uv pip compile ... -o gives misleading errors on race conditions #6239
Description
I completely understand if this is a closedwontfix scenario since I was obviously using it wrong, but figured I'd report it anyways in case you wanted to lock output files more defensively to protect against (perhaps more legitimate) failure modes.
But I recently triggered a situation where uv compile requirements.in -o requirements.txt was being triggered multiple times in parallel (by pre-commit hooks where I forgot to set pass_filenames: false if you're curious), and racing itself while reading the output file leading to misleading errors.
I built a minimal repro script to demonstrate the output:
❯ cat run.sh
#!/bin/bash
for i in {1..200}; do
uv pip compile -q requirements.in -o requirements.txt &
done
wait
❯ cat requirements.in
torch>2
❯ ./run.sh
error: Unexpected '', expected '-c', '-e', '-r' or the start of a requirement at requirements.txt:4:1
error: Unexpected '', expected '-c', '-e', '-r' or the start of a requirement at requirements.txt:2:1
error: error: Unexpected '', expected '-c', '-e', '-r' or the start of a requirement at requirements.txt:12:1
Unexpected '', expected '-c', '-e', '-r' or the start of a requirement at requirements.txt:13:1
error: Unexpected '', expected '-c', '-e', '-r' or the start of a requirement at requirements.txt:1:1
error: Couldn't parse requirement in `requirements.txt` at position 310
Caused by: after parsing '1.13.2', found '', which is not part of a valid version
sympy==1.13.2
^^^^^^^^
error: Unexpected '', expected '-c', '-e', '-r' or the start of a requirement at requirements.txt:2:1
Anyways, thanks for the great project, we're using it now internally and it's been really nice so far.