(crawler) Emulate if-modified-since for domains that don't support the header

This will help reduce the strain on some server software, in particular Discourse.

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

@@ -42,6 +42,7 @@ public class HttpFetcherImpl implements HttpFetcher {
 
     private static final ContentTypeLogic contentTypeLogic = new ContentTypeLogic();
     private final ContentTypeProber contentTypeProber;
+    private final SoftIfModifiedSinceProber softIfModifiedSinceProber;
 
     @Override
     public void setAllowAllContentTypes(boolean allowAllContentTypes) {
@@ -93,6 +94,7 @@ public class HttpFetcherImpl implements HttpFetcher {
         this.userAgentString = userAgent.uaString();
         this.userAgentIdentifier = userAgent.uaIdentifier();
         this.contentTypeProber = new ContentTypeProber(userAgentString, client);
+        this.softIfModifiedSinceProber = new SoftIfModifiedSinceProber(userAgentString, client);
     }
 
     public HttpFetcherImpl(String userAgent) {
@@ -100,6 +102,7 @@ public class HttpFetcherImpl implements HttpFetcher {
         this.userAgentString = userAgent;
         this.userAgentIdentifier = userAgent;
         this.contentTypeProber = new ContentTypeProber(userAgent, client);
+        this.softIfModifiedSinceProber = new SoftIfModifiedSinceProber(userAgent, client);
     }
 
     /**
@@ -166,6 +169,13 @@ public class HttpFetcherImpl implements HttpFetcher {
                 return new HttpFetchResult.ResultNone();
             }
         }
+        else {
+            // Possibly do a soft probe to see if the URL has been modified since the last time we crawled it
+            // if we have reason to suspect ETags are not supported by the server.
+            if (softIfModifiedSinceProber.probeModificationTime(url, contentTags)) {
+                return new HttpFetchResult.Result304Raw();
+            }
+        }
 
         var getBuilder = new Request.Builder().get();
 

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

@@ -0,0 +1,49 @@
+package nu.marginalia.crawl.retreival.fetcher;
+
+import com.google.common.base.Strings;
+import nu.marginalia.model.EdgeUrl;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+
+import java.io.IOException;
+import java.util.Objects;
+
+public class SoftIfModifiedSinceProber {
+
+    private final String userAgentString;
+    private final OkHttpClient client;
+
+    public SoftIfModifiedSinceProber(String userAgentString, OkHttpClient httpClient) {
+        this.userAgentString = userAgentString;
+        this.client = httpClient;
+    }
+
+    /** Implement a soft probe of the last modified time of the given URL with a HEAD request.
+     * This is used to detect if the URL has been modified since the last time we crawled it.
+     */
+    public boolean probeModificationTime(EdgeUrl url, ContentTags tags) throws IOException  {
+        var headBuilder = new Request.Builder().head()
+                .addHeader("User-agent", userAgentString)
+                .addHeader("Accept-Encoding", "gzip")
+                .url(url.toString());
+
+        // This logic is only applicable if we only have a last-modified time, but no ETag.
+        if (Strings.isNullOrEmpty(tags.lastMod()))
+            return false;
+        if (!Strings.isNullOrEmpty(tags.etag()))
+            return false;
+
+        var head = headBuilder.build();
+        var call = client.newCall(head);
+
+        try (var rsp = call.execute()) {
+            if (rsp.code() != 200) {
+                return false;
+            }
+
+            var contentTypeHeader = rsp.header("Last-Modified");
+            return Objects.equals(contentTypeHeader, tags.lastMod());
+        }
+    }
+
+}