Fix solution in Link Checker in Concurrency Morning exercises (#904)

* Fix solution in Link Checker in Concurrency Morning exercises.

This change fixes the following issues with the current solution:

1. It is not listed on the "solutions" page.
2. It is not multi-threaded and does not use channels.

---------

Co-authored-by: Dustin J. Mitchell <djmitche@google.com>

src/exercises/concurrency/link-checker.md

@@ -57,12 +57,13 @@ Your `src/main.rs` file should look something like this:
 ```rust,compile_fail
 {{#include link-checker.rs:setup}}
 
-{{#include link-checker.rs:extract_links}}
+{{#include link-checker.rs:visit_page}}
 
 fn main() {
+    let client = Client::new();
     let start_url = Url::parse("https://www.google.org").unwrap();
-    let response = get(start_url).unwrap();
-    match extract_links(response) {
+    let crawl_command = CrawlCommand{ url: start_url, extract_links: true };
+    match visit_page(&client, &crawl_command) {
         Ok(links) => println!("Links: {links:#?}"),
         Err(err) => println!("Could not extract links: {err:#}"),
     }

src/exercises/concurrency/link-checker.rs

@@ -12,9 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::{sync::Arc, sync::Mutex, sync::mpsc, thread};
+
 // ANCHOR: setup
-use reqwest::blocking::{get, Response};
-use reqwest::Url;
+use reqwest::{blocking::Client, Url};
 use scraper::{Html, Selector};
 use thiserror::Error;
 
@@ -22,68 +23,159 @@ use thiserror::Error;
 enum Error {
     #[error("request error: {0}")]
     ReqwestError(#[from] reqwest::Error),
+    #[error("bad http response: {0}")]
+    BadResponse(String),
 }
 // ANCHOR_END: setup
 
-// ANCHOR: extract_links
-fn extract_links(response: Response) -> Result<Vec<Url>, Error> {
-    let base_url = response.url().to_owned();
-    let document = response.text()?;
-    let html = Html::parse_document(&document);
-    let selector = Selector::parse("a").unwrap();
+// ANCHOR: visit_page
+#[derive(Debug)]
+struct CrawlCommand {
+    url: Url,
+    extract_links: bool,
+}
 
-    let mut valid_urls = Vec::new();
-    for element in html.select(&selector) {
-        if let Some(href) = element.value().attr("href") {
-            match base_url.join(href) {
-                Ok(url) => valid_urls.push(url),
-                Err(err) => {
-                    println!("On {base_url}: could not parse {href:?}: {err} (ignored)",);
+fn visit_page(client: &Client, command: &CrawlCommand) -> Result<Vec<Url>, Error> {
+    println!("Checking {:#}", command.url);
+    let response = client.get(command.url.clone()).send()?;
+    if !response.status().is_success() {
+        return Err(Error::BadResponse(response.status().to_string()));
+    }
+
+    let mut link_urls = Vec::new();
+    if !command.extract_links {
+        return Ok(link_urls);
+    }
+
+    let base_url = response.url().to_owned();
+    let body_text = response.text()?;
+    let document = Html::parse_document(&body_text);
+
+    let selector = Selector::parse("a").unwrap();
+    let href_values = document
+        .select(&selector)
+        .filter_map(|element| element.value().attr("href"));
+    for href in href_values {
+        match base_url.join(href) {
+            Ok(link_url) => {
+                link_urls.push(link_url);
+            }
+            Err(err) => {
+                println!("On {base_url:#}: ignored unparsable {href:?}: {err}");
+            }
+        }
+    }
+    Ok(link_urls)
+}
+// ANCHOR_END: visit_page
+
+struct CrawlState {
+    domain: String,
+    visited_pages: std::collections::HashSet<String>,
+}
+
+impl CrawlState {
+    fn new(start_url: &Url) -> CrawlState {
+        let mut visited_pages = std::collections::HashSet::new();
+        visited_pages.insert(start_url.as_str().to_string());
+        CrawlState {
+            domain: start_url.domain().unwrap().to_string(),
+            visited_pages,
+        }
+    }
+
+    /// Determine whether links within the given page should be extracted.
+    fn should_extract_links(&self, url: &Url) -> bool {
+        let Some(url_domain) = url.domain() else {
+            return false;
+        };
+        url_domain == self.domain
+    }
+
+    /// Mark the given page as visited, returning true if it had already
+    /// been visited.
+    fn mark_visited(&mut self, url: &Url) -> bool {
+        self.visited_pages.insert(url.as_str().to_string())
+    }
+}
+
+type CrawlResult = Result<Vec<Url>, (Url, Error)>;
+fn spawn_crawler_threads(
+    command_receiver: mpsc::Receiver<CrawlCommand>,
+    result_sender: mpsc::Sender<CrawlResult>,
+    thread_count: u32,
+) {
+    let command_receiver = Arc::new(Mutex::new(command_receiver));
+
+    for _ in 0..thread_count {
+        let result_sender = result_sender.clone();
+        let command_receiver = command_receiver.clone();
+        thread::spawn(move || {
+            let client = Client::new();
+            loop {
+                let command_result = {
+                    let receiver_guard = command_receiver.lock().unwrap();
+                    receiver_guard.recv()
+                };
+                let Ok(crawl_command) = command_result else {
+                    // The sender got dropped. No more commands coming in.
+                    break;
+                };
+                let crawl_result = match visit_page(&client, &crawl_command) {
+                    Ok(link_urls) => Ok(link_urls),
+                    Err(error) => Err((crawl_command.url, error)),
+                };
+                result_sender.send(crawl_result).unwrap();
+            }
+        });
+    }
+}
+
+fn control_crawl(
+    start_url: Url,
+    command_sender: mpsc::Sender<CrawlCommand>,
+    result_receiver: mpsc::Receiver<CrawlResult>,
+) -> Vec<Url> {
+    let mut crawl_state = CrawlState::new(&start_url);
+    let start_command = CrawlCommand { url: start_url, extract_links: true };
+    command_sender.send(start_command).unwrap();
+    let mut pending_urls = 1;
+
+    let mut bad_urls = Vec::new();
+    while pending_urls > 0 {
+        let crawl_result = result_receiver.recv().unwrap();
+        pending_urls -= 1;
+
+        match crawl_result {
+            Ok(link_urls) => {
+                for url in link_urls {
+                    if crawl_state.mark_visited(&url) {
+                        let extract_links = crawl_state.should_extract_links(&url);
+                        let crawl_command = CrawlCommand { url, extract_links };
+                        command_sender.send(crawl_command).unwrap();
+                        pending_urls += 1;
+                    }
                 }
             }
-        }
-    }
-
-    Ok(valid_urls)
-}
-// ANCHOR_END: extract_links
-
-fn check_links(url: Url) -> Result<Vec<Url>, Error> {
-    println!("Checking {url}");
-
-    let response = get(url.to_owned())?;
-
-    if !response.status().is_success() {
-        return Ok(vec![url.to_owned()]);
-    }
-
-    let links = extract_links(response)?;
-    for link in &links {
-        println!("{link}, {:?}", link.domain());
-    }
-
-    let mut failed_links = Vec::new();
-    for link in links {
-        if link.domain() != url.domain() {
-            println!("Checking external link: {link}");
-            let response = get(link.clone())?;
-            if !response.status().is_success() {
-                println!("Error on {url}: {link} failed: {}", response.status());
-                failed_links.push(link);
+            Err((url, error)) => {
+                bad_urls.push(url);
+                println!("Got crawling error: {:#}", error);
+                continue;
             }
-        } else {
-            println!("Checking link in same domain: {link}");
-            failed_links.extend(check_links(link)?)
         }
     }
+    bad_urls
+}
 
-    Ok(failed_links)
+fn check_links(start_url: Url) -> Vec<Url> {
+    let (result_sender, result_receiver) = mpsc::channel::<CrawlResult>();
+    let (command_sender, command_receiver) = mpsc::channel::<CrawlCommand>();
+    spawn_crawler_threads(command_receiver, result_sender, 16);
+    control_crawl(start_url, command_sender, result_receiver)
 }
 
 fn main() {
-    let start_url = Url::parse("https://www.google.org").unwrap();
-    match check_links(start_url) {
-        Ok(links) => println!("Links: {links:#?}"),
-        Err(err) => println!("Could not extract links: {err:#}"),
-    }
+    let start_url = reqwest::Url::parse("https://www.google.org").unwrap();
+    let bad_urls = check_links(start_url);
+    println!("Bad URLs: {:#?}", bad_urls);
 }

src/exercises/concurrency/solutions-morning.md

@@ -8,3 +8,10 @@
 {{#include dining-philosophers.rs}}
 ```
 
+## Link Checker
+
+([back to exercise](link-checker.md))
+
+```rust,compile_fail
+{{#include link-checker.rs}}
+```