Use os.UserCacheDir as first fallback if cacheDir is not set

We will now try

1. cacheDir (or, commonly set in environment as `HUGO_CACHEDIR`)
2. if on Netlify we use `/opt/build/cache/hugo_cache/`
3. os.UserCacheDir
4. A temp dir

Storing the cache, especially the module cache, in a temporary idea has had lots of hard to debug issues, especially on MacOS,
which this commit tries to fix.

This should also make it easier to locate the Hugo cache:

>UserCacheDir returns the default root directory to use for user-specific cached data. Users should create their own
application-specific subdirectory within this one and use that.
>
>On Unix systems, it returns $XDG_CACHE_HOME as specified by
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if non-empty, else $HOME/.cache. On Darwin, it
returns $HOME/Library/Caches. On Windows, it returns %LocalAppData%. On Plan 9, it returns $home/lib/cache.
>
>If the location cannot be determined (for example, $HOME is not defined), then it will return an error.

Fixes gohugoio#11286
Fixes gohugoio#11291

config/allconfig/allconfig.go

@@ -729,6 +729,7 @@ func fromLoadConfigResult(fs afero.Fs, logger loggers.Logger, res config.LoadCon
 	cfg := res.Cfg
 
 	all := &Config{}
+
 	err := decodeConfigFromParams(fs, bcfg, cfg, all, nil)
 	if err != nil {
 		return nil, err
@@ -871,6 +872,10 @@ func fromLoadConfigResult(fs afero.Fs, logger loggers.Logger, res config.LoadCon
 
 	bcfg.PublishDir = all.PublishDir
 	res.BaseConfig = bcfg
+	all.CommonDirs.CacheDir = bcfg.CacheDir
+	for _, l := range langConfigs {
+		l.CommonDirs.CacheDir = bcfg.CacheDir
+	}
 
 	cm := &Configs{
 		Base:                  all,

helpers/path.go

@@ -394,6 +394,7 @@ func OpenFileForWriting(fs afero.Fs, filename string) (afero.File, error) {
 // The dir will be created if it does not exist.
 func GetCacheDir(fs afero.Fs, cacheDir string) (string, error) {
 	cacheDir = cacheDirDefault(cacheDir)
+
 	if cacheDir != "" {
 		exists, err := DirExists(cacheDir, fs)
 		if err != nil {
@@ -408,12 +409,22 @@ func GetCacheDir(fs afero.Fs, cacheDir string) (string, error) {
 		return cacheDir, nil
 	}
 
+	const hugoCacheBase = "hugo_cache"
+
+	userCacheDir, err := os.UserCacheDir()
+	if err == nil {
+		cacheDir := filepath.Join(userCacheDir, hugoCacheBase)
+		if err := fs.Mkdir(cacheDir, 0777); err == nil || os.IsExist(err) {
+			return cacheDir, nil
+		}
+	}
+
 	// Fall back to a cache in /tmp.
 	userName := os.Getenv("USER")
 	if userName != "" {
-		return GetTempDir("hugo_cache_"+userName, fs), nil
+		return GetTempDir(hugoCacheBase+"_"+userName, fs), nil
 	} else {
-		return GetTempDir("hugo_cache", fs), nil
+		return GetTempDir(hugoCacheBase, fs), nil
 	}
 }
 
@@ -433,7 +444,7 @@ func cacheDirDefault(cacheDir string) string {
 		return "/opt/build/cache/hugo_cache/"
 	}
 
-	// This will fall back to an hugo_cache folder in the tmp dir, which should work fine for most CI
+	// This will fall back to an hugo_cache folder in either os.UserCacheDir or the tmp dir, which should work fine for most CI
 	// providers. See this for a working CircleCI setup:
 	// https://github.com/bep/hugo-sass-test/blob/6c3960a8f4b90e8938228688bc49bdcdd6b2d99e/.circleci/config.yml
 	// If not, they can set the HUGO_CACHEDIR environment variable or cacheDir config key.

main_test.go

@@ -375,8 +375,23 @@ func testSetupFunc() func(env *testscript.Env) error {
 	return func(env *testscript.Env) error {
 		var keyVals []string
 		keyVals = append(keyVals, "HUGO_TESTRUN", "true")
-		hugoCachedDir := filepath.Join(env.WorkDir, "hugocache")
-		keyVals = append(keyVals, "HUGO_CACHEDIR", hugoCachedDir)
+		keyVals = append(keyVals, "HUGO_CACHEDIR", filepath.Join(env.WorkDir, "hugocache"))
+		xdghome := filepath.Join(env.WorkDir, "xdgcachehome")
+		keyVals = append(keyVals, "XDG_CACHE_HOME", xdghome)
+		home := filepath.Join(env.WorkDir, "home")
+		keyVals = append(keyVals, "HOME", home)
+
+		if runtime.GOOS == "darwin" {
+			if err := os.MkdirAll(filepath.Join(home, "Library", "Caches"), 0777); err != nil {
+				return err
+			}
+		}
+
+		if runtime.GOOS == "linux" {
+			if err := os.MkdirAll(xdghome, 0777); err != nil {
+				return err
+			}
+		}
 
 		keyVals = append(keyVals, "SOURCE", sourceDir)
 

testscripts/commands/config__cachedir.txt

@@ -0,0 +1,18 @@
+
+[windows] skip
+
+env HUGO_CACHEDIR=
+hugo config
+
+[darwin] stdout 'home/Library/Caches/hugo_cache'
+[linux] stdout 'xdgcachehome/hugo_cache'
+
+# Repeat it to make sure it handles an existing hugo_cache dir.
+hugo config
+
+[darwin] stdout 'home/Library/Caches/hugo_cache'
+[linux] stdout 'xdgcachehome/hugo_cache'
+
+-- hugo.toml --
+baseURL="https://example.com/"
+title="My New Hugo Site"