Support preadv/pwritev for multi-thread file IO.

docs/en/tutorial-09-http_file_server.md

@@ -199,5 +199,4 @@ Linux operating system supports a set of asynchronous IO system calls with high
 We have implemented a set of posix aio interfaces to support other UNIX systems, and used the sigevent notification method of threads, but it is no longer in use because of its low efficiency.   
 Currently, for non-Linux systems, asynchronous IO is always simulated by multi-threading. When an IO task arrives, a thread is created in real time to execute IO tasks, and then a callback is used to return to the handler thread pool.   
 Multi-threaded IO is also the only choice in macOS, because macOS does not have good sigevent support and posix aio will not work in macOS.   
-Multi-threaded IO does not support preadv and pwritev tasks. When these two tasks are created and run, you will get an ENOSYS error in the callback.   
 Some UNIX systems do not support fdatasync. In this case, an fdsync task is equivalent to an fsync task.

docs/tutorial-09-http_file_server.md

@@ -195,6 +195,5 @@ Linux操作系统支持一套效率很高，CPU占用非常少的异步IO系统
 我们曾经实现过一套posix aio接口用于支持其它UNIX系统，并使用线程的sigevent通知方式，但由于其效率太低，已经不再使用了。  
 目前，对于非Linux系统，异步IO一律是用多线程实现，在IO任务到达时，实时创建线程执行IO任务，callback回到handler线程池。  
 多线程IO也是macOS下的唯一选择，因为macOS没有良好的sigevent支持，posix aio行不通。  
-多线程IO不支持preadv和pwritev两种任务，创建并运行这两种任务，会在callback里得到一个ENOSYS错误。  
 某些UNIX系统不支持fdatasync调用，这种情况下，fdsync任务将等价于fsync任务。
 

src/kernel/IOService_thread.cc

@@ -18,6 +18,7 @@
 
 #include <sys/uio.h>
 #include <errno.h>
+#include <dlfcn.h>
 #include <unistd.h>
 #include <pthread.h>
 #include "list.h"
@@ -87,6 +88,7 @@ void IOSession::prep_fdsync(int fd)
 
 int IOService::init(int maxevents)
 {
+	void *p;
 	int ret;
 
 	if (maxevents <= 0)
@@ -96,18 +98,30 @@ int IOService::init(int maxevents)
 	}
 
 	ret = pthread_mutex_init(&this->mutex, NULL);
-	if (ret == 0)
+	if (ret)
 	{
-		this->maxevents = maxevents;
-		this->nevents = 0;
-		INIT_LIST_HEAD(&this->session_list);
-		this->pipe_fd[0] = -1;
-		this->pipe_fd[1] = -1;
-		return 0;
+		errno = ret;
+		return -1;
 	}
 
-	errno = ret;
-	return -1;
+	p = dlsym(RTLD_DEFAULT, "preadv");
+	if (p)
+		this->preadv = (ssize_t (*)(int, const struct iovec *, int, off_t))p;
+	else
+		this->preadv = IOService::preadv_emul;
+
+	p = dlsym(RTLD_DEFAULT, "pwritev");
+	if (p)
+		this->pwritev = (ssize_t (*)(int, const struct iovec *, int, off_t))p;
+	else
+		this->pwritev = IOService::pwritev_emul;
+
+	this->maxevents = maxevents;
+	this->nevents = 0;
+	INIT_LIST_HEAD(&this->session_list);
+	this->pipe_fd[0] = -1;
+	this->pipe_fd[1] = -1;
+	return 0;
 }
 
 void IOService::deinit()
@@ -216,9 +230,12 @@ void *IOService::io_routine(void *arg)
 		ret = fdatasync(fd);
 		break;
 	case IO_CMD_PREADV:
+		ret = service->preadv(fd, (const struct iovec *)session->buf,
+							  session->count, session->offset);
+		break;
 	case IO_CMD_PWRITEV:
-		errno = ENOSYS;
-		ret = -1;
+		ret = service->pwritev(fd, (const struct iovec *)session->buf,
+							   session->count, session->offset);
 		break;
 	default:
 		errno = EINVAL;
@@ -249,3 +266,49 @@ void *IOService::aio_finish(void *ptr, void *context)
 	return session;
 }
 
+ssize_t IOService::preadv_emul(int fd, const struct iovec *iov, int iovcnt,
+							   off_t offset)
+{
+	size_t total = 0;
+	ssize_t n;
+	int i;
+
+	for (i = 0; i < iovcnt; i++)
+	{
+		n = pread(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		if (n < 0)
+			return total == 0 ? -1 : total;
+
+		total += n;
+		if ((size_t)n < iov[i].iov_len)
+			return total;
+
+		offset += n;
+	}
+
+	return total;
+}
+
+ssize_t IOService::pwritev_emul(int fd, const struct iovec *iov, int iovcnt,
+								off_t offset)
+{
+	size_t total = 0;
+	ssize_t n;
+	int i;
+
+	for (i = 0; i < iovcnt; i++)
+	{
+		n = pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		if (n < 0)
+			return total == 0 ? -1 : total;
+
+		total += n;
+		if ((size_t)n < iov[i].iov_len)
+			return total;
+
+		offset += n;
+	}
+
+	return total;
+}
+

src/kernel/IOService_thread.h

@@ -105,6 +105,14 @@ private:
 	static void *io_routine(void *arg);
 	static void *aio_finish(void *ptr, void *context);
 
+private:
+	static ssize_t preadv_emul(int fd, const struct iovec *iov, int iovcnt,
+							   off_t offset);
+	static ssize_t pwritev_emul(int fd, const struct iovec *iov, int iovcnt,
+								off_t offset);
+	ssize_t (*preadv)(int, const struct iovec *, int, off_t);
+	ssize_t (*pwritev)(int, const struct iovec *, int, off_t);
+
 public:
 	virtual ~IOService() { }
 	friend class Communicator;