Make Debian upstream repository configurable.

[vpalatin]

Support overriding the Debian handler's upstream (e.g. Ubuntu archives)

Tested with PROXY_UPSTREAM_DEBIAN=http://archive.ubuntu.com/ubuntu to get Ubuntu Resolute packages.

[vpalatin]

The lint failed with: lint: internal/config/config.go#L442 cyclomatic complexity 31 of func (*Config).LoadFromEnv is high (> 30) (gocyclo)
I can definitely add another commit here refactoring LoadFromEnv(), e.g. with a setEnvString helper function

func setEnvString(dst *string, key string) {
       if v := os.Getenv(key); v != "" {
               *dst = v
       }

is this the preferred way ? (in the same PR ? in the same commit or a different commit ?)

[Copilot]

🟡 Changes recommended

The env override refactor misplaces the LoadFromEnv GoDoc comment and the new upstream configurability path should be exercised by tests within the changed regions.

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Pull request overview

This PR adds configuration support for overriding the Debian/APT handler’s upstream base URL (e.g., pointing at Ubuntu archives) and wires that through server startup and environment-variable loading.

Changes:

• Extend NewDebianHandler to accept a configurable upstream URL (defaulting to the existing Debian upstream when unset).
• Add Upstream.Debian to config defaults and PROXY_UPSTREAM_DEBIAN environment override support.
• Update affected tests and call sites to use the new Debian handler constructor signature.

File summaries

File	Description
internal/server/server.go	Passes configured Debian upstream into the Debian handler at server start.
internal/handler/debian.go	Adds an upstream URL parameter with defaulting + trailing-slash normalization.
internal/handler/download_test.go	Updates Debian handler construction in download cache-miss test.
internal/handler/notfound_ecosystems_test.go	Updates Debian handler construction in upstream-not-found test table.
internal/handler/debian_test.go	Updates Debian handler construction in routes test.
internal/config/config.go	Adds Upstream.Debian, default value, env override, and refactors env parsing helpers.
internal/config/config_test.go	Verifies default and env override behavior for Upstream.Debian.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 3
• Review effort level: Lite

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[andrew]

The setEnvString/setEnvBool helper approach is exactly right, and having it in this PR is fine — no need to split it out. Commit layout doesn't matter either way, happy to squash on merge.

One small thing the refactor introduced: the LoadFromEnv doc comment (the block listing the PROXY_* vars) now sits directly above setEnvString with no blank line between them, so godoc attaches it to the helper and LoadFromEnv itself ends up undocumented. Moving that block back down to sit on func (c *Config) LoadFromEnv() (with the two helpers defined above it) would sort it — that's what the two Copilot comments on config.go are getting at.

The third Copilot suggestion on download_test.go isn't needed; the config test already covers the override and strings.TrimSuffix doesn't need its own assertion. I've resolved that one.

[vpalatin]

One small thing the refactor introduced: the LoadFromEnv doc comment (the block listing the PROXY_* vars) now sits directly above setEnvString with no blank line between them, so godoc attaches it to the helper and LoadFromEnv itself ends up undocumented. Moving that block back down to sit on func (c *Config) LoadFromEnv() (with the two helpers defined above it) would sort it — that's what the two Copilot comments on config.go are getting at.

The updated version should now fix the comment/doc

[andrew]

Looks good.