curl_multi_poll: a sister to curl_multi_wait() that waits more

Repeatedly we see problems where using curl_multi_wait() is difficult or
just awkward because if it has no file descriptor to wait for
internally, it returns immediately and leaves it to the caller to wait
for a small amount of time in order to avoid occasional busy-looping.

This is often missed or misunderstood, leading to underperforming
applications.

This change introduces curl_multi_poll() as a replacement drop-in
function that accepts the exact same set of arguments. This function
works identically to curl_multi_wait() - EXCEPT - for the case when
there's nothing to wait for internally, as then this function will by
itself wait for a "suitable" short time before it returns. This
effectiely avoids all risks of busy-looping and should also make it less
likely that apps "over-wait".

This also changes the curl tool to use this funtion internally when
doing parallel transfers and changes curl_easy_perform() to use it
internally.

Closes #4163

4 files changed, 32 insertions, 39 deletions

diff --git a/lib/Makefile.am b/lib/Makefile.am
index 839751edf..5d41c2be3 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -65,7 +65,7 @@ endif
 # Prevent LIBS from being used for all link targets
 LIBS = $(BLANK_AT_MAKETIME)
 
-VERSIONINFO=-version-info 9:0:5
+VERSIONINFO=-version-info 10:0:6
 # This flag accepts an argument of the form current[:revision[:age]]. So,
 # passing -version-info 3:12:1 sets current to 3, revision to 12, and age to
 # 1.
diff --git a/lib/easy.c b/lib/easy.c
index 616ad28b8..b8d94c740 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -602,27 +602,11 @@ static CURLcode easy_transfer(struct Curl_multi *multi)
 
   while(!done && !mcode) {
     int still_running = 0;
-    bool gotsocket = FALSE;
-
-    mcode = Curl_multi_wait(multi, NULL, 0, 1000, NULL, &gotsocket);
-
-    if(!mcode) {
-      if(!gotsocket) {
-        long sleep_ms;
-
-        /* If it returns without any filedescriptor instantly, we need to
-           avoid busy-looping during periods where it has nothing particular
-           to wait for */
-        curl_multi_timeout(multi, &sleep_ms);
-        if(sleep_ms) {
-          if(sleep_ms > 1000)
-            sleep_ms = 1000;
-          Curl_wait_ms((int)sleep_ms);
-        }
-      }
 
+    mcode = curl_multi_poll(multi, NULL, 0, 1000, NULL);
+
+    if(!mcode)
       mcode = curl_multi_perform(multi, &still_running);
-    }
 
     /* only read 'still_running' if curl_multi_perform() return OK */
     if(!mcode && !still_running) {
diff --git a/lib/multi.c b/lib/multi.c
index 6e16eafe0..e5c483c56 100755
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -974,12 +974,12 @@ CURLMcode curl_multi_fdset(struct Curl_multi *multi,
 
 #define NUM_POLLS_ON_STACK 10
 
-CURLMcode Curl_multi_wait(struct Curl_multi *multi,
-                          struct curl_waitfd extra_fds[],
-                          unsigned int extra_nfds,
-                          int timeout_ms,
-                          int *ret,
-                          bool *gotsocket) /* if any socket was checked */
+static CURLMcode Curl_multi_wait(struct Curl_multi *multi,
+                                 struct curl_waitfd extra_fds[],
+                                 unsigned int extra_nfds,
+                                 int timeout_ms,
+                                 int *ret,
+                                 bool extrawait) /* when no socket, wait */
 {
   struct Curl_easy *data;
   curl_socket_t sockbunch[MAX_SOCKSPEREASYHANDLE];
@@ -993,9 +993,6 @@ CURLMcode Curl_multi_wait(struct Curl_multi *multi,
   struct pollfd a_few_on_stack[NUM_POLLS_ON_STACK];
   struct pollfd *ufds = &a_few_on_stack[0];
 
-  if(gotsocket)
-    *gotsocket = FALSE;
-
   if(!GOOD_MULTI_HANDLE(multi))
     return CURLM_BAD_HANDLE;
 
@@ -1124,9 +1121,19 @@ CURLMcode Curl_multi_wait(struct Curl_multi *multi,
     free(ufds);
   if(ret)
     *ret = retcode;
-  if(gotsocket && (extra_fds || curlfds))
+  if(!extrawait || extra_fds || curlfds)
     /* if any socket was checked */
-    *gotsocket = TRUE;
+    ;
+  else {
+    long sleep_ms = 0;
+
+    /* Avoid busy-looping when there's nothing particular to wait for */
+    if(!curl_multi_timeout(multi, &sleep_ms) && sleep_ms) {
+      if(sleep_ms > timeout_ms)
+        sleep_ms = timeout_ms;
+      Curl_wait_ms((int)sleep_ms);
+    }
+  }
 
   return CURLM_OK;
 }
@@ -1137,7 +1144,16 @@ CURLMcode curl_multi_wait(struct Curl_multi *multi,
                           int timeout_ms,
                           int *ret)
 {
-  return Curl_multi_wait(multi, extra_fds, extra_nfds, timeout_ms, ret, NULL);
+  return Curl_multi_wait(multi, extra_fds, extra_nfds, timeout_ms, ret, FALSE);
+}
+
+CURLMcode curl_multi_poll(struct Curl_multi *multi,
+                          struct curl_waitfd extra_fds[],
+                          unsigned int extra_nfds,
+                          int timeout_ms,
+                          int *ret)
+{
+  return Curl_multi_wait(multi, extra_fds, extra_nfds, timeout_ms, ret, TRUE);
 }
 
 /*
diff --git a/lib/multiif.h b/lib/multiif.h
index 57b6145eb..0755a7cd2 100644
--- a/lib/multiif.h
+++ b/lib/multiif.h
@@ -89,11 +89,4 @@ CURLMcode Curl_multi_add_perform(struct Curl_multi *multi,
                                  struct Curl_easy *data,
                                  struct connectdata *conn);
 
-CURLMcode Curl_multi_wait(struct Curl_multi *multi,
-                          struct curl_waitfd extra_fds[],
-                          unsigned int extra_nfds,
-                          int timeout_ms,
-                          int *ret,
-                          bool *gotsocket); /* if any socket was checked */
-
 #endif /* HEADER_CURL_MULTIIF_H */