internal/env: relocate from app

This package is much cleaner to stub independently, and makes no sense to lump into app.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/env/env.go

@@ -0,0 +1,66 @@
+// Package env provides the [Paths] struct for efficiently building paths from the environment.
+package env
+
+import (
+	"log"
+	"os"
+	"strconv"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+)
+
+// Paths holds paths copied from the environment and is used to create [hst.Paths].
+type Paths struct {
+	// TempDir is returned by [os.TempDir].
+	TempDir *check.Absolute
+	// RuntimePath is copied from $XDG_RUNTIME_DIR.
+	RuntimePath *check.Absolute
+}
+
+// Copy expands [Paths] into [hst.Paths].
+func (env *Paths) Copy(v *hst.Paths, userid int) {
+	if env == nil || env.TempDir == nil || v == nil {
+		panic("attempting to use an invalid Paths")
+	}
+
+	v.TempDir = env.TempDir
+	v.SharePath = env.TempDir.Append("hakurei." + strconv.Itoa(userid))
+
+	if env.RuntimePath == nil {
+		// fall back to path in share since hakurei has no hard XDG dependency
+		v.RunDirPath = v.SharePath.Append("run")
+		v.RuntimePath = v.RunDirPath.Append("compat")
+	} else {
+		v.RuntimePath = env.RuntimePath
+		v.RunDirPath = env.RuntimePath.Append("hakurei")
+	}
+}
+
+// CopyPaths returns a populated [Paths].
+func CopyPaths() *Paths { return CopyPathsFunc(log.Fatalf, os.TempDir, os.Getenv) }
+
+// CopyPathsFunc returns a populated [Paths],
+// using the provided [log.Fatalf], [os.TempDir], [os.Getenv] functions.
+func CopyPathsFunc(
+	fatalf func(format string, v ...any),
+	tempdir func() string,
+	getenv func(key string) string,
+) *Paths {
+	const xdgRuntimeDir = "XDG_RUNTIME_DIR"
+
+	var env Paths
+
+	if tempDir, err := check.NewAbs(tempdir()); err != nil {
+		fatalf("invalid TMPDIR: %v", err)
+		panic("unreachable")
+	} else {
+		env.TempDir = tempDir
+	}
+
+	if a, err := check.NewAbs(getenv(xdgRuntimeDir)); err == nil {
+		env.RuntimePath = a
+	}
+
+	return &env
+}