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netwatch-notify: work around race condition

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This used to crash every now and then with:

> script;error script error: no such item (4)

I guess this is caused by querying the dns cache just exactly when a
record expires. The chance is maximized: The script is started by
scheduler every minute, and the record's ttl is a multiple of a minute.

Let's query records that are not about to expire immediately, and
try again.
This commit is contained in:
Christian Hesse 2024-02-26 23:18:02 +01:00
parent c2f5272f18
commit 93bed1b081
1 changed files with 12 additions and 8 deletions
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20
netwatch-notify.rsc
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@ -80,14 +80,18 @@ $ScriptLock $0;
:do {
:local Resolve [ :resolve ($HostInfo->"resolve") ];
:if ($Resolve != $HostVal->"host" && \
[ :len [ /ip/dns/cache/find where name=($HostInfo->"resolve") data=[ :tostr ($HostVal->"host") ] ] ] = 0) do={
$LogPrintExit2 info $0 ("Name '" . $HostInfo->"resolve" . [ $IfThenElse \
($HostInfo->"resolve" != $HostInfo->"name") ("' for " . $Type . " '" . \
$HostInfo->"name") "" ] . "' resolves to different address " . $Resolve . \
", updating.") false;
/tool/netwatch/set host=$Resolve $Host;
:set ($Metric->"resolve-failcnt") 0;
:set ($HostVal->"status") "unknown";
[ :len [ /ip/dns/cache/find where name=($HostInfo->"resolve") data=[ :tostr ($HostVal->"host") ] ttl>0s ] ] = 0) do={
:delay 1500ms;
:resolve ($HostInfo->"resolve");
:if ([ :len [ /ip/dns/cache/find where name=($HostInfo->"resolve") data=[ :tostr ($HostVal->"host") ] ] ] = 0) do={
$LogPrintExit2 info $0 ("Name '" . $HostInfo->"resolve" . [ $IfThenElse \
($HostInfo->"resolve" != $HostInfo->"name") ("' for " . $Type . " '" . \
$HostInfo->"name") "" ] . "' resolves to different address " . $Resolve . \
", updating.") false;
/tool/netwatch/set host=$Resolve $Host;
:set ($Metric->"resolve-failcnt") 0;
:set ($HostVal->"status") "unknown";
}
}
} on-error={
:set ($Metric->"resolve-failcnt") ($Metric->"resolve-failcnt" + 1);