audit Bar.SetTotal calls in copy.go

[mtrmac]

• Some seem to be unnecessary — createProgressBar already uses blobInfo.Size
• copyLayer, after a copy is done, calls SetTotal with the source info (which might not contain a size), when it certainly has a size from the updated result.

It’s probably just bugs, but review also the history in case some of those are non-obvious and necessary.

[vrothberg]

IIRC, those were "needed" for bars to be filled. I'm sure some are not needed and maybe there's a better way.

[vrothberg]

Ah, yes. There are two cases where I used the SetTotal "trick" for the bars to finish rendering and display the "done" state:

1. In case the blob is already on present and copying is skipped
2. For v2s1 images where the blob sizes are unknown; unconditionally calling SetTotal was attractive instead of carrying the information around whether we know the size or not.

[mtrmac]

Notes to self, so that I don’t forget. Referring to the ≥ 7.4.1 MPB API:

• If the source has a size, we need

  // from createProgressBar: bState.total is set to sourceSize, bState.triggerComplete is set
  bar.SetCurrent(sourceSize)     // Even if the blob already exists on the destination and we didn’t need to actually copy it, report 100% progress.

  Notably we don’t want to use destSize at all, because we can’t change bState.total any more, and we want the progress bar to go to 100%
• If the source does not have a size, we need

  // from createProgressBar: bState.total is 0, bState.triggerComplete is unset
  bar.SetTotal(destSize, true) // Report the final size and 100% progress

• The partial pull case is the same as the ordinary cases, except that we also set the “refill” value.
• The TryReusingBlob success case is special (bState.total set to 0 does not immediately trigger a complete state)

[mtrmac]

• If the source does not have a size …

we should, if possible, still count in “source size” units, not the “destination size” units (to use a consistent measure in the hypothetical case of a single (multi-arch?) image that contains both size-known and size-unknown blobs).