c - libssh2 SFTP 管道化多个文件 : sftp_read() internal error

Question

我正在尝试重现一个问题，当我们的一些客户在使用 SFTP 协议(protocol)下载时会收到错误的文件内容。

根据我们的 SFTP 服务器 (CrushFTP) 日志，他们可能会在一个 session 中打开多个文件，然后使用一些流水线来下载文件。我不知道他们使用什么类型的库，因为他们为此使用了一些 SAAS 提供商。

我正在尝试使用 libssh2 重现该行为，但在第一个打开的文件返回 LIBSSH2_ERROR_EAGAIN 后，在第二个打开的文件上异步调用 libssh2_sftp_read 时出现 sftp_read() 内部错误 - 即使在本地主机上也是如此连接到 OpenSSH。

浏览时SSH File Transfer Protocol IETF Draft我可以看到该协议(protocol)允许同时打开多个文件并使用多个 SSH_FXP_READ 请求请求它们的内容，而无需等待响应。

下面是我用于测试的代码(SSCCE，但相当长 - C 非常冗长)- 使用 gcc sftp_multifile.c -lssh2 -Wall -g -o sftp_multifile 编译并测试:

./sftp_multifile localhost 22 testusername testpassword /usr/share/dict/words /usr/share/doc/words/readme.txt

connect try: ai_family=10 ai_socktype=1 ai_protocol=6 addr=::1 port=22
opening: /usr/share/dict/words
opening: /usr/share/doc/words/readme.txt
reading: /usr/share/dict/words from 10134400 to 140736927200896
read result: -37
reading: /usr/share/doc/words/readme.txt from 10135536 to 140736927201920
read result: -31
Bad read result: -31
Read error: /usr/share/doc/words/readme.txt: sftp_read() internal error

是我的代码出错了，还是 libssh2 只是不支持流水线化 libssh2_sftp_read 用于多个打开的文件，或者这只是 libssh2 中的一个错误，应该报告给它的维护者？

#include <libssh2.h>
#include <libssh2_sftp.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <poll.h>
#include <arpa/inet.h>

#define CHUNK_SIZE (1024)

int sftp_connect(char const* hostname, char const* port)
{
    struct addrinfo* addrinfo_result, *addrinfo_current;
    int sfd, s;
    struct addrinfo addrinfo_hints = {0};
    addrinfo_hints.ai_family = AF_UNSPEC;
    addrinfo_hints.ai_socktype = SOCK_STREAM;
    s = getaddrinfo(hostname, port, &addrinfo_hints, &addrinfo_result);
    if (s != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
        exit(EXIT_FAILURE);
    }
    
    for (addrinfo_current = addrinfo_result; addrinfo_current != NULL; addrinfo_current = addrinfo_current->ai_next) {
        char s[INET6_ADDRSTRLEN];
        sfd = socket(addrinfo_current->ai_family, addrinfo_current->ai_socktype, addrinfo_current->ai_protocol);
        if (sfd == -1) continue;
        {
            void* addr;
            in_port_t port;
            switch(addrinfo_current->ai_family) {
                case AF_INET: {
                    struct sockaddr_in* sockaddr = (struct sockaddr_in*)addrinfo_current->ai_addr;
                    addr = &(sockaddr->sin_addr);
                    port = ntohs(sockaddr->sin_port);
                    break;
                }
                case AF_INET6: {
                    struct sockaddr_in6* sockaddr = (struct sockaddr_in6*)addrinfo_current->ai_addr;
                    addr = &(sockaddr->sin6_addr);
                    port = ntohs(sockaddr->sin6_port);
                    break;
                }
                default:
                    fprintf(stderr, "unknown family: %d\n", addrinfo_current->ai_family);
                    exit(EXIT_FAILURE);
            }
            inet_ntop(addrinfo_current->ai_family, addr, s, INET6_ADDRSTRLEN);
            fprintf(stderr, "connect try: ai_family=%d ai_socktype=%d ai_protocol=%d addr=%s port=%d\n", addrinfo_current->ai_family, addrinfo_current->ai_socktype, addrinfo_current->ai_protocol, s, port);
        }
        if (connect(sfd, addrinfo_current->ai_addr, addrinfo_current->ai_addrlen) != 0) {
            fprintf(stderr, "connect: %s\n", strerror(errno));
            close(sfd);
            continue;
        }
        break;
    }
    if (addrinfo_current == NULL) {
        fprintf(stderr, "connection failed\n");
        exit(EXIT_FAILURE);
    }
    freeaddrinfo(addrinfo_result);
    return sfd;
}

void retrieve_files(int socket, LIBSSH2_SESSION* session, LIBSSH2_SFTP *sftp_session, int filec, char *filev[])
{
    LIBSSH2_SFTP_HANDLE* sftp_handles[filec];
    int finished[filec];
    int toread = filec;
    for(int i=0; i<filec; i++) {
        fprintf(stderr, "opening: %s\n", filev[i]);
        sftp_handles[i] = libssh2_sftp_open(sftp_session, filev[i], LIBSSH2_FXF_READ, 0);
        if (!sftp_handles[i]) {
            char* errmsg;
            libssh2_session_last_error(session, &errmsg, NULL, 0);
            fprintf(stderr, "Failure opening remote file: %s: %s\n", filev[i], errmsg);
            exit(EXIT_FAILURE);
        }
        finished[i]=0;
    }
    char buffer[CHUNK_SIZE];
    libssh2_session_set_blocking(session, 0);
    while (toread) {
        for(int i=0; i<filec; i++) {
            if (finished[i]) {
                continue;
            } else {
                int read_result;
                do {
                    fprintf(stderr, "reading: %s from %ld to %ld\n", filev[i], (long)sftp_handles[i], (long)(buffer+CHUNK_SIZE*i));
                    read_result = libssh2_sftp_read(sftp_handles[i], buffer, CHUNK_SIZE);
                    fprintf(stderr, "read result: %d\n", read_result);
                    if (read_result > 0) {
                        printf("%s: ", filev[i]);
                        fwrite(buffer, sizeof(char), read_result, stdout);
                        printf("\n");
                    } else {
                        break;
                    }
                } while (1);
                if (read_result == LIBSSH2_ERROR_EAGAIN) {
                    continue;
                } else if (read_result == 0) {
                    finished[i] = 1;
                    toread--;
                } else {
                    fprintf(stderr, "Bad read result: %d\n", read_result);
                    char* errmsg;
                    libssh2_session_last_error(session, &errmsg, NULL, 0);
                    fprintf(stderr, "Read error: %s: %s\n", filev[i], errmsg);
                    exit(EXIT_FAILURE);
                }
            }
        }
        if (toread) {
            struct pollfd fds[1] = {0};
            fds[0].fd = socket;
            fds[0].events = POLLIN;
            poll(fds, 1, 3000);
        }
    }
}

int main(int argc, char *argv[])
{
    LIBSSH2_SESSION *session;
    LIBSSH2_SFTP *sftp_session;
    int rc, sock;
    
    if (argc<6) {
        fprintf(stderr, "Usage: %s hostname port username password file1 [file2] [file3...]\n", argv[0]);
        return EXIT_FAILURE;
    }
    
    sock = sftp_connect(argv[1], argv[2]);
    session = libssh2_session_init();
    if (!session) {
        return EXIT_FAILURE;
    }
    rc = libssh2_session_handshake(session, sock);
    if (rc) {
        char* errmsg;
        libssh2_session_last_error(session, &errmsg, NULL, 0);
        fprintf(stderr, "Failure establishing SSH session: %s\n", errmsg);
        return EXIT_FAILURE;
    }
    rc = libssh2_userauth_password(session, argv[3], argv[4]);
    if (rc) {
        char* errmsg;
        libssh2_session_last_error(session, &errmsg, NULL, 0);
        fprintf(stderr, "%s\n", errmsg);
        return EXIT_FAILURE;
    }
    
    sftp_session = libssh2_sftp_init(session);
    if (sftp_session == NULL) {
        char* errmsg;
        libssh2_session_last_error(session, &errmsg, NULL, 0);
        fprintf(stderr, "Unable to init SFTP session: %s\n", errmsg);
        return EXIT_FAILURE;
    }

    retrieve_files(sock, session, sftp_session, argc-5, argv+5);

    libssh2_session_set_blocking(session, 1);
    libssh2_sftp_shutdown(sftp_session);
    libssh2_session_disconnect(session, "");
    libssh2_session_free(session);
    libssh2_exit();
    return EXIT_SUCCESS;
}

Answer 1

很有趣。我在 Arch 上使用 libssh2 1.8.1-1。我将你的代码粘贴到 sftp_multifile.c 中，用

创建了一个 Makefile

sftp_multifile: sftp_multifile.c
    gcc -g -O -Wall $< -o $@ -lssh2

并将 args 写入 55728938.<count>(不打算在此处发布 ;-)。只有一个文件，我得到

$ ./sftp_multifile $(cat 55728938.one)
connect try: ai_family=10 ai_socktype=1 ai_protocol=6 addr=::1 port=22
opening: path/text1
reading: path/text1 from 94221524604816 to 140729241245632
read result: -37
reading: path/text1 from 94221524604816 to 140729241245632
read result: 21
reading: path/text1 from 94221524604816 to 140729241245632
read result: -37
reading: path/text1 from 94221524604816 to 140729241245632
read result: 0
path/text1: So much depends
upon

...我觉得不错。但是对于两个或更多文件，

$ ./sftp_multifile $(cat 55728938.two)
connect try: ai_family=10 ai_socktype=1 ai_protocol=6 addr=::1 port=22
opening: path/text1
opening: path/text2
reading: path/text1 from 94473513555856 to 140727557764112
read result: -37
reading: path/text2 from 94473513556256 to 140727557765136
read result: -31
Bad read result: -31
Read error: path/text2: sftp_read() internal error

$ ./sftp_multifile $(cat 55728938.three)
connect try: ai_family=10 ai_socktype=1 ai_protocol=6 addr=::1 port=22
opening: path/text1
opening: path/text2
opening: path/text3
reading: path/text1 from 94652164568976 to 140727538608784
read result: -37
reading: path/text2 from 94652164569376 to 140727538609808
read result: -31
Bad read result: -31
Read error: path/text2: sftp_read() internal error

这和你看到的一致。由于打包的 libssh2 不是用调试符号构建的，所以 gdb 帮不上什么忙。我建议通过 libssh2 mailing list 解决这个问题。

编辑添加:

删除 libssh2_session_set_blocking(session, 0); 行似乎可以解决问题。

$ make && ./sftp_multifile $(cat 55728938.four)
gcc -g -O -Wall sftp_multifile.c -o sftp_multifile -lssh2
connect try: ai_family=10 ai_socktype=1 ai_protocol=6 addr=::1 port=22
opening: /home/pi/stackoverflow/text1
opening: /home/pi/stackoverflow/text2
opening: /home/pi/stackoverflow/text3
opening: /home/pi/stackoverflow/text4
4 of 4
reading: /home/pi/stackoverflow/text1 from 5453288 to 2129740008
read result: 21
/home/pi/stackoverflow/text1: So much depends
upon

reading: /home/pi/stackoverflow/text1 from 5453288 to 2129740008
read result: 0
3 of 4
reading: /home/pi/stackoverflow/text2 from 5453640 to 2129741032
read result: 19
/home/pi/stackoverflow/text2: a red wheel
barrow

reading: /home/pi/stackoverflow/text2 from 5453640 to 2129741032
read result: 0
2 of 4
reading: /home/pi/stackoverflow/text3 from 5455408 to 2129742056
read result: 23
/home/pi/stackoverflow/text3: glazed with rain
water

reading: /home/pi/stackoverflow/text3 from 5455408 to 2129742056
read result: 0
1 of 4
reading: /home/pi/stackoverflow/text4 from 5455760 to 2129743080
read result: 27
/home/pi/stackoverflow/text4: beside the white
chickens.

reading: /home/pi/stackoverflow/text4 from 5455760 to 2129743080
read result: 0

所有四个文件，存在并说明。 Arch ( libssh2-1.8 ) 和 Raspbian ( libssh-1.7 ) 上的结果相同。