Findit identified the culprit r552159 with confidence 70.0% in the config "chromium.memory / Linux ASan LSan Tests (1)"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyzQELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKWAWNocm9taXVtLm1lbW9yeS9MaW51eCBBU2FuIExTYW4gVGVzdHMgKDEpLzQ0OTAyL2Jyb3dzZXJfdGVzdHMvVTFKRFgwVjRkR1Z5Ym1Gc1EyeGxZWEpMWlhrdlJXNWpjbmx3ZEdWa1RXVmthV0ZVWlhOMExsQnNZWGxpWVdOclgxWnBaR1Z2UVhWa2FXOWZWMlZpVFM4dwwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM


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Fails here: https://ci.chromium.org/buildbot/chromium.memory/Linux%20ASan%20LSan%20Tests%20(1)/44902

Also a similar flake for MSE_ExternalClearKey/EncryptedMediaTest.Playback_VP9Video_WebM_Subsample a couple of weeks ago: https://ci.chromium.org/buildbot/chromium.memory/Linux%20ASan%20LSan%20Tests%20%281%29/44439

Pretty rare, though.

Looking at the log from #3, the test did pass ("Set document title to: ENDED, updated title: ENDED"). The log also contains:
[       OK ] SRC_ExternalClearKey/EncryptedMediaTest.Playback_VideoAudio_WebM/0 (20536 ms)

However, LSAN found a bunch of leaks. The first one is shown below (full result attached as file). I have to assume this is a teardown ordering issue. The leak below goes through CSSParserImpl, but the test (https://cs.chromium.org/chromium/src/media/test/data/eme_player.html) has no CSS. None of the leaks appear to originate from the code that is exercised by these tests (modules/encryptedmedia or other media/ code).

Adding tag Blink>CSS in case somebody from CSS has better ideas on why there are leaks from CSS code.

==14185==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 35776 byte(s) in 344 object(s) allocated from:
    #0 0x7d5e913 in __interceptor_malloc /b/build/slave/linux_upload_clang/build/src/third_party/llvm/compiler-rt/lib/asan/asan_malloc_linux.cc:88:3
    #1 0x115d7e2d in PartitionAllocGenericFlags base/allocator/partition_allocator/partition_alloc.h:947:18
    #2 0x115d7e2d in Alloc base/allocator/partition_allocator/partition_alloc.h:967
    #3 0x115d7e2d in WTF::Partitions::FastMalloc(unsigned long, char const*) third_party/blink/renderer/platform/wtf/allocator/partitions.h:121
    #4 0x20f1725d in operator new third_party/blink/renderer/core/css/css_variable_data.h:21:3
    #5 0x20f1725d in Create third_party/blink/renderer/core/css/css_variable_data.h:30
    #6 0x20f1725d in blink::CSSVariableParser::ParseDeclarationValue(WTF::AtomicString const&, blink::CSSParserTokenRange, bool) third_party/blink/renderer/core/css/parser/css_variable_parser.cc:132
    #7 0x20e8df8e in blink::CSSParserImpl::ConsumeVariableValue(blink::CSSParserTokenRange, WTF::AtomicString const&, bool, bool) third_party/blink/renderer/core/css/parser/css_parser_impl.cc:970:11
    #8 0x20e9b8c2 in blink::CSSParserImpl::ConsumeDeclaration(blink::CSSParserTokenRange, blink::CSSParserImpl::RangeOffset const&, blink::StyleRuleBase::RuleType) third_party/blink/renderer/core/css/parser/css_parser_impl.cc:946:5
    #9 0x20e8fbcc in blink::CSSParserImpl::ConsumeDeclarationList(blink::CSSParserTokenStream&, blink::StyleRuleBase::RuleType) third_party/blink/renderer/core/css/parser/css_parser_impl.cc:875:9
    #10 0x20ea50ea in blink::CSSParserImpl::ConsumeStyleRule(blink::CSSParserTokenStream&) third_party/blink/renderer/core/css/parser/css_parser_impl.cc:827:3
    #11 0x20e9579b in blink::CSSParserImpl::ConsumeQualifiedRule(blink::CSSParserTokenStream&, blink::CSSParserImpl::AllowedRulesType) third_party/blink/renderer/core/css/parser/css_parser_impl.cc:528:12
    #12 0x20e967ef in ConsumeRuleList<(lambda at ../../third_party/blink/renderer/core/css/parser/css_parser_impl.cc:263:34)> third_party/blink/renderer/core/css/parser/css_parser_impl.cc:441:16
    #13 0x20e967ef in blink::CSSParserImpl::ParseStyleSheet(WTF::String const&, blink::CSSParserContext const*, blink::StyleSheetContents*, bool) third_party/blink/renderer/core/css/parser/css_parser_impl.cc:262
    #14 0x212ac3ad in ParseSheet third_party/blink/renderer/core/css/style_engine.cc:699:28
    #15 0x212ac3ad in blink::StyleEngine::CreateSheet(blink::Element&, WTF::String const&, WTF::TextPosition, blink::StyleEngineContext&) third_party/blink/renderer/core/css/style_engine.cc:668
    #16 0x2295bfaa in blink::StyleElement::CreateSheet(blink::Element&, WTF::String const&) third_party/blink/renderer/core/css/style_element.cc:166:43
    #17 0x2295b665 in Process third_party/blink/renderer/core/css/style_element.cc:110:10
    #18 0x2295b665 in blink::StyleElement::FinishParsingChildren(blink::Element&) third_party/blink/renderer/core/css/style_element.cc:102
    #19 0x22956d58 in blink::HTMLStyleElement::FinishParsingChildren() third_party/blink/renderer/core/html/html_style_element.cc:70:21
    #20 0x224a2aba in blink::HTMLElementStack::PopCommon() third_party/blink/renderer/core/html/parser/html_element_stack.cc:501:10
    #21 0x224a3918 in blink::HTMLElementStack::Pop() third_party/blink/renderer/core/html/parser/html_element_stack.cc:179:3
    #22 0x225c77d5 in blink::HTMLTreeBuilder::ProcessEndTag(blink::AtomicHTMLToken*) third_party/blink/renderer/core/html/parser/html_tree_builder.cc:2050:29
    #23 0x225abb77 in blink::HTMLTreeBuilder::ProcessToken(blink::AtomicHTMLToken*) third_party/blink/renderer/core/html/parser/html_tree_builder.cc:359:7
    #24 0x225a3799 in blink::HTMLTreeBuilder::ConstructTree(blink::AtomicHTMLToken*) third_party/blink/renderer/core/html/parser/html_tree_builder.cc:313:5
    #25 0x2242c8ee in blink::HTMLDocumentParser::ConstructTreeFromCompactHTMLToken(blink::CompactHTMLToken const&) third_party/blink/renderer/core/html/parser/html_document_parser.cc:750:18
    #26 0x2242a807 in blink::HTMLDocumentParser::ProcessTokenizedChunkFromBackgroundParser(std::__1::unique_ptr<blink::HTMLDocumentParser::TokenizedChunk, std::__1::default_delete<blink::HTMLDocumentParser::TokenizedChunk> >) third_party/blink/renderer/core/html/parser/html_document_parser.cc:534:5
    #27 0x22422e45 in blink::HTMLDocumentParser::PumpPendingSpeculations() third_party/blink/renderer/core/html/parser/html_document_parser.cc:608:9
    #28 0x22422042 in blink::HTMLDocumentParser::ResumeParsingAfterYield() third_party/blink/renderer/core/html/parser/html_document_parser.cc:274:3
    #29 0x13917f77 in Run base/callback.h:95:12
    #30 0x13917f77 in RunInternal third_party/blink/renderer/platform/wtf/functional.h:258
    #31 0x13917f77 in WTF::ThreadCheckingCallbackWrapper<base::OnceCallback<void ()>, void ()>::Run() third_party/blink/renderer/platform/wtf/functional.h:245
    #32 0x206cdf63 in Run base/callback.h:95:12
    #33 0x206cdf63 in blink::TaskHandle::Runner::Run(blink::TaskHandle const&) third_party/blink/renderer/platform/web_task_runner.cc:75
    #34 0x206cf1b6 in Invoke<base::WeakPtr<blink::TaskHandle::Runner>, blink::TaskHandle> base/bind_internal.h:447:12
    #35 0x206cf1b6 in MakeItSo<void (blink::TaskHandle::Runner::*)(const blink::TaskHandle &), base::WeakPtr<blink::TaskHandle::Runner>, blink::TaskHandle> base/bind_internal.h:550
    #36 0x206cf1b6 in RunImpl<void (blink::TaskHandle::Runner::*)(const blink::TaskHandle &), std::__1::tuple<base::WeakPtr<blink::TaskHandle::Runner>, blink::TaskHandle>, 0, 1> base/bind_internal.h:604
    #37 0x206cf1b6 in base::internal::Invoker<base::internal::BindState<void (blink::TaskHandle::Runner::*)(blink::TaskHandle const&), base::WeakPtr<blink::TaskHandle::Runner>, blink::TaskHandle>, void ()>::RunOnce(base::internal::BindStateBase*) base/bind_internal.h:572
    #38 0x13917f77 in Run base/callback.h:95:12
    #39 0x13917f77 in RunInternal third_party/blink/renderer/platform/wtf/functional.h:258
    #40 0x13917f77 in WTF::ThreadCheckingCallbackWrapper<base::OnceCallback<void ()>, void ()>::Run() third_party/blink/renderer/platform/wtf/functional.h:245
    #41 0x145d59ad in Run base/callback.h:95:12
    #42 0x145d59ad in base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) base/debug/task_annotator.cc:101
    #43 0x13a76644 in blink::scheduler::internal::ThreadControllerImpl::DoWork(blink::scheduler::internal::SequencedTaskSource::WorkType) third_party/blink/renderer/platform/scheduler/base/thread_controller_impl.cc:162:21
    #44 0x13a7be5b in Invoke<const base::WeakPtr<blink::scheduler::internal::ThreadControllerImpl> &, const blink::scheduler::internal::SequencedTaskSource::WorkType &> base/bind_internal.h:447:12
    #45 0x13a7be5b in MakeItSo<void (blink::scheduler::internal::ThreadControllerImpl::*const &)(blink::scheduler::internal::SequencedTaskSource::WorkType), const base::WeakPtr<blink::scheduler::internal::ThreadControllerImpl> &, const blink::scheduler::internal::SequencedTaskSource::WorkType &> base/bind_internal.h:550
    #46 0x13a7be5b in RunImpl<void (blink::scheduler::internal::ThreadControllerImpl::*const &)(blink::scheduler::internal::SequencedTaskSource::WorkType), const std::__1::tuple<base::WeakPtr<blink::scheduler::internal::ThreadControllerImpl>, blink::scheduler::internal::SequencedTaskSource::WorkType> &, 0, 1> base/bind_internal.h:604
    #47 0x13a7be5b in base::internal::Invoker<base::internal::BindState<void (blink::scheduler::internal::ThreadControllerImpl::*)(blink::scheduler::internal::SequencedTaskSource::WorkType), base::WeakPtr<blink::scheduler::internal::ThreadControllerImpl>, blink::scheduler::internal::SequencedTaskSource::WorkType>, void ()>::Run(base::internal::BindStateBase*) base/bind_internal.h:586
    #48 0x145d59ad in Run base/callback.h:95:12
    #49 0x145d59ad in base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) base/debug/task_annotator.cc:101

Deleted: LogDog.log 1.2 MB

LogDog.log 1.2 MB View Download

Findit identified the culprit r564933 with confidence 70.0% in the config "chromium.memory / Linux MSan Tests"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyxAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKNAWNocm9taXVtLm1lbW9yeS9MaW51eCBNU2FuIFRlc3RzLzEwMjU4L2Jyb3dzZXJfdGVzdHMvVTFKRFgwVjRkR1Z5Ym1Gc1EyeGxZWEpMWlhrdlJXNWpjbmx3ZEdWa1RXVmthV0ZVWlhOMExsQnNZWGxpWVdOclgxWnBaR1Z2UVhWa2FXOWZWMlZpVFM4dwwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM


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Feedback is welcome! Please use component Tools>Test>FindIt>Flakiness

The whole test suite looks flaky - I've just hit a flake in CDM_9/ECKEncryptedMediaTest.DecryptOnly_VideoOnly_MP4_VP9/0 [1] and SRC_ExternalClearKey/EncryptedMediaTest.Playback_VP9Video_WebM_Subsample/0 [2]

[1] https://ci.chromium.org/p/chromium/builders/luci.chromium.try/mac_chromium_rel_ng/70810
[2] https://ci.chromium.org/p/chromium/builders/luci.chromium.try/mac_chromium_rel_ng/70730