Flag -maxheap

[stephen-soltesz]

This change adds a new flag (-maxheap) to packet-headers that limits the total RAM used by the process. This change requires a new feature of go1.19 debug.SetMemoryLimit.

From local testing, in response to a SYN flood event:

• SetMemoryLimit alone is not sufficient to prevent eventual RAM exhaustion. New savers are created faster than the timeout and GC removes them.
• Conditional creation of saver.StartNew alone is not sufficient to prevent RAM exhaustion either because the GC does not activate frequently enough.

Shorter settings for -uuidwaittimeout (down to 100ms) had no effect on the eventual RAM exhaustion.

When total RAM used is greater than maxHeap, new streams will be ignored. In effect, this both limits new memory allocations and may prevent some legitimate traces from ever being collected, or collected incompletely.

Collection will resume after adequate RAM is available. The setting for -maxheap should be greater or equal to -maxidleram.

Resolves:

• https://github.com/m-lab/ops-tracker/issues/1682

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 24

• 32 of 41 (78.05%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.4%) to 98.602%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
demuxer/tcp.go	21	30	70.0%

Totals
Change from base Build 8:	-1.4%
Covered Lines:	635
Relevant Lines:	644

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💛 - Coveralls

[robertodauria]

Reviewed 6 of 6 files at r2, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @stephen-soltesz)

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demuxer/tcp.go line 67 at r2 (raw file):

	ms := runtime.MemStats{}
	runtime.ReadMemStats(&ms)
	// log.Println(ms.HeapAlloc, ">", maxHeap, "==", ms.HeapAlloc > maxHeap)

Nit: commented LOC

[stephen-soltesz]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @robertodauria)

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demuxer/tcp.go line 67 at r2 (raw file):

Previously, robertodauria (Roberto D'Auria) wrote…

Nit: commented LOC

Thank you.