block/curl: Implement the libcurl timer callback interface

libcurl versions 7.16.0 and later have a timer callback interface which
must be implemented in order for libcurl to make forward progress (it
will sometimes rely on being called back on the timeout if there are
no file descriptors registered). Implement the callback, and use a
QEMU AIO timer to ensure we prod libcurl again when it asks us to.

Based on Peter's original patch plus my fix to add curl_multi_timeout_do.
Should compile just fine even on older versions of libcurl.

I also tried copy-on-read and streaming:

    $ ./qemu-img create -f qcow2 -o \
         backing_file=http://download.fedoraproject.org/pub/fedora/linux/releases/20/Live/x86_64/Fedora-Live-Desktop-x86_64-20-1.iso \
         foo.qcow2 1G
    $ x86_64-softmmu/qemu-system-x86_64 \
         -drive if=none,file=foo.qcow2,copy-on-read=on,id=cd \
         -device ide-cd,drive=cd --enable-kvm -m 1024

Direct http usage is probably too slow, but with copy-on-read ultimately
the image does boot!

After some time, streaming gets canceled by an EIO, which needs further
investigation.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/curl.c

@@ -34,6 +34,11 @@
 #define DPRINTF(fmt, ...) do { } while (0)
 #endif
 
+#if LIBCURL_VERSION_NUM >= 0x071000
+/* The multi interface timer callback was introduced in 7.16.0 */
+#define NEED_CURL_TIMER_CALLBACK
+#endif
+
 #define PROTOCOLS (CURLPROTO_HTTP | CURLPROTO_HTTPS | \
                    CURLPROTO_FTP | CURLPROTO_FTPS | \
                    CURLPROTO_TFTP)
@@ -77,6 +82,7 @@ typedef struct CURLState
 
 typedef struct BDRVCURLState {
     CURLM *multi;
+    QEMUTimer timer;
     size_t len;
     CURLState states[CURL_NUM_STATES];
     char *url;
@@ -87,6 +93,23 @@ typedef struct BDRVCURLState {
 static void curl_clean_state(CURLState *s);
 static void curl_multi_do(void *arg);
 
+#ifdef NEED_CURL_TIMER_CALLBACK
+static int curl_timer_cb(CURLM *multi, long timeout_ms, void *opaque)
+{
+    BDRVCURLState *s = opaque;
+
+    DPRINTF("CURL: timer callback timeout_ms %ld\n", timeout_ms);
+    if (timeout_ms == -1) {
+        timer_del(&s->timer);
+    } else {
+        int64_t timeout_ns = (int64_t)timeout_ms * 1000 * 1000;
+        timer_mod(&s->timer,
+                  qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + timeout_ns);
+    }
+    return 0;
+}
+#endif
+
 static int curl_sock_cb(CURL *curl, curl_socket_t fd, int action,
                         void *s, void *sp)
 {
@@ -209,20 +232,10 @@ static int curl_find_buf(BDRVCURLState *s, size_t start, size_t len,
     return FIND_RET_NONE;
 }
 
-static void curl_multi_do(void *arg)
+static void curl_multi_read(BDRVCURLState *s)
 {
-    BDRVCURLState *s = (BDRVCURLState *)arg;
-    int running;
-    int r;
     int msgs_in_queue;
 
-    if (!s->multi)
-        return;
-
-    do {
-        r = curl_multi_socket_all(s->multi, &running);
-    } while(r == CURLM_CALL_MULTI_PERFORM);
-
     /* Try to find done transfers, so we can free the easy
      * handle again. */
     do {
@@ -266,6 +279,41 @@ static void curl_multi_do(void *arg)
     } while(msgs_in_queue);
 }
 
+static void curl_multi_do(void *arg)
+{
+    BDRVCURLState *s = (BDRVCURLState *)arg;
+    int running;
+    int r;
+
+    if (!s->multi) {
+        return;
+    }
+
+    do {
+        r = curl_multi_socket_all(s->multi, &running);
+    } while(r == CURLM_CALL_MULTI_PERFORM);
+
+    curl_multi_read(s);
+}
+
+static void curl_multi_timeout_do(void *arg)
+{
+#ifdef NEED_CURL_TIMER_CALLBACK
+    BDRVCURLState *s = (BDRVCURLState *)arg;
+    int running;
+
+    if (!s->multi) {
+        return;
+    }
+
+    curl_multi_socket_action(s->multi, CURL_SOCKET_TIMEOUT, 0, &running);
+
+    curl_multi_read(s);
+#else
+    abort();
+#endif
+}
+
 static CURLState *curl_init_state(BDRVCURLState *s)
 {
     CURLState *state = NULL;
@@ -473,12 +521,20 @@ static int curl_open(BlockDriverState *bs, QDict *options, int flags,
     curl_easy_cleanup(state->curl);
     state->curl = NULL;
 
+    aio_timer_init(bdrv_get_aio_context(bs), &s->timer,
+                   QEMU_CLOCK_REALTIME, SCALE_NS,
+                   curl_multi_timeout_do, s);
+
     // Now we know the file exists and its size, so let's
     // initialize the multi interface!
 
     s->multi = curl_multi_init();
     curl_multi_setopt(s->multi, CURLMOPT_SOCKETDATA, s);
     curl_multi_setopt(s->multi, CURLMOPT_SOCKETFUNCTION, curl_sock_cb);
+#ifdef NEED_CURL_TIMER_CALLBACK
+    curl_multi_setopt(s->multi, CURLMOPT_TIMERDATA, s);
+    curl_multi_setopt(s->multi, CURLMOPT_TIMERFUNCTION, curl_timer_cb);
+#endif
     curl_multi_do(s);
 
     qemu_opts_del(opts);
@@ -597,6 +653,9 @@ static void curl_close(BlockDriverState *bs)
     }
     if (s->multi)
         curl_multi_cleanup(s->multi);
+
+    timer_del(&s->timer);
+
     g_free(s->url);
 }