-ddump-json doesn't interact as expected with -ddump-to-file

Usually with dump flags, if you set ddump-to-file then the dumped file will be outputted in the current directory if -dumpdir or -ddump-file-prefix is not set. This doesn't work with -ddump-json, if you do not specify -ddump-file-prefix then the output is always on stdout.

Clearly stated, here are the two problems which I think have the same solution.

1. -ddump-to-file -ddump-json should output a file in the currect directory.
2. -ddump-to-file -ddump-json does not respect -dumpdir

It seems that a stale set of DynFlags is used to direct the output of -ddump-json. The relevant function is DynFlags.jsonLogFinaliser.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

The problem is that dumpPrefix is set in DriverPipeline but then this modified version of DynFlags is not propagated outwards and so it is not set at the time jsonLogFinaliser calls dumpSDoc. This means that chooseDumpFile fails as dumpPrefix is not set and the output is instead forwarded onto stdout.

There is quiet a fundamental problem that even if we propagate this information outwards, functions like parUpsweep_one operate in IO, on an exception this means that the updated dflags are not set and the output it still printed on stdout.

Well, the real problem here is that the way things are currently engineered, -ddump-json collects error messages for the entire compilation and then dumps them in one go. dumpSDoc is set up to dump things on a per-module basis, as such the log files are given the name of the current module they are dumping. Perhaps a better design for -ddump-json would be to output a JSON log on a module-by-module basis rather than collecting all the output.

This would complicate the implementation a little as the messages would need to be sorted by module.

added Pnormal label

added dump flags label

marked this issue as related to #22959