cmd/utils: Disallow --lightserv in light mode

[tsarpaul]

#15492

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Thank you for your contribution! Your commits seem to not adhere to the repository coding standards

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• Commit: 2f86c1d69d841723e012381f0c53ea940a07fb44

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[karalabe]

Sweet, just a couple of polishes and it's good to go.

[karalabe]

Minor suggestion, instead of nesting when the condition holds, let's continue when it doens't hold. This reduces the nesting depth of the entire code by one level:

flag, ok := arg.(cli.Flag)
if !ok {
	continue;
}

[karalabe]

I think a panic here would be better. It shouts louder that something was coded wrong:

panic("unsupported flag argument type")

[GitCop]

Thank you for your contribution! Your commits seem to not adhere to the repository coding standards

• Commit: ce45110

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• Commit: 642612c

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[GitCop]

Thank you for your contribution! Your commits seem to not adhere to the repository coding standards

• Commit: ce45110

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• Commit: 642612c

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[rkeene]

This change appears to disallow all use of --syncmode with --lightserv.

$ geth --syncmode="full" --port="30303" --lightserv=50 --cache=4096 --maxpeers=128
Fatal: Flags --lightserv, --syncmode can't be used at the same time

Due to the change here:

go-ethereum/cmd/utils/flags.go

Line 986 in e4c9fd2

checkExclusive(ctx, LightServFlag, SyncModeFlag, "light")

Since checkExclusive does not discard the option from matching with respect to the string argument (the string argument, "light", only affects the formatting of the error message)