Inconsistent via comments for backtracking resolver on constrained requirements

[mkniewallner]

pip-compile generates inconsistent via comments when using backtracking resolver on constrained requirements.

When a constrained <constrained_requirements_file>.txt file is not yet generated, -c <requirements_file.txt> references are correctly added to via comments, but they are removed when re-compiling the requirements when the constrained <constrained_requirements_file>.txt file already exists.

This seems to apply only to the backtracking resolver, since I wasn't able to reproduce the issue using the legacy resolver.

Environment Versions

1. OS Type: Linux (x86_64)
2. Python version: 3.10.8
3. pip version: 22.3
4. pip-tools version: 6.9.0 (also tried on master branch, at commit 1fb2e69)

Steps to replicate

I've created a reproduction repository (https://github.com/mkniewallner/pip-compile-inconsistent-via-backtracking-resolver) with a README and commits history that highlight the issue, but basically:

1. Create a requirements.in file with the following content:

starlette==0.21.0

2. Create a requirements-dev.in file with the following content:

-c requirements.txt

httpx==0.22.0

3. Run pip-compile --resolver=backtracking requirements.in
4. Run pip-compile --resolver=backtracking requirements-dev.in
5. Run pip-compile --resolver=backtracking requirements-dev.in again

Expected result

pip-compile writes the following requirements-dev.txt for both runs of pip-compile --resolver=backtracking requirements-dev.in:

#
# This file is autogenerated by pip-compile with python 3.10
# To update, run:
#
#    pip-compile --resolver=backtracking requirements-dev.in
#
anyio==3.6.2
    # via
    #   -c requirements.txt
    #   httpcore
certifi==2022.9.24
    # via
    #   httpcore
    #   httpx
charset-normalizer==3.0.0
    # via httpx
h11==0.12.0
    # via httpcore
httpcore==0.14.7
    # via httpx
httpx==0.22.0
    # via -r requirements-dev.in
idna==3.4
    # via
    #   -c requirements.txt
    #   anyio
    #   rfc3986
rfc3986[idna2008]==1.5.0
    # via httpx
sniffio==1.3.0
    # via
    #   -c requirements.txt
    #   anyio
    #   httpcore
    #   httpx

Actual result

The first time pip-compile --resolver=backtracking requirements-dev.in is run (where requirements-dev.txt doesn't yet exist), the correct requirements-dev.txt is generated:

#
# This file is autogenerated by pip-compile with python 3.10
# To update, run:
#
#    pip-compile --resolver=backtracking requirements-dev.in
#
anyio==3.6.2
    # via
    #   -c requirements.txt
    #   httpcore
certifi==2022.9.24
    # via
    #   httpcore
    #   httpx
charset-normalizer==3.0.0
    # via httpx
h11==0.12.0
    # via httpcore
httpcore==0.14.7
    # via httpx
httpx==0.22.0
    # via -r requirements-dev.in
idna==3.4
    # via
    #   -c requirements.txt
    #   anyio
    #   rfc3986
rfc3986[idna2008]==1.5.0
    # via httpx
sniffio==1.3.0
    # via
    #   -c requirements.txt
    #   anyio
    #   httpcore
    #   httpx

The second time pip-compile --resolver=backtracking requirements-dev.in is run (where requirements-dev.txt exists), the following diff is applied:

diff --git a/requirements-dev.txt b/requirements-dev.txt
index eaaa0f0..70ed934 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -5,9 +5,7 @@
 #    pip-compile --resolver=backtracking requirements-dev.in
 #
 anyio==3.6.2
-    # via
-    #   -c requirements.txt
-    #   httpcore
+    # via httpcore
 certifi==2022.9.24
     # via
     #   httpcore
@@ -22,14 +20,12 @@ httpx==0.22.0
     # via -r requirements-dev.in
 idna==3.4
     # via
-    #   -c requirements.txt
     #   anyio
     #   rfc3986
 rfc3986[idna2008]==1.5.0
     # via httpx
 sniffio==1.3.0
     # via
-    #   -c requirements.txt
     #   anyio
     #   httpcore
     #   httpx