Subprocess dropping output

[itingliu]

Summary:
The experimental Subprocess implementation drops part of the output in some cases

Steps To Reproduce:

1. Download attached programs
2. Run ./sample (do go build sample.go first if you want to build it from source)
3. Run swift run Repro

Results:
When invoked directly the program outputs three lines:

% ./sample
State Type Name Start Time End Time Frame In Frame Out Logs ID
Started Terrible1 Foobar 2024-09-30T08:29:13+09:00 2024-09-30T08:29:13+09:00 0914390189048023984 abc-123
Completed Terrible2 Foobar 2024-09-30T08:29:13+09:00 2024-09-30T08:29:13+09:00 0914390189048023984 abc-123

When invoked through Subprocess, the output only contains one line:

% swift run Repro
Building for debugging...
[1/1] Write swift-version--58304C5D6DBC2206.txt
Build of product 'Repro' complete! (0.31s)
State Type Name Start Time End Time Frame In Frame Out Logs ID

Seen on the main branch of https://github.com/iCharlesHu/swift-experimental-subprocess, commit 0f93295

Reported in 136937639