plansdk: use nixpkgs mirror when in cloud VM (#521)

This change is a bit hacky. I'd like to eventually clean this up to work
better with the various nix commands. It also reverts the previous env
var change since it's no longer needed.

When generating the various *.nix files, check to see if the cloud
mirror is reachable. If it is, download nixpkgs archives from there
instead of GitHub.

Also update the default commit hash to be the current nixpkgs unstable,
which is also in the cache. This guarantees that any new devbox projects
created after this release will hit the cache in the cloud.

Author: gcurtis

devbox.json

@@ -12,6 +12,6 @@
     }
   },
   "nixpkgs": {
-    "commit": "aa7e3b940ad90ba58a5d8c0a2269ec557c9ecc70"
+    "commit": "3954218cf613eba8e0dcefa9abe337d26bc48fd0"
   }
 }

internal/impl/tmpl/development.nix.tmpl

@@ -1,7 +1,9 @@
 let
-  mirrorURL = builtins.getEnv "DEVBOX_NIXPKGS_URL";
   pkgs = import (fetchTarball {
-    url = if mirrorURL == "" then "{{ .NixpkgsInfo.URL }}" else mirrorURL;
+    url = "{{ .NixpkgsInfo.URL }}";
+    {{- if .NixpkgsInfo.Sha256 }}
+    sha256 = "{{ .NixpkgsInfo.Sha256 }}";
+    {{- end }}
   }) {
     {{- if .NixOverlays }}
       overlays = [

internal/impl/tmpl/runtime.nix.tmpl

@@ -1,7 +1,9 @@
 let
-  mirrorURL = builtins.getEnv "DEVBOX_NIXPKGS_URL";
   pkgs = import (fetchTarball {
-    url = if mirrorURL == "" then "{{ .NixpkgsInfo.URL }}" else mirrorURL;
+    url = "{{ .NixpkgsInfo.URL }}";
+    {{- if .NixpkgsInfo.Sha256 }}
+    sha256 = "{{ .NixpkgsInfo.Sha256 }}";
+    {{- end }}
   }) {
     {{- if .NixOverlays }}
       overlays = [

internal/impl/tmpl/shell.nix.tmpl

@@ -1,7 +1,11 @@
 let
-  mirrorURL = builtins.getEnv "DEVBOX_NIXPKGS_URL";
   pkgs = import (fetchTarball {
-    url = if mirrorURL == "" then "{{ .NixpkgsInfo.URL }}" else mirrorURL;
+    # Commit hash as of 2022-08-16
+    # `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
+    url = "{{ .NixpkgsInfo.URL }}";
+    {{- if .NixpkgsInfo.Sha256 }}
+    sha256 = "{{ .NixpkgsInfo.Sha256 }}";
+    {{- end }}
   }) {
     {{- if .NixOverlays }}
       overlays = [

internal/planner/plansdk/plansdk.go

@@ -5,7 +5,9 @@ package plansdk
 
 import (
 	"fmt"
+	"net/http"
 	"os"
+	"time"
 
 	"github.com/imdario/mergo"
 	"go.jetpack.io/devbox/internal/cuecfg"
@@ -91,11 +93,34 @@ type NixpkgsInfo struct {
 	Sha256 string
 }
 
-// The commit hash for nixos-22.11 on 2022-12-06 from status.nixos.org
-const DefaultNixpkgsCommit = "52e3e80afff4b16ccb7c52e9f0f5220552f03d04"
+// The commit hash for nixpkgs-unstable on 2023-01-25 from status.nixos.org
+const DefaultNixpkgsCommit = "3954218cf613eba8e0dcefa9abe337d26bc48fd0"
 
 func GetNixpkgsInfo(commitHash string) (*NixpkgsInfo, error) {
+	mirror := nixpkgsMirrorURL(commitHash)
+	if mirror != "" {
+		return &NixpkgsInfo{
+			URL: mirror,
+		}, nil
+	}
 	return &NixpkgsInfo{
 		URL: fmt.Sprintf("https://github.com/nixos/nixpkgs/archive/%s.tar.gz", commitHash),
 	}, nil
 }
+
+func nixpkgsMirrorURL(commitHash string) string {
+	// Use DEVBOX_REGION as a hint to see if we're in Devbox Cloud.
+	if os.Getenv("DEVBOX_REGION") == "" {
+		return ""
+	}
+
+	// Check that the mirror is responsive and has the tar file. We can't
+	// leave this up to Nix because fetchTarball will retry indefinitely.
+	client := &http.Client{Timeout: 3 * time.Second}
+	mirrorURL := fmt.Sprintf("http://[fdaa:0:a780:0:1::2]:8081/nixos/nixpkgs/archive/%s.tar.gz", commitHash)
+	resp, err := client.Head(mirrorURL)
+	if err != nil || resp.StatusCode != http.StatusOK {
+		return ""
+	}
+	return mirrorURL
+}