Partial fix for #12681 caching content (#12702)

This is a partial fix for #12681. It does not fix the actual release race on a single content, but by restricting the shrinking to a single thread, make the race less likely to occur.

There is a more comprehensive fix in #12678 and #12704 for 12.1,

jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/content/CachingHttpContentFactory.java

@@ -68,6 +68,7 @@ public class CachingHttpContentFactory implements HttpContent.Factory
     private final HttpContent.Factory _authority;
     private final ConcurrentHashMap<String, CachingHttpContent> _cache = new ConcurrentHashMap<>();
     private final AtomicLong _cachedSize = new AtomicLong();
+    private final AtomicBoolean _shrinking = new AtomicBoolean();
     private final ByteBufferPool _bufferPool;
     private int _maxCachedFileSize = DEFAULT_MAX_CACHED_FILE_SIZE;
     private int _maxCachedFiles = DEFAULT_MAX_CACHED_FILES;
@@ -148,35 +149,46 @@ public void setUseDirectByteBuffers(boolean useDirectByteBuffers)
 
     private void shrinkCache()
     {
-        // While we need to shrink
-        int numCacheEntries = _cache.size();
-        while (numCacheEntries > 0 && (numCacheEntries > _maxCachedFiles || _cachedSize.get() > _maxCacheSize))
+        // Only 1 thread shrinking at once
+        if (_shrinking.compareAndSet(false, true))
         {
-            // Scan the entire cache and generate an ordered list by last accessed time.
-            SortedSet<CachingHttpContent> sorted = new TreeSet<>((c1, c2) ->
+            try
             {
-                long delta = NanoTime.elapsed(c2.getLastAccessedNanos(), c1.getLastAccessedNanos());
-                if (delta != 0)
-                    return delta < 0 ? -1 : 1;
-
-                delta = c1.getContentLengthValue() - c2.getContentLengthValue();
-                if (delta != 0)
-                    return delta < 0 ? -1 : 1;
-
-                return c1.getKey().compareTo(c2.getKey());
-            });
-            sorted.addAll(_cache.values());
-
-            // TODO: Can we remove the buffers from the content before evicting.
-            // Invalidate least recently used first
-            for (CachingHttpContent content : sorted)
+                // While we need to shrink
+                int numCacheEntries = _cache.size();
+                while (numCacheEntries > 0 && (numCacheEntries > _maxCachedFiles || _cachedSize.get() > _maxCacheSize))
+                {
+                    // Scan the entire cache and generate an ordered list by last accessed time.
+                    SortedSet<CachingHttpContent> sorted = new TreeSet<>((c1, c2) ->
+                    {
+                        long delta = NanoTime.elapsed(c2.getLastAccessedNanos(), c1.getLastAccessedNanos());
+                        if (delta != 0)
+                            return delta < 0 ? -1 : 1;
+
+                        delta = c1.getContentLengthValue() - c2.getContentLengthValue();
+                        if (delta != 0)
+                            return delta < 0 ? -1 : 1;
+
+                        return c1.getKey().compareTo(c2.getKey());
+                    });
+                    sorted.addAll(_cache.values());
+
+                    // TODO: Can we remove the buffers from the content before evicting.
+                    // Invalidate least recently used first
+                    for (CachingHttpContent content : sorted)
+                    {
+                        if (_cache.size() <= _maxCachedFiles && _cachedSize.get() <= _maxCacheSize)
+                            break;
+                        removeFromCache(content);
+                    }
+
+                    numCacheEntries = _cache.size();
+                }
+            }
+            finally
             {
-                if (_cache.size() <= _maxCachedFiles && _cachedSize.get() <= _maxCacheSize)
-                    break;
-                removeFromCache(content);
+                _shrinking.set(false);
             }
-
-            numCacheEntries = _cache.size();
         }
     }