fix(inputs.mongodb): resolve SIGSEGV when restarting MongoDB node

[dkhamitov]

Below is an example of the SIGSEGV I occasionally get when restarting a MongoDB node that is being monitored by Telegraf MongoDB plugin. I think it rather happens during the node shutdown, though. Because according to the stacktrace, it's the newStat variable that doesn't have Locks populated as the code expects. The proposed fix is quite straightforward. It just adds adds necessary nil checks that are the same (symmetric) as those performed on the oldStat data.

The stacktrace below is for Telegraf 1.24.2. The line with "nil pointer dereference" can be found at

telegraf/plugins/inputs/mongodb/mongostat.go

Line 1227 in 9550e7a

readWaitCountDiff := newStat.Locks["Collection"].AcquireWaitCount.Read - oldStat.Locks["Collection"].AcquireWaitCount.Read

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x2637e51]

goroutine 6343563 [running]:
[github.com/influxdata/telegraf/plugins/inputs/mongodb.NewStatLine(](http://github.com/influxdata/telegraf/plugins/inputs/mongodb.NewStatLine(){{0xc0eeb390fb1262f5, 0x1a554c27316d, 0x95c0b40}, 0xc00167a360, 0xc0027761a0, 0xc001db02d0, 0xc000760060, 0xc0029b8e88, 0xc00076e2d0, 0xc0019800e8, ...}, ...)
	/go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongostat.go:1227 +0x1871
[github.com/influxdata/telegraf/plugins/inputs/mongodb.(*Server).gatherData(0xc00146a5a0,](http://github.com/influxdata/telegraf/plugins/inputs/mongodb.(*Server).gatherData(0xc00146a5a0,) {0x671b7a0, 0xc0009a6460}, 0x1, 0x1, 0x1, 0x1, {0x9603620, 0x0, 0x0})
	/go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb_server.go:363 +0x79d
[github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather.func1(0xc002622850?)](http://github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather.func1(0xc002622850?))
	/go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:164 +0x1a9
created by [github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather](http://github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather)
	/go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:155 +0x65

Also, quite often but not always, I can catch the following error in the plugin before the crash:

2023-02-01T12:11:10Z E! [inputs.mongodb] failed to gather data: %!w(mongo.CommandError={13436 node is not in primary or recovering state [] NotPrimaryOrSecondary <nil> [180 0 0 0 3 116 111 112 111 108 111 103 121 86 101 114 115 105 111 110 0 45 0 0 0 7 112 114 111 99 101 115 115 73 100 0 99 218 86 207 130 25 202 195 230 35 33 178 18 99 111 117 110 116 101 114 0 0 0 0 0 0 0 0 0 0 1 111 107 0 0 0 0 0 0 0 0 0 2 101 114 114 109 115 103 0 43 0 0 0 110 111 100 101 32 105 115 32 110 111 116 32 105 110 32 112 114 105 109 97 114 121 32 111 114 32 114 101 99 111 118 101 114 105 110 103 32 115 116 97 116 101 0 16 99 111 100 101 0 124 52 0 0 2 99 111 100 101 78 97 109 101 0 22 0 0 0 78 111 116 80 114 105 109 97 114 121 79 114 83 101 99 111 110 100 97 114 121 0 0]})

Required for all PRs

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #12611

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

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Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[powersj]

Thanks for the PR and tests! Is there an issue that is associated with this fix that we can close as well?

[dkhamitov]

Hi @powersj, I haven't found any related issues, neither open nor closed.

[srebhan]

@dkhamitov I took the liberty to open an issue with the information you provided. In the future, please open an issue first describing the problem and then reference this issue in your PR. This is to allow other users with a similar problem to easier discover the solution (your PR) as they might only search in issues...

[srebhan]

Looks good to me. Thanks @dkhamitov for tracking this down and fixing it!

[dkhamitov]

@srebhan

In the future, please open an issue first describing the problem and then reference this issue in your PR

Got it. Thanks!