The "hang" is actually this loop

Lines 861 to 867 in 0f3eced

    
           loop { 
        
               let mask = (scale * max_rand + low).ge_mask(high); 
        
               if !mask.any() { 
        
                   break; 
        
               } 
        
               scale = scale.decrease_masked(mask); 
        
           }

taking a very long time to complete. That loop will require roughly spacing(low)/(2*spacing(scale)) iterations to complete, where spacing(x) is the unit of least precision of x. For the values in the MWE, this is 17179869184 iterations! Here's the calculation in a Python session:

In [52]: import numpy as np

In [53]: low = 0.99999999997819644

In [54]: high = 1.0

In [55]: scale = high - low

In [56]: np.spacing(low)/(2*np.spacing(scale))
Out[56]: 17179869184.0

The problem is that in the loop, scale is repeatedly decreased by the smallest possible amount until scale*max_rand + low is less than high. For inputs where high - low is much smaller than low, this will require a large number of iterations.

	loop {
	let mask = (scale * max_rand + low).ge_mask(high);
	if !mask.any() {
	break;
	}
	scale = scale.decrease_masked(mask);
	}

Uh oh!

Uniform Generator hangs for certain limits. #1299

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