Android okhttp3 进行socket connect&poll的底层实现跟踪

由《Android okhttp3 创建Socket的底层实现追踪》可知，okhttp3在okhttp3.internal.io.RealConnection#connectSocket中通过rawSocket.setSoTimeout(readTimeout)调用java.net.Socket#checkOpenAndCreate创建了socket，底层是在/bionic/libc/arch-arm/syscalls/socket.S汇编代码中调用系统调用实现的，其中rawSocket是PlainSocketImpl.java的实例。

下面跟踪socket connect的底层实现
1.okhttp3.internal.io.RealConnection#connectSocket

try {      Platform.get().connectSocket(rawSocket, route.socketAddress(), connectTimeout);    } catch (ConnectException e) {      throw new ConnectException("Failed to connect to " + route.socketAddress());    }

2.
okhttp3.internal#connectSocket

public void connectSocket(Socket socket, InetSocketAddress address,      int connectTimeout) throws IOException {    socket.connect(address, connectTimeout);}

/libcore/luni/src/main/java/java/net/Socket.java#connect

    public void connect(SocketAddress remoteAddr, int timeout) throws IOException {        。。。。        synchronized (connectLock) {            try {                。。。。                impl.connect(remoteAddr, timeout);                isConnected = true;                cacheLocalAddress();            } catch (IOException e) {                impl.close();                throw e;            }        }    }

3./libcore/luni/src/main/java/java/net/PlainSocketImpl.java

protected void connect(SocketAddress remoteAddr, int timeout) throws IOException {        InetSocketAddress inetAddr = (InetSocketAddress) remoteAddr;        connect(inetAddr.getAddress(), inetAddr.getPort(), timeout);}private void connect(InetAddress anAddr, int aPort, int timeout) throws IOException {        InetAddress normalAddr = anAddr.isAnyLocalAddress() ? InetAddress.getLocalHost() : anAddr;        if (streaming && usingSocks()) {            socksConnect(anAddr, aPort, 0);        } else {            IoBridge.connect(fd, normalAddr, aPort, timeout);        }        super.address = normalAddr;        super.port = aPort;}

4./libcore/luni/src/main/java/libcore/io/IoBridge.java

    public static boolean connect(FileDescriptor fd, InetAddress inetAddress, int port, int timeoutMs) throws SocketException, SocketTimeoutException {        try {            return connectErrno(fd, inetAddress, port, timeoutMs);        } catch (ErrnoException errnoException) {            throw new ConnectException(connectDetail(inetAddress, port, timeoutMs, errnoException), errnoException);        } catch (SocketException ex) {            throw ex; // We don't want to doubly wrap these.        } catch (SocketTimeoutException ex) {            throw ex; // We don't want to doubly wrap these.        } catch (IOException ex) {            throw new SocketException(ex);        }    }

5./libcore/luni/src/main/java/libcore/io/IoBridge.java#connectErrno

    private static boolean connectErrno(FileDescriptor fd, InetAddress inetAddress, int port, int timeoutMs) throws ErrnoException, IOException {        // With no timeout, just call connect(2) directly.        if (timeoutMs == 0) {            Libcore.os.connect(fd, inetAddress, port);            return true;        }        // For connect with a timeout, we:        //   1. set the socket to non-blocking,        //   2. connect(2),        //   3. loop using poll(2) to decide whether we're connected, whether we should keep        //      waiting, or whether we've seen a permanent failure and should give up,        //   4. set the socket back to blocking.        // 1. set the socket to non-blocking.        IoUtils.setBlocking(fd, false);        // 2. call connect(2) non-blocking.        long finishTimeMs = System.currentTimeMillis() + timeoutMs;        try {            Libcore.os.connect(fd, inetAddress, port);            IoUtils.setBlocking(fd, true); // 4. set the socket back to blocking.            return true; // We connected immediately.        } catch (ErrnoException errnoException) {            if (errnoException.errno != EINPROGRESS) {                throw errnoException;            }            // EINPROGRESS means we should keep trying...        }        // 3. loop using poll(2).        int remainingTimeoutMs;        do {            remainingTimeoutMs = (int) (finishTimeMs - System.currentTimeMillis());            if (remainingTimeoutMs <= 0) {                throw new SocketTimeoutException(connectDetail(inetAddress, port, timeoutMs, null));            }        } while (!IoBridge.isConnected(fd, inetAddress, port, timeoutMs, remainingTimeoutMs));        IoUtils.setBlocking(fd, true); // 4. set the socket back to blocking.        return true; // Or we'd have thrown.    }

connectErrno实现了connect+poll的逻辑，注释已经说明的很清楚了。
有timeout的时候，进行非阻塞connect，然后用poll进行事件轮询直到timeout

6.
/libcore/luni/src/main/java/libcore/io/BlockGuardOs.java

public void connect(FileDescriptor fd, InetAddress address, int port) throws ErrnoException, SocketException {        BlockGuard.getThreadPolicy().onNetwork();        os.connect(fd, address, port);    }

/libcore/luni/src/main/java/libcore/io/ForwardingOs.java

public void connect(FileDescriptor fd, InetAddress address, int port) throws ErrnoException, SocketException { os.connect(fd, address, port); }

7./libcore/luni/src/main/java/libcore/io/Posix.java

public native void connect(FileDescriptor fd, InetAddress address, int port) throws ErrnoException, SocketException;

8./libcore/luni/src/main/native/libcore_io_Posix.cpp

static void Posix_connect(JNIEnv* env, jobject, jobject javaFd, jobject javaAddress, jint port) {    sockaddr_storage ss;    socklen_t sa_len;    if (!inetAddressToSockaddr(env, javaAddress, port, ss, sa_len)) {        return;    }    const sockaddr* sa = reinterpret_cast<const sockaddr*>(&ss);    // We don't need the return value because we'll already have thrown.    (void) NET_FAILURE_RETRY(env, int, connect, javaFd, sa, sa_len);}#define NET_FAILURE_RETRY(jni_env, return_type, syscall_name, java_fd, ...) ({ \    return_type _rc = -1; \    do { \        { \            int _fd = jniGetFDFromFileDescriptor(jni_env, java_fd); \            AsynchronousSocketCloseMonitor _monitor(_fd); \            _rc = syscall_name(_fd, __VA_ARGS__); \        } \        if (_rc == -1) { \            if (jniGetFDFromFileDescriptor(jni_env, java_fd) == -1) { \                jniThrowException(jni_env, "java/net/SocketException", "Socket closed"); \                break; \            } else if (errno != EINTR) { \                /* TODO: with a format string we could show the arguments too, like strace(1). */ \                throwErrnoException(jni_env, # syscall_name); \                break; \            } \        } \    } while (_rc == -1); \    _rc; })

9./bionic/libc/arch-arm/syscalls/connect.S

ENTRY(connect)    mov     ip, r7    ldr     r7, =__NR_connect    swi     #0    mov     r7, ip    cmn     r0, #(MAX_ERRNO + 1)    bxls    lr    neg     r0, r0    b       __set_errnoEND(connect)

可以看出，最终socket的connect也是在汇编代码中调用系统调用实现的。
接着看下poll的实现。

10./libcore/luni/src/main/java/libcore/io/IoBridge.java#isConnected

    public static boolean isConnected(FileDescriptor fd, InetAddress inetAddress, int port, int timeoutMs, int remainingTimeoutMs) throws IOException {        ErrnoException cause;        try {            StructPollfd[] pollFds = new StructPollfd[] { new StructPollfd() };            pollFds[0].fd = fd;            pollFds[0].events = (short) POLLOUT;            int rc = Libcore.os.poll(pollFds, remainingTimeoutMs);            if (rc == 0) {                return false; // Timeout.            }            int connectError = Libcore.os.getsockoptInt(fd, SOL_SOCKET, SO_ERROR);            if (connectError == 0) {                return true; // Success!            }            throw new ErrnoException("isConnected", connectError); // The connect(2) failed.        } catch (ErrnoException errnoException) {            if (!fd.valid()) {                throw new SocketException("Socket closed");            }            if (errnoException.errno == EINTR) {                return false; // Punt and ask the caller to try again.            } else {                cause = errnoException;            }        }        String detail = connectDetail(inetAddress, port, timeoutMs, cause);        if (cause.errno == ETIMEDOUT) {            throw new SocketTimeoutException(detail, cause);        }        throw new ConnectException(detail, cause);    }

通过poll监控fd可写（POLLOUT）这个事件

11.
/libcore/luni/src/main/java/libcore/io/BlockGuardOs.java

public int poll(StructPollfd[] fds, int timeoutMs) throws ErrnoException {        // Greater than 0 is a timeout in milliseconds and -1 means "block forever",        // but 0 means "poll and return immediately", which shouldn't be subject to BlockGuard.        if (timeoutMs != 0) {            BlockGuard.getThreadPolicy().onNetwork();        }        return os.poll(fds, timeoutMs);    }

/libcore/luni/src/main/java/libcore/io/ForwardingOs.java

public int poll(StructPollfd[] fds, int timeoutMs) throws ErrnoException { return os.poll(fds, timeoutMs); }

12./libcore/luni/src/main/java/libcore/io/Posix.java

public native int poll(StructPollfd[] fds, int timeoutMs) throws ErrnoException;

13./libcore/luni/src/main/native/libcore_io_Posix.cpp

static jint Posix_poll(JNIEnv* env, jobject, jobjectArray javaStructs, jint timeoutMs) {    static jfieldID fdFid = env->GetFieldID(JniConstants::structPollfdClass, "fd", "Ljava/io/FileDescriptor;");    static jfieldID eventsFid = env->GetFieldID(JniConstants::structPollfdClass, "events", "S");    static jfieldID reventsFid = env->GetFieldID(JniConstants::structPollfdClass, "revents", "S");    // Turn the Java libcore.io.StructPollfd[] into a C++ struct pollfd[].    size_t arrayLength = env->GetArrayLength(javaStructs);    UniquePtr<struct pollfd[]> fds(new struct pollfd[arrayLength]);    memset(fds.get(), 0, sizeof(struct pollfd) * arrayLength);    size_t count = 0; // Some trailing array elements may be irrelevant. (See below.)    for (size_t i = 0; i < arrayLength; ++i) {        ScopedLocalRef<jobject> javaStruct(env, env->GetObjectArrayElement(javaStructs, i));        if (javaStruct.get() == NULL) {            break; // We allow trailing nulls in the array for caller convenience.        }        ScopedLocalRef<jobject> javaFd(env, env->GetObjectField(javaStruct.get(), fdFid));        if (javaFd.get() == NULL) {            break; // We also allow callers to just clear the fd field (this is what Selector does).        }        fds[count].fd = jniGetFDFromFileDescriptor(env, javaFd.get());        fds[count].events = env->GetShortField(javaStruct.get(), eventsFid);        ++count;    }    // Since we don't know which fds -- if any -- are sockets, be conservative and register    // all fds for asynchronous socket close monitoring.    std::vector<AsynchronousSocketCloseMonitor*> monitors;    for (size_t i = 0; i < count; ++i) {        monitors.push_back(new AsynchronousSocketCloseMonitor(fds[i].fd));    }    int rc = poll(fds.get(), count, timeoutMs);    for (size_t i = 0; i < monitors.size(); ++i) {        delete monitors[i];    }    if (rc == -1) {        throwErrnoException(env, "poll");        return -1;    }    // Update the revents fields in the Java libcore.io.StructPollfd[].    for (size_t i = 0; i < count; ++i) {        ScopedLocalRef<jobject> javaStruct(env, env->GetObjectArrayElement(javaStructs, i));        if (javaStruct.get() == NULL) {            return -1;        }        env->SetShortField(javaStruct.get(), reventsFid, fds[i].revents);    }    return rc;}

14./bionic/libc/arch-arm/syscalls/poll.S

ENTRY(poll)    mov     ip, r7    ldr     r7, =__NR_poll    swi     #0    mov     r7, ip    cmn     r0, #(MAX_ERRNO + 1)    bxls    lr    neg     r0, r0    b       __set_errnoEND(poll)

跟connect一样，最后也是汇编实现系统调用