Ben Kelly [:bkelly, not reviewing] Reporter
	Description • 8 years ago

STR:

1) Open a fresh nightly browser
2) Open a content tab (like example.com)
3) open about:memory in a tab
4) Minimize memory
5) Save verbose gc/cc logs from about:memory tab
6) Wait until all 4 files are written
7) Look at the 2 files generated for the content process.  They will have 0 bytes.

	Ben Kelly [:bkelly, not reviewing] Reporter
	Comment 1 • 8 years ago

Change step (2) to open many content tabs.  In my case I needed a few gmail tabs, irccloud, telegram, etc.  If you only have one or two minimal content tabs the bug does not trigger.

It seems if the child process writing is delayed enough compared to the parent then the file is truncated.

	Eric Rahm [:erahm]
	Comment 2 • 8 years ago

I'll try to repro.

	Andrew McCreight [:mccr8]
	Comment 3 • 8 years ago

Have you had a chance to look at this, Eric?

	Eric Rahm [:erahm]
	Comment 4 • 8 years ago

No, but I can reboot into Windows today.

	Eric Rahm [:erahm]
	Comment 5 • 8 years ago

Okay I repro'd at least. Same deal: just one page everything was fine, open a few and the files are 0 bytes, but there are rather large "incomplete" files. The timeout seems rather short, like less than 10s. I'll dig in further.

	Eric Rahm [:erahm]
	Comment 6 • 8 years ago

about:memory is using nsIMemoryInfoDumper.dumpGCAndCCLogsToFile [1]. That calls |ContentParent::CycleCollectWithLogs| in a loop for the children and then |nsJSContext::CycleCollectNow| in the parent process.

If we look at |CycleCollectWithLogsParent| we can see the |Recv__delete__| call is being hit (we get the callback that they're done) but no explicit closing of the logs [2]. The implication here is that the message itself is timing out.

[1] http://searchfox.org/mozilla-central/rev/8cf1367dd89cc36ef8f025dfc6af6d5c086838a7/xpcom/base/nsMemoryInfoDumper.cpp#358
[2] http://searchfox.org/mozilla-central/rev/8cf1367dd89cc36ef8f025dfc6af6d5c086838a7/dom/ipc/ContentParent.cpp#420-449

	Eric Rahm [:erahm]
	Comment 7 • 8 years ago

I should also note that the "imcomplete" files are fine. So if anyone is waiting for this to be fixed before investigating memory issues just use the reported filename prefixed with "incomplete-", ie: "C:\Users\moz21268\AppData\Local\Temp\incomplete-cc-edges.8128.1477527117.log