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diff --git a/gfx/src/nsFontCache.h b/gfx/src/nsFontCache.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef _NS_FONTCACHE_H_
+#define _NS_FONTCACHE_H_
+
+#include <stdint.h>
+#include <sys/types.h>
+#include "mozilla/RefPtr.h"
+#include "nsCOMPtr.h"
+#include "nsFontMetrics.h"
+#include "nsIObserver.h"
+#include "nsISupports.h"
+#include "nsTArray.h"
+#include "nsThreadUtils.h"
+#include "prtime.h"
+
+class gfxUserFontSet;
+class nsAtom;
+class nsPresContext;
+struct nsFont;
+
+class nsFontCache final : public nsIObserver {
+ public:
+ nsFontCache() : mContext(nullptr) {}
+
+ NS_DECL_THREADSAFE_ISUPPORTS
+ NS_DECL_NSIOBSERVER
+
+ void Init(nsPresContext* aContext);
+ void Destroy();
+
+ already_AddRefed<nsFontMetrics> GetMetricsFor(
+ const nsFont& aFont, const nsFontMetrics::Params& aParams);
+
+ void FontMetricsDeleted(const nsFontMetrics* aFontMetrics);
+ void Compact();
+
+ // Flush aFlushCount oldest entries, or all if aFlushCount is negative
+ void Flush(int32_t aFlushCount = -1);
+
+ void UpdateUserFonts(gfxUserFontSet* aUserFontSet);
+
+ protected:
+ // If the array of cached entries is about to exceed this threshold,
+ // we'll discard the oldest ones so as to keep the size reasonable.
+ // In practice, the great majority of cache hits are among the last
+ // few entries; keeping thousands of older entries becomes counter-
+ // productive because it can then take too long to scan the cache.
+ static constexpr int32_t kMaxCacheEntries = 128;
+
+ // Number of cache misses before we assume that a font fingerprinting attempt
+ // is being made. Usually fingerprinters will lookup the same font-family
+ // three times, as "sans-serif", "serif" and "monospace".
+ static constexpr int32_t kFingerprintingCacheMissThreshold = 3 * 20;
+ // We assume that fingerprinters will lookup a large number of fonts in a
+ // short amount of time.
+ static constexpr PRTime kFingerprintingTimeout =
+ PRTime(PR_USEC_PER_SEC) * 3; // 3 seconds
+
+ static_assert(kFingerprintingCacheMissThreshold < kMaxCacheEntries);
+
+ ~nsFontCache() = default;
+
+ nsPresContext* mContext; // owner
+ RefPtr<nsAtom> mLocaleLanguage;
+
+ // We may not flush older entries immediately the array reaches
+ // kMaxCacheEntries length, because this usually happens on a stylo
+ // thread where we can't safely delete metrics objects. So we allocate an
+ // oversized autoarray buffer here, so that we're unlikely to overflow
+ // it and need separate heap allocation before the flush happens on the
+ // main thread.
+ AutoTArray<nsFontMetrics*, kMaxCacheEntries * 2> mFontMetrics;
+
+ bool mFlushPending = false;
+
+ class FlushFontMetricsTask : public mozilla::Runnable {
+ public:
+ explicit FlushFontMetricsTask(nsFontCache* aCache)
+ : mozilla::Runnable("FlushFontMetricsTask"), mCache(aCache) {}
+ NS_IMETHOD Run() override {
+ // Partially flush the cache, leaving the kMaxCacheEntries/2 most
+ // recent entries.
+ mCache->Flush(mCache->mFontMetrics.Length() - kMaxCacheEntries / 2);
+ mCache->mFlushPending = false;
+ return NS_OK;
+ }
+
+ private:
+ RefPtr<nsFontCache> mCache;
+ };
+
+ PRTime mLastCacheMiss = 0;
+ uint64_t mCacheMisses = 0;
+ bool mReportedProbableFingerprinting = false;
+};
+
+#endif /* _NS_FONTCACHE_H_ */