router: implement ETags/If-None-Match based on build time

Essentially caching based on when the program was built.

static.go

@@ -4,6 +4,8 @@ import (
 	"io/fs"
 	"net/http"
 	"strings"
+
+	"github.com/gin-gonic/gin"
 )
 
 // Since the gin-static middleware uses a version of http.Filesystem with an extra Exists() func, we extend it here.
@@ -30,3 +32,30 @@ func (f httpFS) Exists(prefix string, filepath string) bool {
 	}
 	return false
 }
+
+var (
+	etag = buildTimeUnix
+)
+
+// Use unix build time as the ETag for a request, allowing caching of static files.
+// Copied from gin-contrib/static:
+// https://github.com/gin-gonic/contrib/blob/2b1292699c15c6bc6ee8f0e801a4d0b4e807f366/static/static.go
+func serveTaggedStatic(urlPrefix string, fs httpFS) gin.HandlerFunc {
+	fileserver := http.FileServer(fs)
+	if urlPrefix != "" {
+		fileserver = http.StripPrefix(urlPrefix, fileserver)
+	}
+	return func(gc *gin.Context) {
+		if fs.Exists(urlPrefix, gc.Request.URL.Path) {
+			gc.Header("Cache-Control", "no-cache")
+			gc.Header("ETag", buildTimeUnix)
+			ifNoneMatchTag := gc.Request.Header.Get("If-None-Match")
+			if ifNoneMatchTag != "" && ifNoneMatchTag == etag && (gc.Request.Method == http.MethodGet || gc.Request.Method == http.MethodHead) {
+				gc.AbortWithStatus(http.StatusNotModified)
+			} else {
+				fileserver.ServeHTTP(gc.Writer, gc.Request)
+				gc.Abort()
+			}
+		}
+	}
+}