Crash Dumps Archiving

Testing framework and buildbot/LUCI infrastructure have support for archiving crash dumps and crashed binaries for unexpected crashes for postmortem debugging.

This feature is enabled by passing --copy-coredumps to tools/test.py and is supported on all three major OSes we run our tests on.

Currently crash dump archiving should be enabled on the majority of the VM bots, including those running in CQ.

Known limitations

• We don't archive crash dumps produced by subprocesses (e.g. service tests);
• We don't produce and archive crash dumps for hanged processes.

Maintainers

For any questions about this infrastructure contact vegorov@, kustermann@ or athom@.

Downloading dumps

To find if a bot managed to upload any crashdumps open its log and search for Archiving coredumps for crash. Depending on the bot configuration you might need to look for uploaded artifacts in two different locations.

Isolate Server

Swarming bots upload dumps to the isolate server. In this case you will see something like this in the log:

INFO: Core dump archiving is activated
Archiving coredumps for crash (if possible):
----> Crash(language_2/call_method_implicit_tear_off_test/02: binary.release_x64_dart.exe 1556)
+++ Copying 6e3e4b40-206e-4ae6-99e8-c2c94cc21a98.dmp.tar.gz to output_directory (C:\b\s\w\ioz_p_ei)
+++ Copying release_x64_dart.exe.tar.gz to output_directory (C:\b\s\w\ioz_p_ei)

If you see +++ Copying ... to output_directory this means that the bot is using isolate storage. In this case you need to look for shard #... isolated out link under the failed step.

This link would bring you to isolate server file browser where you can see the artifacts you can download.

One caveat is that file browser might fail to download large files, so you might need to use cas tool if you need to download those.

cas tool is installed using cipd:

cipd install "infra/tools/luci/cas/\${platform}" -root ~/cas_tool

Then you point cas at digest hash for the directory to be downloaded:

~/cas_tool/cas download -cas-instance projects/chromium-swarm/instances/default_instance -digest <digest_hash> -dir downloaded

where <digest_hash> is ‘Digest’ value from the isolate server browser listing.

Cloud Storage

Other bots upload collected artifacts into a dart-temp-crash-archive Cloud Storage bucket. In this case you will see +++ Uploaded gs://dart-temp-crash-archive/... in the output log. For example:

Archiving coredumps for crash (if possible):
----> Crash(language_2/mixin_interface_check_test/mixin: binary.release_ia32_dart.exe 10620)

--- Uploading into gs://dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/ (https://storage.cloud.google.com/dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/) ---
Running command:  ['C:\\b\\s\\w\\ir\\cache\\vpython\\2d366b\\Scripts\\python.exe', 'C:\\b\\s\\w\\ir\\cache\\builder\\sdk\\third_party\\gsutil\\gsutil', 'cp', 'release_ia32_dart.exe.tar.gz', 'gs://dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/release_ia32_dart.exe.tar.gz']
+++ Uploaded gs://dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/release_ia32_dart.exe.tar.gz (https://storage.cloud.google.com/dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/release_ia32_dart.exe.tar.gz)
Running command:  ['C:\\b\\s\\w\\ir\\cache\\vpython\\2d366b\\Scripts\\python.exe', 'C:\\b\\s\\w\\ir\\cache\\builder\\sdk\\third_party\\gsutil\\gsutil', 'cp', 'a92618b1-0f99-4c87-a9fb-adb3e44ad458.dmp.tar.gz', 'gs://dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/a92618b1-0f99-4c87-a9fb-adb3e44ad458.dmp.tar.gz']
+++ Uploaded gs://dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/a92618b1-0f99-4c87-a9fb-adb3e44ad458.dmp.tar.gz (https://storage.cloud.google.com/dart-temp-crash-archive/0da77e10-d349-4c45-a146-0ce052972b03/a92618b1-0f99-4c87-a9fb-adb3e44ad458.dmp.tar.gz)
--- Done ---

In this case you can use gsutil.py from depot_tools (or gsutil from Cloud SDK) to download these artifacts:

$ gsutil.py cp gs://dart-temp-crash-archive/<src-path> <dst-path>

Analyzing Dumps

Linux

$ gdb <binary> <core>

Because bots are running slightly different Linux versions that what is available on development machines gdb might not be able to properly resolve symbols and unwind the stack by itself.

In this case you can:

• spin up a Cloud Instance with a matching OS version and debug the dump on that instance;
• create a sysroot that matches operating system on the bots and tell gdb to use that sysroot with set sysroot command.

ARM/ARM64

If you have a core produced on ARM or ARM64 bot and you would like to inspect it on a non-ARM development machine, then you will need gdb-multiarch and sysroot that matches OS version on the bot. Then you can use set sysroot ... and set architecture ... to configure gdb-multiarch before loading core.

Mac

$ lldb <binary> -c <core>

Windows

Googlers can find WinDBG and CDB in %depot_tools%\win_toolchain\vs_files\%toolchain_version%\win_sdk\Debuggers\x64\.

For quick investigation you can do

> cdb.exe -z <coredump.dmp>
...
0:000> !uniqstack
...

Implementation details

Implementation of crash dump archiving consists of two parts:

• Code in the testing framework (UnexpectedCrashLogger in pkg/test_runner/lib/src/test_progress.dart) that detects unexpected crashes, that is situations when test unexpectedly completes with a Crash status. In this case it logs a line about this crash into unexpected-crashes file in the root of the SDK checkout. This line has format test,pid,binary and identifies which test crashed, what was the process id and which binary crashed.
• Code in the Python wrapper around testing framework (see CoreDumpArchiver in tools/utils.py) is responsible for
  • Before tests: configuring OS and/or environment to setup crashdump archiving in the OS specific way (see classes with names ending at CoreDumpEnabler);
  • After tests: trying to locate crashdumps written by OS or crash reporting library and match those to unexpected crashes reporting by testing framework (see classes with names ending at CoreDumpArchiver);
  • Selecting a subset of crashdumps and uploading those into a persistent storage (isolate server or Cloud Storage bucket).

Linux

We rely on builtin ability of the OS to generate core files (man core) when the program is terminated abnormally.

We adjust core resource limits to unlimited before running tests. See man setrlimit for more information (search for RLIMIT_CORE).

We verify that /proc/sys/kernel/core_pattern is set to core.%p which means that crash dumps would be produced in the current working directory (root of the SDK checkout for most tests) and mangled with the pid of the crashed process.

Core dump archiving code then uses pids to match unexpected crashes with core dumps.

See LinuxCoreDumpArchiver and LinuxCoreDumpEnabler in tools/utils.py.

Mac

Just like on Linux we rely on OS to produce core files (man core) and similarly set core resource limits to unlimited.

Core files are also mangled with PID of the crashing process.

The only difference from Linux is that on Mac core files are written into /cores directory instead of current working directory.

See MacOSCoreDumpArchiver and PosixCoreDumpEnabler in tools/utils.py.

Windows

On Windows we rely on Crashpad library to produce minidumps of the crashed process, because configuring builtin crash reporting functionality requires registry changes that are global, impossible without Administrator privileges and are overwritten by buildbot fleet management software running in background.

Crashpad integration for dart.exe and run_vm_tests.exe binaries is controlled by dart_use_crashpad GN argument.

This argument is set to true when tools/gn.py is run with --use-crashpad flag or with DART_USE_CRASHPAD environment variable set.

If the binary is built with Crashpad integration it checks two environment variables on startup:

• DART_CRASHPAD_HANDLER should contain path to crashpad_handler.exe Crashpad binary that implements out-of-process crash monitoring;
• DART_CRASHPAD_CRASHES_DIR should contain path to a folder, which would be used as Crashpad crash database.

If both variables are set then Crashpad handler will be started. If the handler catches main binary crashing it would write minidump into %DART_CRASHPAD_CRASHES_DIR%\crashes from where it would be picked up by crash archiving machinery.

We match crash dumps to unexpected crashes by parsing them and extracting process id of the crashed process (see utils/minidump.py library).

See WindowsCoreDumpEnabler and WindowsCoreDumpArchiver in tools/utils.py.

Note: DART_CRASHPAD_CRASHES_DIR is set by WindowsCoreDumpEnabler in tools/utils.py, while DART_CRASHPAD_HANDLER is set by TestSuite in pkg/test_runner/lib/src/test_suite.dart.

Note: Crashpad is optimized for end-user crash reporting use case and does not write full crash dumps.