session_manager sends periodic “liveness checks” to Chrome in the form of D-Bus method calls and kills Chrome if it doesn't respond in a timely manner. Chrome handles D-Bus messages on its UI thread, so a failure to reply indicates that the UI thread is blocked.
session_manager‘s portion of the implementation lives in liveness_checker_impl.h and liveness_checker_impl.cc. By default, session_manager makes an asynchronous CheckLiveness D-Bus method call to org.chromium.LivenessService every 60 seconds. If a response hasn’t been received for the previous call at this point, session_manager sends a SIGABRT signal to Chrome's browser process.
Chrome exports the org.chromium.LivenessService D-Bus service and replies immediately to CheckLiveness method calls.
On developer systems (as indicated by the is_developer_end_user command), Chrome will not be killed by the hang detector since it may be unresponsive due to having been stopped by a debugger.
When session_manager detects that Chrome is hanging, messages similar to the following should be logged to /var/log/messages (timestamps omitted):
WARNING session_manager[1492]: [WARNING:liveness_checker_impl.cc(64)] Browser hang detected! WARNING session_manager[1492]: [WARNING:liveness_checker_impl.cc(68)] Aborting browser process. INFO session_manager[1492]: [INFO:browser_job.cc(164)] Terminating process: Browser did not respond to DBus liveness check. INFO session_manager[1492]: [INFO:system_utils_impl.cc(93)] Sending 6 to 1529 as 1000 WARNING crash_reporter[4409]: Received crash notification for chrome[1529] user 1000 (called directly) WARNING session_manager[1492]: [WARNING:browser_job.cc(172)] Aborting child process 1529's process group 3 seconds after sending signal INFO session_manager[1492]: [INFO:browser_job.cc(156)] Terminating process group: Browser took more than 3 seconds to exit after signal. INFO session_manager[1492]: [INFO:system_utils_impl.cc(93)] Sending 6 to -1529 as 1000 ... INFO session_manager[1492]: [INFO:child_exit_handler.cc(77)] Handling 1529 exit. ERR session_manager[1492]: [ERROR:child_exit_handler.cc(85)] Exited with signal 6 INFO session_manager[1492]: [INFO:session_manager_service.cc(289)] Exiting process is chrome. INFO session_manager[1492]: [INFO:browser_job.cc(156)] Terminating process group: Ensuring browser processes are gone.
Breakpad handles SIGABRT within Chrome, and if crash reporting is enabled, a crash report should be sent. The browser process's stack trace will hopefully indicate the reason for the hang.
Note also that the chrome.txt file linked in the crash report (which is actually a gzipped text file) includes the tail portions of the session_manager and Chrome user and system log files. If a SIGABRT crash was caused by the hang detector, you'll probably see the above log messages in chrome.txt.
Similar code originally lived in chromeos-wm, the long-gone Chrome OS X11 window manager. The pings were sent via X11 back then. See issue 217814 and other bugs of similar vintage for more details.
The original implementation of this within session_manager was tracked by issue 217825, and the code hasn't changed much since then. The feature was briefly disabled in 2013 and before being reenabled (issue 221008) and updated to send SIGFPE rather than SIGABRT (to make it easier to distinguish between hangs and CHECKs or LOG(FATAL)s within Chrome). In 2015, session_manager was updated to send SIGABRT rather than SIGFPE (issue 284601). The CheckLiveness D-Bus method was moved from org.chromium.LibCrosService into a new org.chromium.LivenessService service also provided by Chrome (issue 644322).
