I split this out from Issue 872667. Repro:

Needs: Current Canary or Dev (71)

1.) Download threads.zip
2.) Host the files locally on localhost:8000
3.) Enable WebAssembly Threads in about:flags
4.) open localhost:8000/threads.html
5.) Open devtools and make sure that the sourcemaps are recognized (threads.c)
6.) Set breakpoint at "printf("x increment finished\n");"
7.) Set breakpoint at "printf("y increment finished\n");"
8.) Breakpoint "printf("x increment finished\n");" is never hit

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Kozy@'s comment:

The issue mentioned in the last comment looks unrelated to this one, and any worker with source map is affected.

We do not pause execution to wait until our frontend loads source map so we intentionally may skip some breakpoints between page starts worker and frontend loads source map. For the page, we store some additional information between reloads and can use it to make all breakpoints work after reload; unfortunately, we can not use this way for workers.

We can introduce new instrumentation breakpoint type, e.g., breakOnScriptWithSourceMapParsed. The frontend will load the source map and resume execution. I will prepare CL.

If you put breakpoints at line 40, 43 or 51, they work.
 

Deleted: threads.zip 227 KB

threads.zip 227 KB Download