(crawler) Strip W/-prefix from the etag when supplied as If-None-Match

code/processes/crawling-process/java/nu/marginalia/crawl/retreival/fetcher/ContentTags.java

@@ -18,7 +18,38 @@ public record ContentTags(String etag, String lastMod) {
 
     /** Paints the tags onto the request builder. */
     public void paint(Request.Builder getBuilder) {
-        if (etag != null) getBuilder.addHeader("If-None-Match", etag);
+
-        if (lastMod != null) getBuilder.addHeader("If-Modified-Since", lastMod);
+        System.out.println(ifNoneMatch() + " " + ifModifiedSince());
+
+        if (etag != null) {
+            getBuilder.addHeader("If-None-Match", ifNoneMatch());
+        }
+
+        if (lastMod != null) {
+            getBuilder.addHeader("If-Modified-Since", ifModifiedSince());
+        }
+    }
+
+    private String ifNoneMatch() {
+        // Remove the W/ prefix if it exists
+
+        //'W/' (case-sensitive) indicates that a weak validator is used. Weak etags are
+        // easy to generate, but are far less useful for comparisons. Strong validators
+        // are ideal for comparisons but can be very difficult to generate efficiently.
+        // Weak ETag values of two representations of the same resources might be semantically
+        // equivalent, but not byte-for-byte identical. This means weak etags prevent caching
+        // when byte range requests are used, but strong etags mean range requests can
+        // still be cached.
+        // - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag
+
+        if (null != etag && etag.startsWith("W/")) {
+            return etag.substring(2);
+        } else {
+            return etag;
+        }
+    }
+
+    private String ifModifiedSince() {
+        return lastMod;
     }
 }