I was wondering who was the first person to spin a double bass during a performance, and who popularized it?
I'm not entirely sure whether by "spinning a double bass" you mean spinning it on its pin while holding the neck or literally spinning it around during a performance.
I found a video clip (credited to Archive Films - Editorial on Getty Images) from 1936 of "Curly" of The Three Stooges spinning a double bass and bumping into it for comedic purposes. If he was doing it for comedy then, maybe it is safe to assume that this was common in performances even in the mid-1930s?
If you mean literally spinning the bass, then it is possible that the credit goes to Marshall Lytle, who played for the band Bill Haley & His Comets in the 1950s. Watch him spin the bass around in a live performance in Brussels, Belgium in 1958, towards the end of the video.
Marshall Lytle generally seems to be credited for this kind of histrionics on the instrument. The following extract is from the entry on the "Double Bass" in the Encyclopaedic Dictionary of World Musical Instruments, Vol. 1 (on page 183):
The double bass is also favoured over the electric bass guitar in many rockabilly and psychobilly bands. In such bands the bassist often plays with great showmanship, using [sic] slapping technique, sometimes spinning the bass around or even physically climbing onto the instrument while performing; this style was pioneered c. 1953 by Marshall Lytle, the bassist for Bill Haley & His Comets, and modern performers of such stunts include Lee Rocker of the Stray Cats, Scott Owen from The Living End and Jimbo from Reverend Horton Heat.
Source: P. S. Ganguly (ed.), Encyclopaedic Dictionary of World Musical Instruments, Vol. 1. Global Vision Publishing House, Daryaganj, New Delhi, 2008. ISBN: 978-81-8220-291-4. Available at Google Books.
Two slap bass players who are rarely mentioned alongside the more exalted Rockabilly Bull Fiddle slingers are Al Rex and Marshal Lyttle [sic] of Bill Haley’s Comets. Listen to the early Comets’ material and to their work when they were known as The Saddlemen and that distinctive rhythmic clicking of the bass strings that permeates every song. That percussive slap came from Al Rex who joined the Comets in 1949 and was then replaced in 1951 by Marshal Lyttle [sic] who followed on where Rex left off, after a few bass lessons from Bill Haley himself. It’s Marshal who is slapping away on the immortal “Rock Around The Clock”. When Marshal quit the Comets in 1955 he was replaced by none other than Al Rex! Obviously ol’ Bill Haley was partial to slap bass in his band. Both Al and Marshal were pioneers of all sorts of stage gymnastics with their uprights. . .a tradition which lives on with today’s Rockabilly bassists.
Source: The Wheelgrinders, Rockabilly Slap Bass. The Wheelgrinders, Wordpress, https://thewheelgrinders.com/2016/11/17/rockabilly-slap-bass/, 2016-11-17. Accessed: 2020-5-12.
As the band focused on the art of performance, particularly for engaging the audience during television appearances, Lytle took advantage of the sheer size of the upright bass. Developing into quite the showman, Lytle would experiment with playing the bass in any way possible, including holding it above his head, standing on its side, playing it on the floor, and spinning it around. This set a new standard for performing and, as country and blues morphed into rock ’n’ roll, which later transformed into rockabilly, many bass players took their cue from Lytle’s energetic and unconventional style.
Source: Ryan Madora, Bass Players to Know: Marshall Lytle. No Treble, LLC, https://www.notreble.com/buzz/2014/11/28/bass-players-to-know-marshall-lytle/, 2014-11-28. Accessed: 2020-5-12.
All emphases in the above quotes are mine.