Set useOpenSsl in Flags lazily to reduce server startup time (line#2184)

Motivation:
Related line#1645
`useOpenSsl` in `Flags` is used only when TLS is enabled. However, it takes a lot to instantiate `OpenSsl` class so it adds up a tremendous time to the server startup time. We should fix to instantiate it only when it needs.

Modification:
- Set `useOpenSsl` in `Flags` lazily
- Check whether the OS is Linux before calling `Epoll.isAvailable()`
- Add `OsType`

Result:
- Reduced server startup time to 80 percent (20% decreased)

core/src/main/java/com/linecorp/armeria/common/Flags.java

@@ -45,6 +45,7 @@
 import com.linecorp.armeria.client.retry.RetryingRpcClient;
 import com.linecorp.armeria.common.util.Exceptions;
 import com.linecorp.armeria.common.util.Sampler;
+import com.linecorp.armeria.common.util.SystemInfo;
 import com.linecorp.armeria.internal.SslContextUtil;
 import com.linecorp.armeria.server.RoutingContext;
 import com.linecorp.armeria.server.ServerBuilder;
@@ -117,9 +118,8 @@ public final class Flags {
     private static final boolean HAS_WSLENV = System.getenv("WSLENV") != null;
     private static final boolean USE_EPOLL = getBoolean("useEpoll", isEpollAvailable(),
                                                         value -> isEpollAvailable() || !value);
-
-    private static final boolean USE_OPENSSL = getBoolean("useOpenSsl", OpenSsl.isAvailable(),
-                                                          value -> OpenSsl.isAvailable() || !value);
+    @Nullable
+    private static Boolean useOpenSsl;
 
     private static final boolean DUMP_OPENSSL_INFO = getBoolean("dumpOpenSslInfo", false);
 
@@ -312,34 +312,15 @@ public final class Flags {
         } else if (USE_EPOLL) {
             logger.info("Using /dev/epoll");
         }
-
-        if (!OpenSsl.isAvailable()) {
-            final Throwable cause = Exceptions.peel(OpenSsl.unavailabilityCause());
-            logger.info("OpenSSL not available: {}", cause.toString());
-        } else if (USE_OPENSSL) {
-            logger.info("Using OpenSSL: {}, 0x{}",
-                        OpenSsl.versionString(),
-                        Long.toHexString(OpenSsl.version() & 0xFFFFFFFFL));
-
-            if (dumpOpenSslInfo()) {
-                final SSLEngine engine = SslContextUtil.createSslContext(
-                        SslContextBuilder::forClient,
-                        false,
-                        unused -> {}).newEngine(ByteBufAllocator.DEFAULT);
-                logger.info("All available SSL protocols: {}",
-                            ImmutableList.copyOf(engine.getSupportedProtocols()));
-                logger.info("Default enabled SSL protocols: {}", SslContextUtil.DEFAULT_PROTOCOLS);
-                ReferenceCountUtil.release(engine);
-                logger.info("All available SSL ciphers: {}", OpenSsl.availableJavaCipherSuites());
-                logger.info("Default enabled SSL ciphers: {}", SslContextUtil.DEFAULT_CIPHERS);
-            }
-        }
     }
 
     private static boolean isEpollAvailable() {
-        // Netty epoll transport does not work with WSL (Windows Sybsystem for Linux) yet.
-        // TODO(trustin): Re-enable on WSL if https://github.com/Microsoft/WSL/issues/1982 is resolved.
-        return Epoll.isAvailable() && !HAS_WSLENV;
+        if (SystemInfo.isLinux()) {
+            // Netty epoll transport does not work with WSL (Windows Sybsystem for Linux) yet.
+            // TODO(trustin): Re-enable on WSL if https://github.com/Microsoft/WSL/issues/1982 is resolved.
+            return Epoll.isAvailable() && !HAS_WSLENV;
+        }
+        return false;
     }
 
     /**
@@ -426,7 +407,34 @@ public static boolean useEpoll() {
      * {@code -Dcom.linecorp.armeria.useOpenSsl=false} JVM option to disable it.
      */
     public static boolean useOpenSsl() {
-        return USE_OPENSSL;
+        if (useOpenSsl != null) {
+            return useOpenSsl;
+        }
+        final boolean useOpenSsl = getBoolean("useOpenSsl", true);
+        if (!useOpenSsl) {
+            // OpenSSL explicitly disabled
+            return Flags.useOpenSsl = false;
+        }
+        if (!OpenSsl.isAvailable()) {
+            final Throwable cause = Exceptions.peel(OpenSsl.unavailabilityCause());
+            logger.info("OpenSSL not available: {}", cause.toString());
+            return Flags.useOpenSsl = false;
+        }
+        logger.info("Using OpenSSL: {}, 0x{}", OpenSsl.versionString(),
+                    Long.toHexString(OpenSsl.version() & 0xFFFFFFFFL));
+        if (dumpOpenSslInfo()) {
+            final SSLEngine engine = SslContextUtil.createSslContext(
+                    SslContextBuilder::forClient,
+                    false,
+                    unused -> {}).newEngine(ByteBufAllocator.DEFAULT);
+            logger.info("All available SSL protocols: {}",
+                        ImmutableList.copyOf(engine.getSupportedProtocols()));
+            logger.info("Default enabled SSL protocols: {}", SslContextUtil.DEFAULT_PROTOCOLS);
+            ReferenceCountUtil.release(engine);
+            logger.info("All available SSL ciphers: {}", OpenSsl.availableJavaCipherSuites());
+            logger.info("Default enabled SSL ciphers: {}", SslContextUtil.DEFAULT_CIPHERS);
+        }
+        return Flags.useOpenSsl = true;
     }
 
     /**

core/src/main/java/com/linecorp/armeria/common/util/OsType.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2019 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you 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:
+ *
+ *   https://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.
+ */
+
+package com.linecorp.armeria.common.util;
+
+/**
+ * Operating system.
+ */
+public enum OsType {
+    WINDOWS,
+    LINUX,
+    MAC,
+    OTHERS
+}

core/src/main/java/com/linecorp/armeria/common/util/SystemInfo.java

@@ -51,6 +51,8 @@ public final class SystemInfo {
 
     private static boolean JETTY_ALPN_OPTIONAL_OR_AVAILABLE;
 
+    private static final OsType osType;
+
     static {
         int javaVersion = -1;
         try {
@@ -109,6 +111,17 @@ public final class SystemInfo {
                 JETTY_ALPN_OPTIONAL_OR_AVAILABLE = false;
             }
         }
+
+        final String osName = Ascii.toUpperCase(System.getProperty("os.name", ""));
+        if (osName.startsWith("WINDOWS")) {
+            osType = OsType.WINDOWS;
+        } else if (osName.startsWith("LINUX")) {
+            osType = OsType.LINUX;
+        } else if (osName.startsWith("MAC")) {
+            osType = OsType.MAC;
+        } else {
+            osType = OsType.OTHERS;
+        }
     }
 
     /**
@@ -154,8 +167,18 @@ public static long currentTimeMicros() {
         return JavaVersionSpecific.get().currentTimeMicros();
     }
 
-    private static boolean isLinux() {
-        return Ascii.toLowerCase(System.getProperty("os.name", "")).startsWith("linux");
+    /**
+     * Returns the operating system for the currently running process.
+     */
+    public static OsType osType() {
+        return osType;
+    }
+
+    /**
+     * Returns {@code true} if the operating system is Linux.
+     */
+    public static boolean isLinux() {
+        return osType == OsType.LINUX;
     }
 
     private SystemInfo() {}

core/src/main/java/com/linecorp/armeria/server/ServerBuilder.java

@@ -81,7 +81,6 @@
 import io.netty.channel.ChannelOption;
 import io.netty.channel.EventLoopGroup;
 import io.netty.channel.epoll.EpollChannelOption;
-import io.netty.handler.ssl.OpenSsl;
 import io.netty.handler.ssl.SslContext;
 import io.netty.handler.ssl.SslContextBuilder;
 import io.netty.util.DomainNameMapping;
@@ -1576,7 +1575,7 @@ public Server build() {
                 ports = ImmutableList.of(new ServerPort(0, HTTP));
             }
         } else {
-            if ((!OpenSsl.isAvailable() || !Flags.useOpenSsl()) && !SystemInfo.jettyAlpnOptionalOrAvailable()) {
+            if (!Flags.useOpenSsl() && !SystemInfo.jettyAlpnOptionalOrAvailable()) {
                 throw new IllegalStateException(
                         "TLS configured but this is Java 8 and neither OpenSSL nor Jetty ALPN could be " +
                         "detected. To use TLS with Armeria, you must either use Java 9+, enable OpenSSL, " +