueId.");
+    }
+#endif
+
+    [super saveWithTransaction:transaction];
+}
+
 @end
 
 NS_ASSUME_NONNULL_END

SignalServiceKit/src/Messages/Attachments/TSAttachmentStream.m

@@ -524,10 +524,11 @@ typedef void (^OWSLoadedThumbnailSuccess)(OWSLoadedThumbnail *loadedThumbnail);
         self.cachedImageHeight = @(imageSize.height);
 
         [self.dbReadWriteConnection asyncReadWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
-
+            NSString *collection = [TSAttachmentStream collection];
-            NSString *collection = [[self class] collection];
             TSAttachmentStream *latestInstance = [transaction objectForKey:self.uniqueId inCollection:collection];
-            if (latestInstance) {
+            if (![latestInstance isKindOfClass:[TSAttachmentStream class]]) {
+                OWSFailDebug(@"Attachment has unexpected type: %@", latestInstance.class);
+            } else if (latestInstance) {
                 latestInstance.cachedImageWidth = @(imageSize.width);
                 latestInstance.cachedImageHeight = @(imageSize.height);
                 [latestInstance saveWithTransaction:transaction];