[bugs] Fix shell.nix bug and ensure current symlink (#1015)

## Summary

Fixes 2 unrelated bugs:

* Create current symlink if needed. This has no user impact, but is
causing devbox cloud to break. It is also needed for future
multi-profile support.
* Ensure shell.nix works by including correct URL in fetchTarball. This
affects direnv users that have a legacy .envrc file. New/updated users
are unaffected. Fixes #1010

cc: @Lagoja 

## How was it tested?

* Deleted current symlink, and saw it being recreated.
* `nix-shell .devbox/gen/shell.nix`

Author: mikeland73

internal/impl/global.go

@@ -5,6 +5,7 @@ package impl
 
 import (
 	"fmt"
+	"io/fs"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -162,7 +163,21 @@ func (d *Devbox) addFromPull(pullCfg *Config) error {
 
 func GlobalDataPath() (string, error) {
 	path := xdg.DataSubpath(filepath.Join("devbox/global", currentGlobalProfile))
-	return path, errors.WithStack(os.MkdirAll(path, 0755))
+	if err := os.MkdirAll(path, 0755); err != nil {
+		return "", errors.WithStack(err)
+	}
+
+	nixProfilePath := filepath.Join(path, "profile")
+	currentPath := xdg.DataSubpath("devbox/global/current")
+
+	// For now default is always current. In the future we will support multiple
+	// and allow user to switch.
+	err := os.Symlink(nixProfilePath, currentPath)
+	if err != nil && !errors.Is(err, fs.ErrExist) {
+		return "", errors.WithStack(err)
+	}
+
+	return path, nil
 }
 
 func GlobalNixProfilePath() (string, error) {

internal/impl/tmpl/shell.nix.tmpl

@@ -1,7 +1,7 @@
 let
   pkgs = import
     (fetchTarball {
-      url = "{{ .NixpkgsInfo.URL }}";
+      url = "{{ .NixpkgsInfo.TarURL }}";
     })
     { };
   {{- range .Definitions}}

internal/planner/plansdk/plansdk.go

@@ -85,21 +85,22 @@ func WelcomeMessage(s string) string {
 // TODO: We can probably get rid of this once we remove the haskell and php
 // planners
 type NixpkgsInfo struct {
-	URL string
+	URL    string
+	TarURL string
 }
 
 // The commit hash for nixpkgs-unstable on 2023-01-25 from status.nixos.org
 const DefaultNixpkgsCommit = "f80ac848e3d6f0c12c52758c0f25c10c97ca3b62"
 
 func GetNixpkgsInfo(commitHash string) *NixpkgsInfo {
-	mirror := nixpkgsMirrorURL(commitHash)
-	if mirror != "" {
-		return &NixpkgsInfo{
-			URL: mirror,
-		}
+	url := fmt.Sprintf("github:NixOS/nixpkgs/%s", commitHash)
+	if mirror := nixpkgsMirrorURL(commitHash); mirror != "" {
+		url = mirror
 	}
 	return &NixpkgsInfo{
-		URL: fmt.Sprintf("github:NixOS/nixpkgs/%s", commitHash),
+		URL: url,
+		// legacy, used for shell.nix (which is no longer used, but some direnv users still need it)
+		TarURL: fmt.Sprintf("https://github.com/nixos/nixpkgs/archive/%s.tar.gz", commitHash),
 	}
 }