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`pip` sometimes includes debug messages on `stdout`

See original GitHub issue

This is an abbreviated copy of #115 and #116, since those have been filled up with additional debugging.

Summary: In rare cases (currently N=1), we fail to collect environmental dependencies from PipSource. PipSource uses pip-api internally, which boils down to pip list --format=json.

What seems to happen is that the pip list --format=json command doesn’t emit just JSON. Instead, it also emits a trailing log message that looks like this:

Given no hashes to check 181 links for project 'pip': discarding no candidates

According to pip’s source code, that message is produced by a call to log.debug, which should be going to stderr instead of stdout. pip-api only uses stdout from pip subprocesses, so this is a strong indicator that the two streams are being mixed and/or confused somewhere, potentially with a logging override. The original reporter also traced the process and confirmed that the log was produced on stdout.

The original reporter found this on Python 3.9, pip version 20.3.3. I was unable to reproduce it locally, and they were also unable to reliably reproduce it locally.