feat: Revalidate HTTP cache entries once per minute maximum

This is to avoid revalidating HTTP cache too frequently (and have many parallel revalidation tasks)
if revalidation fails or the HTTP request takes some time. The stale period >= 1 hour, so 1 more
minute won't be a problem.

src/net/http.rs

@@ -151,7 +151,7 @@ async fn http_cache_put(context: &Context, url: &str, response: &Response) -> Re
 /// Also returns if the response is stale and should be revalidated in the background.
 async fn http_cache_get(context: &Context, url: &str) -> Result<Option<(Response, bool)>> {
     let now = time();
-    let Some((blob_name, mimetype, encoding, is_stale)) = context
+    let Some((blob_name, mimetype, encoding, stale_timestamp)) = context
         .sql
         .query_row_optional(
             "SELECT blobname, mimetype, encoding, stale
@@ -162,13 +162,14 @@ async fn http_cache_get(context: &Context, url: &str) -> Result<Option<(Response
                 let mimetype: Option<String> = Some(row.get(1)?).filter(|s: &String| !s.is_empty());
                 let encoding: Option<String> = Some(row.get(2)?).filter(|s: &String| !s.is_empty());
                 let stale_timestamp: i64 = row.get(3)?;
-                Ok((blob_name, mimetype, encoding, now > stale_timestamp))
+                Ok((blob_name, mimetype, encoding, stale_timestamp))
             },
         )
         .await?
     else {
         return Ok(None);
     };
+    let is_stale = now > stale_timestamp;
 
     let blob_object = BlobObject::from_name(context, blob_name)?;
     let blob_abs_path = blob_object.to_abs_path();
@@ -195,15 +196,16 @@ async fn http_cache_get(context: &Context, url: &str) -> Result<Option<(Response
     // Update expiration timestamp
     // to prevent deletion of the file still in use.
     //
-    // We do not update stale timestamp here
-    // as we have not revalidated the response.
-    // Stale timestamp is updated only
-    // when the URL is sucessfully fetched.
+    // If the response is stale, the caller should revalidate it in the background, so update
+    // `stale` timestamp to avoid revalidating too frequently (and have many parallel revalidation
+    // tasks) if revalidation fails or the HTTP request takes some time. The stale period >= 1 hour,
+    // so 1 more minute won't be a problem.
+    let stale_timestamp = if is_stale { now + 60 } else { stale_timestamp };
     context
         .sql
         .execute(
-            "UPDATE http_cache SET expires=? WHERE url=?",
-            (expires, url),
+            "UPDATE http_cache SET expires=?, stale=? WHERE url=?",
+            (expires, stale_timestamp, url),
         )
         .await?;
 
@@ -305,8 +307,6 @@ pub async fn read_url_blob(context: &Context, url: &str) -> Result<Response> {
                 }
             });
         }
-
-        // Return stale result.
         return Ok(response);
     }
 
@@ -495,6 +495,22 @@ mod tests {
         );
         assert_eq!(http_cache_get(t, xdc_pixel_url).await?, None);
 
+        // If we get the blob the second time quickly, it shouldn't be stale because it's supposed
+        // that we've already run a revalidation task which will update the blob soon.
+        assert_eq!(
+            http_cache_get(t, xdc_editor_url).await?,
+            Some((xdc_response.clone(), false))
+        );
+        // But if the revalidation task hasn't succeeded after some time, the blob is stale again
+        // even if we continue to get it frequently.
+        for i in (0..100).rev() {
+            SystemTime::shift(Duration::from_secs(6));
+            if let Some((_, true)) = http_cache_get(t, xdc_editor_url).await? {
+                break;
+            }
+            assert!(i > 0);
+        }
+
         // Test that if the file is accidentally removed from the blobdir,
         // there is no error when trying to load the cache entry.
         for entry in std::fs::read_dir(t.get_blobdir())? {