Disk write in a "for" loop with RwMutex never happens

[Gavin Ray]

I've put the code, stripped to a minimal example here:
- https://ldc.godbolt.org/z/fzsx3Tnnn

You can see that the single write + read version of the code 
works just fine:
```
pageData[0..4] = [1, 2, 3, 4]
readData[0..4] = [1, 2, 3, 4]
```

Where here, `pageData` is the data to be written to a file, and 
`readData` is the result of trying to read the freshly written 
file data.

But if the same code is placed inside of a `for` loop, suddenly 
no writes occur:
```d
pageData[0..4] = [0, 0, 0, 0]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 1]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 2]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 3]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 4]
readData[0..4] = [0, 0, 0, 0]

// ...
```

Does anyone know what is happening here? It's really puzzling.