docs(contributing): document how to debug Devbox locally

[mikeland73]

Summary

Resolves #2699.

Several Devbox commands (e.g. devbox shell, devbox services up) do little work themselves — they re-exec a nested devbox subprocess inside the project's shell environment, and that subprocess does the real work. The nested process is resolved from PATH, so it's usually the installed version of Devbox rather than the local build you're editing. This is why devbox run build && dist/devbox services up may silently run old code, and why DEVBOX_DEBUG=1 logs / breakpoints added to a local build don't fire.

The issue author hit exactly this and asked for it to be documented in the repo. This PR adds a "Debugging Devbox Locally" section to CONTRIBUTING.md describing the two workarounds they found:

1. Point the launcher at your local build via DEVBOX_USE_VERSION (most faithful — every invocation, including nested subprocesses, uses your build).
2. The hidden --run-in-current-shell flag (quick to iterate, but bypasses the nested re-exec so behavior differs slightly).

Verification

The documented behavior was confirmed against the code:

• devbox run build → dist/devbox (devbox.json).
• devbox services … re-execs a nested devbox subprocess unless --run-in-current-shell is set (internal/devbox/services.go — runDevboxServicesScript runs the devbox on PATH inside the shell; the flag is registered hidden in internal/boxcli/services.go).
• DEVBOX_USE_VERSION is the version-override env the launcher honors and that Devbox preserves across re-launches (internal/setup/setup.go, internal/devbox/providers/nixcache/setup.go).

Docs-only change; no code paths affected.

cc @jay-aye-see-kay (issue author) — thanks for the detailed write-up of the workaround.

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[Copilot]

Pull request overview

Adds contributor documentation explaining why some Devbox commands don’t execute the locally-built binary (due to nested devbox re-exec from PATH) and how to reliably debug local changes.

Changes:

• Document the nested devbox subprocess behavior that can cause local builds/logs/breakpoints to be ignored.
• Add two debugging workarounds: DEVBOX_USE_VERSION (launcher override) and --run-in-current-shell (skip re-exec).

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[mikeland73]

@claude please review this PR for correctness, and flag anything that would block merging.

[claude]

Claude finished @mikeland73's task in 3m 21s —— View job

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Review of PR #2893

• Gather context (diff, file, prior review comments)
• Verify claims about nested re-exec / launcher behavior against source
• Check XDG fix from commit 1395772 is applied correctly
• Post review findings

What checks out:

• devbox run build → dist/devbox — confirmed via devbox.json's scripts.build.
• devbox services {up,start,stop,restart,ls} genuinely re-exec into a nested devbox subprocess resolved from PATH — confirmed in internal/devbox/services.go:272-275 (runDevboxServicesScript → d.RunScript(..., "devbox", cmdArgs)), unless --run-in-current-shell is passed.
• The XDG fix in 1395772 (cache_dir="${XDG_CACHE_HOME:-$HOME/.cache}/...", CONTRIBUTING.md:85) is correctly applied and addresses the earlier review comment.
• DEVBOX_USE_VERSION is preserved across Devbox's own internal re-execs (internal/setup/setup.go:183, internal/devbox/providers/nixcache/setup.go:202), consistent with the doc's claim that it propagates to nested invocations.

Correctness concerns (worth fixing before merge):

1. devbox shell is a weak/inaccurate motivating example (CONTRIBUTING.md:66-75). The doc opens by citing devbox shell alongside devbox services up as a command that "does little work itself" and re-execs a nested subprocess that "does the real work." That's not what the code shows: Shell() (internal/devbox/devbox.go:227-261) does the real work (computing env, building the nix shell) in-process. The only nested devbox invocations spawned inside the resulting shell are devbox log shell-ready / devbox log shell-interactive in internal/devbox/shellrc.tmpl — lightweight telemetry calls, not "the real work" that would cause DEVBOX_DEBUG logs or breakpoints to silently not fire. The re-exec-masks-your-build problem is real for devbox services * (and similarly for anything invoked via devbox run), but devbox shell doesn't clearly suffer from it the way it's framed here.

2. Workaround Allow overwrites on install, build and start command #2 doesn't apply to the doc's own first example. --run-in-current-shell is registered as a PersistentFlags() only on servicesCommand (internal/boxcli/services.go:138-144), scoped to devbox services * subcommands. It does not exist for devbox shell. So "Many commands accept a hidden --run-in-current-shell flag" (CONTRIBUTING.md:94) overstates its scope, and a reader who hit the devbox shell case from the intro has no way to apply workaround Allow overwrites on install, build and start command #2 to it.

   Suggested fix: either drop devbox shell from the opening example (keep it focused on devbox services/devbox run-style commands, where the claim is fully accurate), or add a caveat that --run-in-current-shell is devbox services-only.

3. Minor / unverifiable in this session: the exact binary cache path ($XDG_CACHE_HOME/devbox/bin/<version>_<os>_<arch>/devbox) is determined by the external launch.sh launcher script, which isn't part of this repo (only referenced as "keep in sync with launch.sh" in internal/vercheck/vercheck.go:29). I didn't have network access in this session to fetch and confirm that script matches the documented path format. Worth double-checking workaround Add quickstart to Readme #1 was tested end-to-end (build → copy to cache dir → DEVBOX_USE_VERSION → confirm the launcher actually picks it up) before merging, since a mismatched path would make the primary workaround silently a no-op.

Nothing here is a functional/build risk since this is docs-only, but #1 and #2 are factual inaccuracies in a doc whose whole purpose is precise debugging instructions, so I'd flag them as blocking for a docs PR.

Fix this →

[mikeland73]

Closing: the root cause was fixed in #2936 — devbox services … now re-execs the currently running binary via os.Executable() instead of resolving devbox from PATH, so devbox run build && dist/devbox <cmd> picks up local changes end to end and the workarounds documented here are no longer needed.