fix: shorebird release can sometimes hang on a flutter build step

[hann-yun]

when i run release & patch, i faced infinity loading..

so i added --verbose, and found

 /Users/$USERNAME/.shorebird/bin/cache/flutter/9904ef00d8085df9b100561035d698dee4d8bf6a/bin/flutter build ipa --release --flavor=deve..

stuck this step. so i directly run that cli.
finally occur flutter build error.

but build via shorebird cli, infinity loading( actually flutter occur build error )

[felangel]

What was the build error when building with Flutter? It would be great to have the full flutter logs so we can reproduce the issue locally, thanks!

[eseidel]

What could be happening is that we're not correctly servicing stderr/stdout in our invocation of flutter. The parent process (in this case shorebird) gives the child process it's stderr/stdin. If (as in our case) the parent gives a pipe or buffer to the child, but then doesn't always read from that pipe/buffer (or doesn't read until the child is finished, as in our case), the pipe/buffer can fill up and then the child's write() calls will block. This can cause a deadlock.

The correct way to handle this is to make sure stderr/stdin are serviced, either by routing them to /dev/null, or to a file or handling the stream callbacks in Dart, if we just wait until the end, we could deadlock if the buffer fills, the child blocks on the write, and then the child never finishes, thus we never get to actually read/print the child's output.

https://github.com/shorebirdtech/shorebird/blob/main/packages/shorebird_cli/lib/src/shorebird_process.dart#L248

Yeah, I suspect if that's big enough, the stderr/stdout stream will just fill up. I'm not sure what the recommended dart pattern is, we may need to invent one.

[eseidel]

It's also possible that the above theory I offered is wrong of course.