1 files changed, 71 insertions, 0 deletions
diff --git a/services/surfaceflinger/DisplayIdGenerator.h b/services/surfaceflinger/DisplayIdGenerator.h
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+++ b/services/surfaceflinger/DisplayIdGenerator.h
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+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <ui/DisplayId.h>
+
+#include <limits>
+#include <optional>
+#include <random>
+#include <unordered_set>
+
+#include <log/log.h>
+
+namespace android {
+
+// Generates pseudo-random IDs of type GpuVirtualDisplayId or HalVirtualDisplayId.
+template <typename Id>
+class DisplayIdGenerator {
+public:
+    explicit DisplayIdGenerator(size_t maxIdsCount = std::numeric_limits<size_t>::max())
+          : mMaxIdsCount(maxIdsCount) {}
+
+    bool inUse() const { return !mUsedIds.empty(); }
+
+    std::optional<Id> generateId() {
+        if (mUsedIds.size() >= mMaxIdsCount) {
+            return std::nullopt;
+        }
+
+        constexpr int kMaxAttempts = 1000;
+
+        for (int attempts = 0; attempts < kMaxAttempts; attempts++) {
+            const Id id{mDistribution(mGenerator)};
+            if (mUsedIds.count(id) == 0) {
+                mUsedIds.insert(id);
+                return id;
+            }
+        }
+
+        LOG_ALWAYS_FATAL("Couldn't generate ID after %d attempts", kMaxAttempts);
+    }
+
+    void releaseId(Id id) { mUsedIds.erase(id); }
+
+private:
+    const size_t mMaxIdsCount;
+
+    std::unordered_set<Id> mUsedIds;
+
+    // Pseudo-random with random seed, in contrast to physical display IDs, which are stable
+    // across reboots. The only ISurfaceComposer exposure for these IDs is a restricted API
+    // for screencap, so there is little benefit in making them unpredictable.
+    std::default_random_engine mGenerator{std::random_device()()};
+    std::uniform_int_distribution<typename Id::BaseId> mDistribution;
+};
+
+} // namespace android