pantsd does not react well to truncated stdout

[stuhood]

./pants --enable-pantsd list :: | head
...seemingly exits cleanly. But the pantsd log does not show the run completing, and the next call in the same workspace will block with the:

Another pants invocation is running. Will wait forever for it to finish before giving up.
If you don't want to wait for the first run to finish, please press Ctrl-C and run this command with PANTS_CONCURRENT=True in the environment.
...

...message, presumably because the first run did not exit cleanly.

[blorente]

This one is very weird, because it happens reliably, but only after a certain amount of ouput has been truncated:

• Small output, but still truncated. I can repeat it as many times as I want:

$ ./pants  --enable-pantsd list src/scala::  | head
src/scala/org/pantsbuild/zinc/analysis:analysis
src/scala/org/pantsbuild/zinc/extractor:extractor
src/scala/org/pantsbuild/zinc/options:options
src/scala/org/pantsbuild/zinc/compiler/native-image-substitutions:native-image-substitutions
src/scala/org/pantsbuild/zinc/compiler/native-image-substitutions:statics
src/scala/org/pantsbuild/scoverage/report:report
src/scala/META-INF/native-image/org/pantsbuild/zinc/compiler:zinc-native-image-configuration
src/scala/org/pantsbuild/zinc/cache:cache
src/scala/org/pantsbuild/zinc/scalautil:scalautil
src/scala/org/pantsbuild/zinc/util:util

• Sightly larger output:

$ ./pants  --enable-pantsd list src/python::  | head
src/python/pants/rules/core:core
src/python/pants/rules/core:tests
src/python/pants/base:build_environment
src/python/pants/base:build_file
src/python/pants/base:build_file_target_factory
src/python/pants/base:build_root
src/python/pants/base:cmd_line_spec_parser
src/python/pants/base:deprecated
src/python/pants/base:exception_sink
src/python/pants/base:exceptions

• Let's truncate it even more:

$ ./pants  --enable-pantsd list src/python::  | head -n 1
src/python/pants/rules/core:core

• Let's have even more output:

$ ./pants  --enable-pantsd list src/::  | head -n 1
src/python/pants/rules/core:core

• It only crashes for :::

$ ./pants  --enable-pantsd list ::  | head -n 1
//:3rdparty_directory
Exception ignored in: <_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'>
BrokenPipeError: [Errno 32] Broken pipe