fix: refuse to start when the cache directory is not writable

An unwritable cache directory degraded silently: media still served, because
the download succeeds before the cache write is attempted, so the only symptom
was one "permission denied" line per request and a cache that never filled.
Every request re-fetched from the CDN.

Probe the directory at startup and exit with the uid and the chown that fixes
it. The container image runs as uid 10000, which is the usual cause with a
bind-mounted cache, so say so in the message and in both compose examples.

1 files changed, 28 insertions, 1 deletions

diff --git a/app/config.go b/app/config.go
index 3b85527..ad07aa3 100755
--- a/app/config.go
+++ b/app/config.go
@@ -58,9 +58,28 @@ var CFG = config{
 
 var lifetimeParsed int64
 
+// checkCacheWritable creates the cache directory if it is missing and confirms
+// this process can actually write into it, returning the error that a real cache
+// write would hit.
+//
+// An unwritable cache directory is otherwise a silent cliff: every media request
+// still succeeds by re-downloading from the CDN, so the only symptom is one
+// "permission denied" line per request and a cache that never fills.
+func checkCacheWritable(path string) error {
+	if err := os.MkdirAll(path, 0700); err != nil {
+		return err
+	}
+	probe := path + "/.skunkyart-write-probe"
+	if err := os.WriteFile(probe, nil, 0600); err != nil {
+		return err
+	}
+	return os.Remove(probe)
+}
+
 // ExecuteConfig loads the config file into CFG, validates it, and starts the
 // cache rotation loop if caching is on. It exits the process on a config that
-// cannot be read or that asks for caching without proxying.
+// cannot be read, that asks for caching without proxying, or that points caching
+// at a directory this process cannot write.
 func ExecuteConfig() {
 	if CFG.cfg != "" {
 		f, err := os.ReadFile(CFG.cfg)
@@ -71,6 +90,14 @@ func ExecuteConfig() {
 		}
 
 		if CFG.Cache.Enabled {
+			if err := checkCacheWritable(CFG.Cache.Path); err != nil {
+				exit("Cache directory is not writable by this process (uid "+
+					strconv.Itoa(os.Getuid())+"): "+err.Error()+
+					"\nGrant that uid write access to the directory, or set cache.enabled to false."+
+					"\nThe official container image runs as uid 10000, so a bind-mounted cache needs:"+
+					"\n  chown -R 10000:10000 <cache dir on the host>", 1)
+			}
+
 			if CFG.Cache.Lifetime != "" {
 				var duration int64
 				day := 24 * time.Hour.Milliseconds()