Fixed slow sftp/scp connection

In curl/docs/examples/fopen.c, the developer didn't check maxfd set by
curl_multi_fdset() and supposed that calling select(0,...) is effectively
equivalent to sleep. While the comment is correct, timeout estimated by
curl_multi_fdset() is frequently too large - in my case 10 seconds. We seldom
need to wait that long.

In curl_multi_fdset.3, the cURL developers recommended to wait at least 100ms
if maxfd is set to 1. This is what I am doing in the revised code. It does
little harm.

diff --git a/kurl.c b/kurl.c
index 40e761705232c73b2015fae9c848400fff7a3f6d..ac92d71f9e2d8c4c48c77c9d3de607f4079a68ea 100644 (file)
--- a/kurl.c
+++ b/kurl.c
@@ -67,9 +67,6 @@ static int fill_buffer(kurl_t *ku) // fill the buffer
        assert(ku->p_buf == ku->l_buf); // buffer is always used up when fill_buffer() is called; otherwise a bug
        ku->off0 += ku->l_buf;
        ku->p_buf = ku->l_buf = 0;
-       // For a normal file, it is not necessary to test ->done_read. For a cURL connection, however,
-       // if we have already read the entire content, this function will be blocked at "select()" below.
-       // Probably there are better means to check if reading is over.
        if (ku->done_reading) return 0;
        if (kurl_isfile(ku)) {
                // The following block is equivalent to "ku->l_buf = read(ku->fd, ku->buf, ku->m_buf)" on Mac.
@@ -90,7 +87,6 @@ static int fill_buffer(kurl_t *ku) // fill the buffer
                        long curl_to = -1;
                        struct timeval to;
                        // the following adaped from docs/examples/fopen.c 
-                       FD_ZERO(&fdr); FD_ZERO(&fdw); FD_ZERO(&fde);
                        to.tv_sec = 10, to.tv_usec = 0; // 10 seconds
                        curl_multi_timeout(ku->multi, &curl_to);
                        if (curl_to >= 0) {
@@ -98,10 +94,15 @@ static int fill_buffer(kurl_t *ku) // fill the buffer
                                if (to.tv_sec > 1) to.tv_sec = 1;
                                else to.tv_usec = (curl_to % 1000) * 1000;
                        }
+                       FD_ZERO(&fdr); FD_ZERO(&fdw); FD_ZERO(&fde);
                        curl_multi_fdset(ku->multi, &fdr, &fdw, &fde, &maxfd);
-                       rc = select(maxfd+1, &fdr, &fdw, &fde, &to);
-                       if (rc >= 0) rc = curl_multi_perform(ku->multi, &n_running);
-                       else break;
+                       if (maxfd >= 0 && (rc = select(maxfd+1, &fdr, &fdw, &fde, &to)) < 0) break;
+                       if (maxfd < 0) { // check curl_multi_fdset.3 about why we wait 100ms here
+                               struct timespec req, rem;
+                               req.tv_sec = 0; req.tv_nsec = 100000000; // this is 100ms
+                               nanosleep(&req, &rem);
+                       }
+                       rc = curl_multi_perform(ku->multi, &n_running);
                } while (n_running && ku->l_buf < CURL_MAX_WRITE_SIZE);
                if (ku->l_buf < CURL_MAX_WRITE_SIZE) ku->done_reading = 1;
        }