[perf] Only compute print-dev-env if needed (#898)

## Summary

Stacked on #897

This significantly improves `run` and `shell` performance (`run` goes
from 500ms -> 150ms without telemetry)

Almost all remaining time for run and shell involves the `exec.Command`
so this is close to as fast as it will get.

## How was it tested?

```bash
devbox add hello
time devbox run which hello
```

Author: mikeland73

internal/cuecfg/hash.go

@@ -0,0 +1,25 @@
+package cuecfg
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"os"
+)
+
+func FileHash(path string) (string, error) {
+	data, err := os.ReadFile(path)
+	if err != nil && !os.IsNotExist(err) {
+		return "", err
+	}
+	hash := sha256.Sum256(data)
+	return hex.EncodeToString(hash[:]), nil
+}
+
+func Hash(s any) (string, error) {
+	json, err := MarshalJSON(s)
+	if err != nil {
+		return "", err
+	}
+	hash := sha256.Sum256(json)
+	return hex.EncodeToString(hash[:]), nil
+}

internal/impl/config.go

@@ -4,8 +4,6 @@
 package impl
 
 import (
-	"crypto/sha256"
-	"encoding/hex"
 	"io"
 	"net/http"
 	"net/url"
@@ -69,12 +67,7 @@ func (c *Config) MergedPackages(w io.Writer) []string {
 }
 
 func (c *Config) Hash() (string, error) {
-	json, err := cuecfg.MarshalJSON(c)
-	if err != nil {
-		return "", err
-	}
-	hash := sha256.Sum256(json)
-	return hex.EncodeToString(hash[:]), nil
+	return cuecfg.Hash(c)
 }
 
 func readConfig(path string) (*Config, error) {

internal/impl/devbox.go

@@ -6,8 +6,6 @@ package impl
 
 import (
 	"context"
-	"crypto/md5"
-	"encoding/hex"
 	"fmt"
 	"io"
 	"os"
@@ -33,6 +31,7 @@ import (
 	"go.jetpack.io/devbox/internal/debug"
 	"go.jetpack.io/devbox/internal/fileutil"
 	"go.jetpack.io/devbox/internal/initrec"
+	"go.jetpack.io/devbox/internal/lockfile"
 	"go.jetpack.io/devbox/internal/nix"
 	"go.jetpack.io/devbox/internal/pkgslice"
 	"go.jetpack.io/devbox/internal/planner"
@@ -796,9 +795,8 @@ func (d *Devbox) computeNixEnv(
 	debug.Log("computed environment PATH is: %s", env["PATH"])
 
 	d.setCommonHelperEnvVars(env)
-	addHashToEnv(env)
 
-	return env, nil
+	return env, addHashToEnv(env)
 }
 
 var nixEnvCache map[string]string
@@ -810,7 +808,18 @@ var nixEnvWithPrintDevEnvCache map[string]string
 func (d *Devbox) nixEnv(ctx context.Context) (map[string]string, error) {
 	var err error
 	if nixEnvCache == nil {
-		nixEnvCache, err = d.computeNixEnv(ctx, false /*usePrintDevEnvCache*/)
+		usePrintDevEnvCache := false
+
+		// If lockfile is up-to-date, we can use the print-dev-env cache.
+		if lock, err := lockfile.Local(d); err != nil {
+			return nil, err
+		} else if upToDate, err := lock.IsUpToDate(); err != nil {
+			return nil, err
+		} else if upToDate {
+			usePrintDevEnvCache = true
+		}
+
+		nixEnvCache, err = d.computeNixEnv(ctx, usePrintDevEnvCache)
 	}
 	return nixEnvCache, err
 }
@@ -1075,11 +1084,11 @@ func (d *Devbox) NixBins(ctx context.Context) ([]string, error) {
 	return lo.Values(bins), nil
 }
 
-func addHashToEnv(env map[string]string) {
-	h := md5.New()
-	for k, v := range env {
-		h.Write([]byte(k))
-		h.Write([]byte(v))
+func addHashToEnv(env map[string]string) error {
+	hash, err := cuecfg.Hash(env)
+	if err == nil {
+		env[devboxShellEnvHashVarName] = hash
+
 	}
-	env[devboxShellEnvHashVarName] = hex.EncodeToString(h.Sum(nil)[:])
+	return err
 }

internal/impl/packages.go

@@ -163,6 +163,11 @@ func (d *Devbox) ensurePackagesAreInstalled(ctx context.Context, mode installMod
 		return err
 	}
 
+	// Force print-dev-env cache to be recomputed.
+	if _, err = d.computeNixEnv(ctx, false /*use cache*/); err != nil {
+		return err
+	}
+
 	return lock.Update()
 }
 

internal/lockfile/lockfile.go renamed to internal/lockfile/local.go

@@ -18,11 +18,13 @@ type localLockFile struct {
 	project                devboxProject
 	ConfigHash             string `json:"config_hash"`
 	NixProfileManifestHash string `json:"nix_profile_manifest_hash"`
+	NixPrintDevEnvHash     string `json:"nix_print_dev_env_hash"`
 }
 
 func (l *localLockFile) equals(other *localLockFile) bool {
 	return l.ConfigHash == other.ConfigHash &&
-		l.NixProfileManifestHash == other.NixProfileManifestHash
+		l.NixProfileManifestHash == other.NixProfileManifestHash &&
+		l.NixPrintDevEnvHash == other.NixPrintDevEnvHash
 }
 
 func (l *localLockFile) IsUpToDate() (bool, error) {
@@ -71,10 +73,16 @@ func forProject(project devboxProject) (*localLockFile, error) {
 		return nil, err
 	}
 
+	printDevEnvCacheHash, err := nix.PrintDevEnvCacheHash(project.ProjectDir())
+	if err != nil {
+		return nil, err
+	}
+
 	newLock := &localLockFile{
 		project:                project,
 		ConfigHash:             configHash,
 		NixProfileManifestHash: nixHash,
+		NixPrintDevEnvHash:     printDevEnvCacheHash,
 	}
 
 	return newLock, nil

internal/nix/nix.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/samber/lo"
+	"go.jetpack.io/devbox/internal/cuecfg"
 	"go.jetpack.io/devbox/internal/debug"
 )
 
@@ -147,6 +148,12 @@ func PrintDevEnv(ctx context.Context, args *PrintDevEnvArgs) (*printDevEnvOut, e
 	return &out, nil
 }
 
+func PrintDevEnvCacheHash(profileDir string) (string, error) {
+	return cuecfg.FileHash(
+		filepath.Join(profileDir, ".devbox", ".nix-print-dev-env-cache"),
+	)
+}
+
 func savePrintDevEnvCache(path string, out printDevEnvOut) error {
 	data, err := json.Marshal(out)
 	if err != nil {

internal/nix/profile.go

@@ -3,8 +3,6 @@ package nix
 import (
 	"bufio"
 	"bytes"
-	"crypto/sha256"
-	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -15,6 +13,7 @@ import (
 	"strings"
 
 	"github.com/fatih/color"
+	"go.jetpack.io/devbox/internal/cuecfg"
 	"go.jetpack.io/devbox/internal/redact"
 )
 
@@ -334,13 +333,7 @@ func readManifest(profilePath string) (manifest, error) {
 }
 
 func ManifestHash(profileDir string) (string, error) {
-	path := filepath.Join(profileDir, ProfilePath, "manifest.json")
-	data, err := os.ReadFile(path)
-	if err != nil && !os.IsNotExist(err) {
-		return "", err
-	}
-	hash := sha256.Sum256(data)
-	return hex.EncodeToString(hash[:]), nil
+	return cuecfg.FileHash(filepath.Join(profileDir, ProfilePath, "manifest.json"))
 }
 
 func nextPriority(profilePath string) string {