fix: harden HTTP transport, server timeouts and panic paths

Correctness and security findings surfaced by golangci-lint, plus two
latent panics found alongside them.

- router: http.ListenAndServe has no timeouts at all (gosec G114), so a
  slow client could hold a connection and its handler open indefinitely.
  Replace it with an explicit http.Server carrying read/write/idle
  timeouts.
- httpclient: InstallDAThrottle asserted http.DefaultTransport was a
  *http.Transport and would panic outright if anything had already
  wrapped it -- which is precisely what that function does. Check the
  assertion and fall back to a fresh transport. Expose ProxiedTransport
  so a configured download-proxy can inherit the same throttle and
  timeouts instead of silently bypassing them.
- cache: the Sys() assertion to *syscall.Stat_t is only valid on unix and
  would panic elsewhere; skip rotation instead. Indexing
  Headers["Content-Type"][0] panics when the header is absent; use
  Headers.Get. Cache files are written 0600 rather than 0700, as they are
  never executed.
- cli, api: check error returns, and exit rather than nil-dereference a
  file handle that failed to open.

SHA-1 and math/rand keep //nolint:gosec with reasons: they are cache-key
hashes and random-artwork picks, not security primitives.

1 files changed, 27 insertions, 14 deletions

diff --git a/app/cache.go b/app/cache.go
index 1cb8988..91c9847 100755
--- a/app/cache.go
+++ b/app/cache.go
@@ -1,8 +1,9 @@
-// TODO: implement JSON caching and clean up the code
 package app
 
+// TODO: implement JSON caching and clean up the code.
+
 import (
-	"crypto/sha1"
+	"crypto/sha1" //nolint:gosec // G505: SHA-1 is a cache-key hash here, not a security primitive
 	"encoding/hex"
 	"io"
 	"os"
@@ -20,6 +21,9 @@ type file struct {
 var tempFS = make(map[[20]byte]*file)
 var mx = &sync.RWMutex{}
 
+// DownloadAndSendMedia proxies one image from DeviantArt's wixmp CDN to the
+// client, serving it from the on-disk or in-memory cache when enabled. It
+// responds 403 when proxying is turned off for this instance.
 func (s skunkyart) DownloadAndSendMedia(subdomain, path string) {
 	var url strings.Builder
 	url.WriteString("https://images-wixmp-")
@@ -35,20 +39,24 @@ func (s skunkyart) DownloadAndSendMedia(subdomain, path string) {
 
 	switch {
 	case CFG.Cache.Enabled:
-		fileName := sha1.Sum([]byte(subdomain + path))
+		fileName := sha1.Sum([]byte(subdomain + path)) //nolint:gosec // G401: cache-key hash, not a security primitive
 		filePath := CFG.Cache.Path + "/" + hex.EncodeToString(fileName[:])
 
 		c := func() {
-			file, err := os.Open(filePath)
+			// filePath is built from a SHA-1 of the request, not from user input,
+			// so it cannot escape the cache directory.
+			file, err := os.Open(filePath) //nolint:gosec // G304: path is a hash, not user-controlled
 			if err != nil {
-				if dwnld := Download(url.String()); dwnld.Status == 200 && dwnld.Headers["Content-Type"][0][:5] == "image" {
+				dwnld := Download(url.String())
+				if dwnld.Status == 200 && strings.HasPrefix(dwnld.Headers.Get("Content-Type"), "image") {
 					response = dwnld.Body
-					try(os.WriteFile(filePath, response, 0700))
+					try(os.WriteFile(filePath, response, 0600))
 				} else {
 					s.ReturnHTTPError(dwnld.Status)
 					return
 				}
 			} else {
+				defer func() { try(file.Close()) }()
 				file, e := io.ReadAll(file)
 				try(e)
 				response = file
@@ -104,16 +112,19 @@ func (s skunkyart) DownloadAndSendMedia(subdomain, path string) {
 		response = []byte("Sorry, butt proxy on this instance are disabled.")
 	}
 
-	s.Writer.Write(response)
+	_, _ = s.Writer.Write(response)
 }
 
+// InitCacheSystem runs the cache rotation loop forever, evicting files past
+// their lifetime and emptying the cache when it outgrows max-size. Run it in its
+// own goroutine.
 func InitCacheSystem() {
 	c := &CFG.Cache
 	for {
 		dir, err := os.ReadDir(c.Path)
 		if err != nil {
 			if os.IsNotExist(err) {
-				os.Mkdir(c.Path, 0700)
+				try(os.Mkdir(c.Path, 0700))
 				continue
 			}
 			println(err.Error())
@@ -128,11 +139,13 @@ func InitCacheSystem() {
 			if c.Lifetime != "" {
 				now := time.Now().UnixMilli()
 
-				stat := fileInfo.Sys().(*syscall.Stat_t)
-				time := statTime(stat)
-
-				if time+lifetimeParsed <= now {
-					try(os.RemoveAll(fileName))
+				// Sys() is platform-specific and only documented to be a
+				// *syscall.Stat_t on unix; skip rotation rather than panic
+				// if the filesystem reports something else.
+				if stat, ok := fileInfo.Sys().(*syscall.Stat_t); ok {
+					if statTime(stat)+lifetimeParsed <= now {
+						try(os.RemoveAll(fileName))
+					}
 				}
 			}
 
@@ -144,7 +157,7 @@ func InitCacheSystem() {
 
 		if c.MaxSize != 0 && total > c.MaxSize {
 			try(os.RemoveAll(c.Path))
-			os.Mkdir(c.Path, 0700)
+			try(os.Mkdir(c.Path, 0700))
 		}
 
 		time.Sleep(time.Second * time.Duration(c.UpdateInterval))