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.

4 files changed, 39 insertions, 4 deletions

diff --git a/SETUP.md b/SETUP.md
index 4cb8aa8..4e7dd93 100755
--- a/SETUP.md
+++ b/SETUP.md
@@ -12,7 +12,10 @@ Time units:
 * `uri` — Instance URI. Example: `"uri":"/art/"` -> https://skunky.ebloid.ru/art/
 * `cache` — Caching system; default is off.
   * `enabled` — Caching system state, requires boolean value
-  * `path` — Path to cache directory, requires absolute filesystem path
+  * `path` — Path to cache directory. It must be writable by the user SkunkyArt
+    runs as, and SkunkyArt refuses to start if it is not. The container image
+    runs as uid 10000, so a bind-mounted cache needs
+    `sudo chown -R 10000:10000 <dir>` on the host.
   * `lifetime` — Cached file life time, requires numeric value, followed by multiplicative suffix (see Time Units for details)
   * `max-size` — Maximum file size in megabytes
   * `update-interval` — Automatic rotation interval
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()
diff --git a/compose.example.yaml b/compose.example.yaml
index 2e2f088..d7c20ea 100755
--- a/compose.example.yaml
+++ b/compose.example.yaml
@@ -14,4 +14,7 @@ services:
       - no-new-privileges:true
     volumes:
       - ./config.json:/config.json:ro
-      - ./cache:/cache # Ensure cache folder has a 10000:10000 ownership.
+      # The image runs as uid 10000, so the host cache dir must be writable by it:
+      #   mkdir -p cache && sudo chown -R 10000:10000 cache
+      # Without this the container exits at startup telling you the same thing.
+      - ./cache:/cache
diff --git a/compose.vpn_example.yml b/compose.vpn_example.yml
index 27cd488..a89262c 100644
--- a/compose.vpn_example.yml
+++ b/compose.vpn_example.yml
@@ -59,7 +59,9 @@ services:
       - no-new-privileges:true
     volumes:
       - ./config.json:/config.json:ro
-      - ./cache:/cache            # ensure this dir is owned 10000:10000
+      # The image runs as uid 10000, so the host cache dir must be writable by it:
+      #   mkdir -p cache && sudo chown -R 10000:10000 cache
+      - ./cache:/cache
     environment:
       # Empty by default = direct. Set SKUNKY_PROXY in .env to route via the VPN.
       - HTTPS_PROXY=${SKUNKY_PROXY:-}