Realm crash on Results.first.getter - crash

Question

Realm 5.0.2
I'm trying to understand how this crash is possible. It is crashing when calling first on a Realm Results object. I'm guessing this is a concurrency issue, but Realm is instantiated on same thread as query, and the Realm objects are immediately converted to app objects. But if it's not a concurrency issue, then ... what?!
0
Thread 1 Queue : com.apple.main-thread (serial)
#0 0x00000001037d8f7c in realm::IndexArray::index_string_all(realm::StringData, std::__1::vector<realm::ObjKey, std::__1::allocator<realm::ObjKey> >&, realm::ClusterColumn const&) const ()
#1 0x00000001036c8fdc in realm::IntegerNode<realm::ArrayIntNull, realm::Equal>::init() ()
#2 0x00000001036a740c in realm::Query::init() const ()
#3 0x00000001036a74dc in realm::Query::find_all(realm::ConstTableView&, unsigned long, unsigned long, unsigned long) const ()
#4 0x0000000103866e7c in realm::ConstTableView::do_sync() ()
#5 0x00000001036a78d0 in realm::Query::find_all(unsigned long, unsigned long, unsigned long) ()
#6 0x00000001036a7d50 in realm::Query::find_all(realm::DescriptorOrdering const&) ()
#7 0x00000001033af2c4 in realm::Results::do_evaluate_query_if_needed(bool)
#8 0x00000001033aeca8 in realm::util::Optional<realm::Obj> realm::Results::try_get<realm::Obj>(unsigned long)
#9 0x00000001033af01c in realm::util::Optional<realm::Obj> realm::Results::first<realm::Obj>()
#10 0x0000000103542b9c in _ZZN5realm7Results5firstI18RLMAccessorContextEEDaRT_ENKUlTyS4_E_clIPNS_3ObjEEES3_S4_
#11 0x0000000103541d38 in _ZN5realmL14switch_on_typeINS_3ObjEZNS_7Results5firstI18RLMAccessorContextEEDaRT_EUlTyS6_E_EES5_NS_12PropertyTypeEOT0_
#12 0x00000001035418c0 in _ZNK5realm7Results8dispatchIZNS0_5firstI18RLMAccessorContextEEDaRT_EUlTyS5_E_EES4_OS5_
#13 0x0000000103541870 in auto realm::Results::first<RLMAccessorContext>(RLMAccessorContext&)
#14 0x0000000103541834 in -[RLMResults firstObject]::$_8::operator()() const
#15 0x000000010353d5c4 in auto translateRLMResultsErrors<-[RLMResults firstObject]::$_8>(-[RLMResults firstObject]::$_8&&, NSString*)
#16 0x000000010353d544 in -[RLMResults firstObject]
#17 0x0000000104968cf4 in Results.first.getter
#18 0x000000010228e97c in static LibraryManager.LibraryPersistence.findPersistentTag(permanentId:temporaryId:)
#19 0x000000010228e500 in LibraryManager.getTag(forAssignment:)
#20 0x000000010220c284 in closure #1 in AudiobookUserSettings.tags.getter
#21 0x000000019eb5e038 in Sequence.compactMap<A>(_:) ()
#22 0x000000010220c1d0 in AudiobookUserSettings.tags.getter
#23 0x0000000102195e70 in Audiobook.tags.getter
#24 0x000000010226407c in FolderManager.updateFromLibrary()
#25 0x00000001022ca78c in LibraryTableViewController.refreshFolders()
#26 0x00000001022c6930 in closure #1 in LibraryTableViewController.refreshView()
#27 0x0000000102150700 in thunk for #escaping #callee_guaranteed () -> () ()
#28 0x0000000104b7605c in _dispatch_call_block_and_release ()
#29 0x0000000104b774d8 in _dispatch_client_callout ()
#30 0x0000000104b85f64 in _dispatch_main_queue_callback_4CF ()
#31 0x00000001911cc8d4 in __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ ()
#32 0x00000001911c758c in __CFRunLoopRun ()
#33 0x00000001911c6bc8 in CFRunLoopRunSpecific ()
#34 0x000000019b5a85cc in GSEventRunModal ()
#35 0x0000000195379744 in UIApplicationMain ()
#36 0x00000001023d838c in main
#37 0x0000000191043384 in start ()
Enqueued from audiobookSettings-persistence (Thread 33) Queue
#0 0x0000000104b7bdd0 in dispatch_async ()
#1 0x00000001c807004c in OS_dispatch_queue.async(group:qos:flags:execute:) ()
#2 0x00000001022c6808 in LibraryTableViewController.refreshView()
#3 0x00000001022c6288 in closure #1 in LibraryTableViewController.viewWillAppear(_:)
The relevant code from the app:
func getTag(forAssignment assignment: TagAssignment) -> Tag? {
let tag = LibraryPersistence.findPersistentTag(permanentId: assignment.permanentTagId,
temporaryId: assignment.temporaryTagId)
return tag?.toTag()
}
static func findPersistentTag(permanentId: Int? = nil, temporaryId: String? = nil) -> PersistentTag? {
guard let userId = getIdForCurrentUser() else {
return nil
}
let realm = try! Realm()
// First try lookup by permanent tag id
if let permanentId = permanentId {
let permanentIdNumber = NSNumber(value: permanentId)
if let stored = realm.objects(PersistentTag.self).filter("permanentId == %# AND userId == %#", permanentIdNumber, userId).first {
return stored
}
}
// Then try temporary tag id
if let temporaryId = temporaryId {
return realm.objects(PersistentTag.self).filter("temporaryId == %# AND userId == %#", temporaryId, userId).first
}
return nil
}
UPDATE: More crash details:
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
#0: 0x0000000103bfc2fd Realm`realm::util::EncryptedFileMapping::read_barrier(void const*, unsigned long, unsigned long (*)(char const*)) + 29
* frame #1: 0x00000001035ce000 Realm`realm::util::do_encryption_read_barrier(addr=0x00007fff87a95540, size=8, header_to_size=(Realm`realm::NodeHeader::get_byte_size_from_header(char const*) at node_header.hpp:201), mapping=0xca4c51df08c18be5)(char const*), realm::util::EncryptedFileMapping*) at file_mapper.hpp:133:14
#2: 0x0000000103b496ee Realm`realm::IndexArray::index_string_all(realm::StringData, std::__1::vector<realm::ObjKey, std::__1::allocator<realm::ObjKey> >&, realm::ClusterColumn const&) const + 398
...

Answer 1

It looks like you're not the only one with this problem.
https://github.com/realm/realm-cocoa/issues/6556
This looks like a problem with 5.0.x, some have downgraded to 4.x to remove the problem. It references another issue (bottom of the report) which has been fixed and merged, but not yet released.
I'd read the bug report and see if this follows your experience, then either (a) follow the workaround, or (b) downgrade to 4.4.x. Watch the fix issue (https://github.com/realm/realm-core/pull/3828) for being included in a new release.