[nixpkgs version] generate nix files containing the nixpkgs version from config (#258)

## Summary

This PR will parameterize the `shell.nix`, `development.nix` and
`runtime.nix` files
to accept a `NixpkgsInfo` struct comprising of `url` and `sha256`
fields.

To pass the `NixPkgsInfo` struct to these templates, we need to add them
to the `ShellPlan`
and `BuildPlan` structs. I don't like this, but it seemed the most
straightforward
approach. 

## How was it tested?

with the hardcoded commit hash
```
➜ DEVBOX_FEATURE_NIXPKG_VERSION=1 DEVBOX_DEBUG=0 devbox shell -- node --version
Installing nix packages. This may take a while...done.
Starting a devbox shell...
v18.7.0
```

From https://status.nixos.org, I got the most recent commit for the
`nixos-22.05` channel:
`b3a8f7ed267e0a7ed100eb7d716c9137ff120fe3`. I set this in the
devbox.json file.

```
> DEVBOX_FEATURE_NIXPKG_VERSION=1 DEVBOX_DEBUG=0 devbox shell -- node --version
Installing nix packages. This may take a while...done.
Starting a devbox shell...
v18.9.1
```
This matches the version seen in https://search.nixos.org

```
>  DEVBOX_FEATURE_NIXPKG_VERSION=1 devbox build && docker run devbox
.... // omitted
Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them
the NODE_MAJOR_VERSION is 18
```

Author: savil

boxcli/plan.go

@@ -47,7 +47,11 @@ func runPlanCmd(_ *cobra.Command, args []string, flags planCmdFlags) error {
 	enc.SetIndent("", "  ")
 	enc.SetEscapeHTML(false)
 
-	shellPlan := box.ShellPlan()
+	shellPlan, err := box.ShellPlan()
+	if err != nil {
+		return err
+	}
+
 	err = enc.Encode(shellPlan)
 	if err != nil {
 		return errors.WithStack(err)

devbox.go

@@ -158,12 +158,18 @@ func (d *Devbox) Build(flags *docker.BuildFlags) error {
 
 // Plan creates a plan of the actions that devbox will take to generate its
 // shell environment.
-func (d *Devbox) ShellPlan() *plansdk.ShellPlan {
+func (d *Devbox) ShellPlan() (*plansdk.ShellPlan, error) {
 	userDefinedPkgs := d.cfg.Packages
 	shellPlan := planner.GetShellPlan(d.srcDir, userDefinedPkgs)
 	shellPlan.DevPackages = userDefinedPkgs
 
-	return shellPlan
+	if nixpkgsInfo, err := plansdk.GetNixpkgsInfo(d.cfg.Nixpkgs.Commit); err != nil {
+		return nil, err
+	} else {
+		shellPlan.NixpkgsInfo = nixpkgsInfo
+	}
+
+	return shellPlan, nil
 }
 
 // Plan creates a plan of the actions that devbox will take to generate its
@@ -174,7 +180,17 @@ func (d *Devbox) BuildPlan() (*plansdk.BuildPlan, error) {
 	if err != nil {
 		return nil, err
 	}
-	return plansdk.MergeUserBuildPlan(userPlan, buildPlan)
+	plan, err := plansdk.MergeUserBuildPlan(userPlan, buildPlan)
+	if err != nil {
+		return nil, err
+	}
+
+	if nixpkgsInfo, err := plansdk.GetNixpkgsInfo(d.cfg.Nixpkgs.Commit); err != nil {
+		return nil, err
+	} else {
+		plan.NixpkgsInfo = nixpkgsInfo
+	}
+	return plan, nil
 }
 
 // Generate creates the directory of Nix files and the Dockerfile that define
@@ -195,7 +211,10 @@ func (d *Devbox) Shell() error {
 	if err := d.ensurePackagesAreInstalled(install); err != nil {
 		return err
 	}
-	plan := d.ShellPlan()
+	plan, err := d.ShellPlan()
+	if err != nil {
+		return err
+	}
 	profileDir, err := d.profileDir()
 	if err != nil {
 		return err
@@ -293,7 +312,11 @@ func (d *Devbox) convertToBuildPlan() *plansdk.BuildPlan {
 }
 
 func (d *Devbox) generateShellFiles() error {
-	return generateForShell(d.srcDir, d.ShellPlan())
+	plan, err := d.ShellPlan()
+	if err != nil {
+		return err
+	}
+	return generateForShell(d.srcDir, plan)
 }
 
 func (d *Devbox) generateBuildFiles() error {

devbox_test.go

@@ -51,7 +51,7 @@ func testShell(t *testing.T, testPath string) {
 		box.srcDir, err = filepath.Rel(currentDir, box.srcDir)
 		assert.NoErrorf(err, "expect to construct relative path from %s relative to base %s", box.srcDir, currentDir)
 
-		shellPlan := box.ShellPlan()
+		shellPlan, err := box.ShellPlan()
 		assert.NoError(err, "devbox shell plan should not fail")
 
 		err = box.generateShellFiles()

planner/plansdk/plansdk.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/imdario/mergo"
 	"github.com/pkg/errors"
+	"go.jetpack.io/devbox/boxcli/featureflag"
 	"go.jetpack.io/devbox/pkgslice"
 )
 
@@ -27,6 +28,7 @@ type PlanError struct {
 
 // Plan tells devbox how to start shell projects.
 type ShellPlan struct {
+	NixpkgsInfo *NixpkgsInfo
 	// Set by devbox.json
 	DevPackages []string `cue:"[...string]" json:"dev_packages,omitempty"`
 	// Init hook on shell start. Currently, Nginx and python pip planners need it for shell.
@@ -42,6 +44,8 @@ type ShellPlan struct {
 
 // Plan tells devbox how to start build projects.
 type BuildPlan struct {
+	NixpkgsInfo *NixpkgsInfo
+
 	NixOverlays []string `cue:"[...string]" json:"nix_overlays,omitempty"`
 	// DevPackages is the slice of Nix packages that devbox makes available in
 	// its development environment. They are also available in shell.
@@ -201,3 +205,31 @@ func FileExists(path string) bool {
 func WelcomeMessage(s string) string {
 	return fmt.Sprintf(`echo "%s";`, s)
 }
+
+// publicly visible so that json marshalling works
+type NixpkgsInfo struct {
+	URL string
+
+	Sha256 string
+}
+
+func GetNixpkgsInfo(commitHash string) (*NixpkgsInfo, error) {
+
+	// If the featureflag is OFF, then we fallback to the hardcoded commit
+	// and ignore any value set in the devbox.json
+	if !featureflag.Get(featureflag.NixpkgVersion).Enabled() {
+		// Commit hash as of 2022-08-16
+		// `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
+		//
+		// sha256 from:
+		// nix-prefetch-url --unpack  https://github.com/nixos/nixpkgs/archive/<commit-hash>.tar.gz
+		return &NixpkgsInfo{
+			URL:    "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz",
+			Sha256: "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy",
+		}, nil
+	}
+
+	return &NixpkgsInfo{
+		URL: fmt.Sprintf("https://github.com/nixos/nixpkgs/archive/%s.tar.gz", commitHash),
+	}, nil
+}

testdata/rust/rust-stable/devbox.json

@@ -7,5 +7,8 @@
     "init_hook": [
       "source init-shell.sh"
     ]
+  },
+  "nixpkgs": {
+    "version": "22.05"
   }
 }

tmpl/development.nix.tmpl

@@ -1,9 +1,9 @@
 let
   pkgs = import (fetchTarball {
-    # Commit hash as of 2022-08-16
-    # `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
-    url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
-    sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
+    url = "{{ .NixpkgsInfo.URL }}";
+    {{- if .NixpkgsInfo.Sha256 }}
+    sha256 = "{{ .NixpkgsInfo.Sha256 }}";
+    {{- end }}
   }) {
     {{- if .NixOverlays }}
       overlays = [

tmpl/runtime.nix.tmpl

@@ -1,9 +1,9 @@
 let
   pkgs = import (fetchTarball {
-    # Commit hash as of 2022-08-16
-    # `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
-    url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
-    sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
+    url = "{{ .NixpkgsInfo.URL }}";
+    {{- if .NixpkgsInfo.Sha256 }}
+    sha256 = "{{ .NixpkgsInfo.Sha256 }}";
+    {{- end }}
   }) {
     {{- if .NixOverlays }}
       overlays = [

tmpl/shell.nix.tmpl

@@ -2,8 +2,10 @@ let
   pkgs = import (fetchTarball {
     # Commit hash as of 2022-08-16
     # `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
-    url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
-    sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
+    url = "{{ .NixpkgsInfo.URL }}";
+    {{- if .NixpkgsInfo.Sha256 }}
+    sha256 = "{{ .NixpkgsInfo.Sha256 }}";
+    {{- end }}
   }) {
     {{- if .NixOverlays }}
       overlays = [