wget: explain clearerr more clearly

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/networking/wget.c b/networking/wget.c
index 48688640a..673113bfc 100644
--- a/networking/wget.c
+++ b/networking/wget.c
@@ -466,6 +466,14 @@ static void NOINLINE retrieve_file_data(FILE *dfp, int output_fd)
 
         polldata.fd = fileno(dfp);
         polldata.events = POLLIN | POLLPRI;
+
+        /* Must use nonblocking I/O, otherwise fread will loop
+         * and *block* until it reads full buffer,
+         * which messes up progress bar and/or timing out.
+         * Because of nonblocking I/O, we need to dance
+         * very carefully around EAGAIN. See explanation at
+         * clearerr() call.
+         */
         ndelay_on(polldata.fd);
 #endif
         progress_meter(PROGRESS_START);
@@ -504,7 +512,7 @@ static void NOINLINE retrieve_file_data(FILE *dfp, int output_fd)
                                 /* Needed for "stalled" indicator */
                                 progress_meter(PROGRESS_BUMP);
                         }
-#endif
+
                         /* fread internally uses read loop, which in our case
                          * is usually exited when we get EAGAIN.
                          * In this case, libc sets error marker on the stream.
@@ -515,6 +523,7 @@ static void NOINLINE retrieve_file_data(FILE *dfp, int output_fd)
                          */
                         clearerr(dfp);
                         errno = 0;
+#endif
                         n = fread(G.wget_buf, 1, rdsz, dfp);
                         /* man fread:
                          * If error occurs, or EOF is reached, the return value