Fix .bs.js timestamp being reset to 1970-01-01 when there are warnings

[cknitt]

Closes #5206.

[cristianoc]

Tested and it works fine for me.
Anything else to test here? Some bigger project or something.

Other than that, please add a line in the CHANGELOG.

[cknitt]

I already tested with a fairly large project.

Rebased and added to CHANGELOG.

[bobzhang]

@cknitt sorry for the late comments, note the fix was correct but inefficient.

Set the .mlast as old, would trigger its dependencies rebuild while nothing changes, what we want is only files with warnings rebuilt, so if you have warnings, the rebuild would take a while.

The proposed fix was tracking cmt[i] file in the generated ninja file, we set the cmt[i] file as old.
Why cmt file? It is a leaf node in the build graph that is not used by any other rule, so only the current artifact is rebuilt.

maybe some kind of additional "marker" file should be created for files with warnings?

cmt[i] file would be a good candidate

cc @cristianoc

[cknitt]

@bobzhang Thanks for the hint, I will have another look at this.

[bobzhang]

So you can split into two steps:

• Change the ninja generator to track cmt/cmti files -- no side effects, no regressions
• set the cmt/cmti as old file

[cknitt]

@bobzhang @cristianoc I have retested the current implementation that sets the timestamp of the .ast file and did not see any dependencies being unnecessarily recompiled.

For example, I have a warning in AuthManager.res and have this in the ninja file:

o src/auth/AuthManager.ast : astj ../../src/auth/AuthManager.res
o src/auth/AuthManager.d : deps src/auth/AuthManager.ast src/auth/AuthManager.iast
o src/auth/AuthManager.cmj ../../src/auth/AuthManager.bs.mjs : mj src/auth/AuthManager.ast src/auth/AuthManager.cmi

Now when I run rescript build again, I can see that AuthManager.res is recompiled.

So IMHO the feature is working fine and there is no inefficiency / performance regression.