Incremental mode on errors

[arcanis]

The rebuild function is only provided when esbuild manages to finish a build. It would be nice if it could instead always return it (even on errors), so that we can decrease the feedback loop when fixing the build.

I tried doing it myself but although it almost works it crashes with the following right after it finishes the rebuild:

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc00010cd58)
        runtime/sema.go:56 +0x45
sync.(*WaitGroup).Wait(0xc00010cd50)
        sync/waitgroup.go:130 +0x65
main.runService(0x7ffeefbff401)
        github.com/evanw/esbuild/cmd/esbuild/service.go:138 +0x636
main.main()
        github.com/evanw/esbuild/cmd/esbuild/main.go:200 +0x3ae

goroutine 18 [chan receive]:
main.runService.func1(0xc00012e140, 0xc00010cd50)
        github.com/evanw/esbuild/cmd/esbuild/service.go:66 +0x4a
created by main.runService
        github.com/evanw/esbuild/cmd/esbuild/service.go:64 +0x1fc

goroutine 19 [chan receive]:
main.(*serviceType).sendRequest(0xc00012e140, 0x148ca60, 0xc066251980, 0x7, 0xc066278088)
        github.com/evanw/esbuild/cmd/esbuild/service.go:163 +0x11e
main.runService.func2(0xc00012e140)
        github.com/evanw/esbuild/cmd/esbuild/service.go:92 +0x47
created by main.runService
        github.com/evanw/esbuild/cmd/esbuild/service.go:89 +0x6ae

[evanw]

Yes, this is a known issue. One reason why I haven't done it yet is that it'll take a breaking API change to fix. I'm not totally sure what the best API design is. One approach is of course to have the API call never throw an error and instead return a result object with an error message in the array of logs. However, people will likely not check for it and then write brittle code that fails silently when an error happens. Incremental mode could work differently than non-incremental mode of course but the current consistency between the two is kind of nice. I definitely need to do something though.

Related: #1037 (about watch mode instead of incremental mode, but it's basically the same problem).

[arcanis]

Wouldn't attaching rebuild to the thrown error (like I attempted in my diff) work without breaking change?