Why Layout Tests search for results the way they do

Here are the results fallback order for chromium-mac (as of April, 2011):

 'leopard': ['chromium-mac-leopard', 'chromium-mac-snowleopard', 'chromium-mac', 'chromium', 
             'mac-leopard', 'mac-snowleopard', 'mac'], 
 'snowleopard': ['chromium-mac-snowleopard', 'chromium-mac', 'chromium', 
                 'mac-snowleopard', 'mac'], 

A common question is why chromium-mac-snowleopard and mac-snowleopard are in the fallback list for leopard.

Here's the logic behind the layout test paths:

Assume you have three tests called foo, bar, and baz and suppose we're a couple of years ago, when Leopard was the most recent version of the Mac. The generic version of a platform always contains the "future" or "latest" version of the results.

LayoutTests/platform/chromium-mac contains: 
  foo-expected.txt  r1.1 
  bar-expected.txt r.1.1 
  baz-expected.txt r.1.1 

Now Snow Leopard comes out, and foo produces different results. You want SL to find the new file, and the other two to find the old file. We put the new file in the "future" version, and move the older file into the directory matching the last platform that passed it:

platform/chromium-mac: 
  foo-expected.txt r1.2 
  bar-expected.txt r1.1 
  baz-expected.txt r1.1 
platform/chromium-mac-leopard: 
  foo-expected.txt r1.1 

SL only looks in the -mac dir, and Leopard looks in -mac-leopard, then -mac. Now Lion comes out, and bar changes, and foo changes again. We can leave the -leopard directory alone, and create a new -snowleopard directory, and only have to move the two failing tests there:

platform/chromium-mac: 
  foo-expected.txt r1.3 
  bar-expected.txt r1.2 
  baz-expected.txt r1.1 
platform/chromium-mac-snowleopard: 
  foo-expected.txt r1.2 
  bar-expected.txt r1.1 
platform/chromium-mac-leopard: 
  foo-expected.txt r1.1 

Lion only looks in -mac, SL looks in -mac-snowleopard, then -mac, and Leopard looks in -leopard, then -snowleopard, then -mac. If Leopard didn't look in the snowleopard directory, then we would have to copy versions of files more places every time a new release came out. This approach minimizes the amount of file shuffling we have to do, and keeps most of the files in platform/chromium-mac. But, it means that version X needs to look in every newer version in order to find the right files.

Another way to think of it is that directory X-1 contains all and only the files that were different between versions X-1 and X, and then realize that this is, in a sense, associative (but not commutative). So, if you want to find the differences between Leopard (Lion-2) and Lion, you can look for delta(Lion-2, Lion-1) then delta(Lion-1, Lion) (but you can't reverse the order).

Also, in Chromium we try to match the upstream baselines where possible (but we prioritize Chromium results over upstream results), so we look in both the chromium version-specific path and then the upstream version-specific path.