The relationship between meter, speed of light and c

The meter is defined as the length covered by light in 1/299792458
seconds (see the NIST website).

How did the metrologists arrive to the 299792458 number? Somehow they
must have decided that the speed of light is exactly 299792458 m/s.
But the light speed measurements are dependent on the definition of
the meter (and the second). How did they break this apparent circular
dependency?

.

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