multi: add curl_multi_wakeup()

This commit adds curl_multi_wakeup() which was previously in the TODO
list under the curl_multi_unblock name.

On some platforms and with some configurations this feature might not be
available or can fail, in these cases a new error code
(CURLM_WAKEUP_FAILURE) is returned from curl_multi_wakeup().

Fixes curl#4418

Author: ngg

docs/TODO

@@ -51,7 +51,6 @@
  2.4 Split connect and authentication process
  2.5 Edge-triggered sockets should work
  2.6 multi upkeep
- 2.7 curl_multi_unblock
 
  3. Documentation
  3.2 Provide cmake config-file
@@ -448,13 +447,6 @@
 
  See https://github.com/curl/curl/issues/3199
 
-2.7 curl_multi_unblock
-
- A portable way to unblock curl_multi_wait from another thread.
-
- See https://github.com/curl/curl/issues/4418 and
- https://github.com/curl/curl/wiki/curl_multi_unblock
-
 3. Documentation
 
 3.2 Provide cmake config-file

docs/libcurl/Makefile.inc

@@ -54,6 +54,7 @@ man_MANS = \
  curl_multi_socket_all.3 \
  curl_multi_strerror.3 \
  curl_multi_timeout.3 \
+ curl_multi_wakeup.3 \
  curl_multi_wait.3 \
  curl_share_cleanup.3 \
  curl_share_init.3 \

docs/libcurl/curl_multi_poll.3

@@ -48,10 +48,16 @@ total number of file descriptors on which interesting events occurred. This
 number can include both libcurl internal descriptors as well as descriptors
 provided in \fIextra_fds\fP.
 
+The \fIcurl_multi_wakeup(3)\fP function can be used from another thread to
+wake up this function and return faster. This is one of the details
+that makes this function different than \fIcurl_multi_wait(3)\fP which cannot
+be woken up this way.
+
 If no extra file descriptors are provided and libcurl has no file descriptor
 to offer to wait for, this function will instead wait during \fItimeout_ms\fP
 milliseconds (or shorter if an internal timer indicates so). This is the
-detail that makes this function different than \fIcurl_multi_wait(3)\fP.
+other detail that makes this function different than
+\fIcurl_multi_wait(3)\fP.
 
 This function is encouraged to be used instead of select(3) when using the
 multi interface to allow applications to easier circumvent the common problem
@@ -107,4 +113,5 @@ CURLMcode type, general libcurl multi interface error code. See
 .SH AVAILABILITY
 This function was added in libcurl 7.66.0.
 .SH "SEE ALSO"
-.BR curl_multi_fdset "(3), " curl_multi_perform "(3), " curl_multi_wait "(3)"
+.BR curl_multi_fdset "(3), " curl_multi_perform "(3), "
+.BR curl_multi_wait "(3), " curl_multi_wakeup "(3)"

docs/libcurl/curl_multi_wakeup.3

@@ -0,0 +1,47 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <[email protected]>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.TH curl_multi_wakeup 3 "17 Nov 2019" "libcurl 7.68.0" "libcurl Manual"
+.SH NAME
+curl_multi_wakeup - wakes up a sleeping curl_multi_poll call
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLMcode curl_multi_wakeup(CURLM *multi_handle);
+.ad
+.SH DESCRIPTION
+This function can be called from any thread and it wakes up a
+sleeping \fIcurl_multi_poll(3)\fP call that is currently (or will be)
+waiting for activity or a timeout.
+
+If the function is called when there is no \fIcurl_multi_poll(3)\fP call,
+it will cause the next call to return immediately.
+
+Calling this function only guarantees to wake up the current (or the next
+if there is no current) \fIcurl_multi_poll(3)\fP call, which means it is
+possible that multiple calls to this function will wake up the same waiting
+operation.
+
+This function has no effect on \fIcurl_multi_wait(3)\fP calls.
+.SH RETURN VALUE
+CURLMcode type, general libcurl multi interface error code.
+.SH "SEE ALSO"
+.BR curl_multi_poll "(3), " curl_multi_wait "(3)"

docs/libcurl/libcurl-errors.3

@@ -297,6 +297,8 @@ An easy handle already added to a multi handle was attempted to get added a
 second time. (Added in 7.32.1)
 .IP "CURLM_RECURSIVE_API_CALL (8)"
 An API function was called from inside a callback.
+.IP "CURLM_WAKEUP_FAILURE (9)"
+Wakeup is unavailable or failed.
 .SH "CURLSHcode"
 The "share" interface will return a CURLSHcode to indicate when an error has
 occurred.  Also consider \fIcurl_share_strerror(3)\fP.

docs/libcurl/symbols-in-versions

@@ -342,6 +342,7 @@ CURLM_INTERNAL_ERROR            7.9.6
 CURLM_OK                        7.9.6
 CURLM_OUT_OF_MEMORY             7.9.6
 CURLM_RECURSIVE_API_CALL        7.59.0
+CURLM_WAKEUP_FAILURE            7.68.0
 CURLM_UNKNOWN_OPTION            7.15.4
 CURLOPTTYPE_FUNCTIONPOINT       7.1
 CURLOPTTYPE_LONG                7.1

include/curl/multi.h

@@ -72,6 +72,7 @@ typedef enum {
                             attempted to get added - again */
   CURLM_RECURSIVE_API_CALL, /* an api function was called from inside a
                                callback */
+  CURLM_WAKEUP_FAILURE, /* wakeup is unavailable or failed */
   CURLM_LAST
 } CURLMcode;
 
@@ -187,6 +188,15 @@ CURL_EXTERN CURLMcode curl_multi_poll(CURLM *multi_handle,
                                       int timeout_ms,
                                       int *ret);
 
+/*
+ * Name:     curl_multi_wakeup()
+ *
+ * Desc:     wakes up a sleeping curl_multi_poll call.
+ *
+ * Returns:  CURLMcode type, general multi error code.
+ */
+CURL_EXTERN CURLMcode curl_multi_wakeup(CURLM *multi_handle);
+
  /*
   * Name:    curl_multi_perform()
   *

lib/multi.c

@@ -46,6 +46,7 @@
 #include "connect.h"
 #include "http_proxy.h"
 #include "http2.h"
+#include "socketpair.h"
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
 #include "curl_memory.h"
@@ -367,6 +368,21 @@ struct Curl_multi *Curl_multi_handle(int hashsize, /* socket hash */
 
   /* -1 means it not set by user, use the default value */
   multi->maxconnects = -1;
+
+#ifdef ENABLE_WAKEUP
+  if(Curl_socketpair(AF_UNIX, SOCK_STREAM, 0, multi->wakeup_pair) < 0) {
+    multi->wakeup_pair[0] = CURL_SOCKET_BAD;
+    multi->wakeup_pair[1] = CURL_SOCKET_BAD;
+  }
+  else if(curlx_nonblock(multi->wakeup_pair[0], TRUE) < 0 ||
+          curlx_nonblock(multi->wakeup_pair[1], TRUE) < 0) {
+    sclose(multi->wakeup_pair[0]);
+    sclose(multi->wakeup_pair[1]);
+    multi->wakeup_pair[0] = CURL_SOCKET_BAD;
+    multi->wakeup_pair[1] = CURL_SOCKET_BAD;
+  }
+#endif
+
   return multi;
 
   error:
@@ -1005,7 +1021,8 @@ static CURLMcode Curl_multi_wait(struct Curl_multi *multi,
                                  unsigned int extra_nfds,
                                  int timeout_ms,
                                  int *ret,
-                                 bool extrawait) /* when no socket, wait */
+                                 bool extrawait, /* when no socket, wait */
+                                 bool use_wakeup)
 {
   struct Curl_easy *data;
   curl_socket_t sockbunch[MAX_SOCKSPEREASYHANDLE];
@@ -1059,6 +1076,12 @@ static CURLMcode Curl_multi_wait(struct Curl_multi *multi,
   curlfds = nfds; /* number of internal file descriptors */
   nfds += extra_nfds; /* add the externally provided ones */
 
+#ifdef ENABLE_WAKEUP
+  if(use_wakeup && multi->wakeup_pair[0] != CURL_SOCKET_BAD) {
+    ++nfds;
+  }
+#endif
+
   if(nfds > NUM_POLLS_ON_STACK) {
     /* 'nfds' is a 32 bit value and 'struct pollfd' is typically 8 bytes
        big, so at 2^29 sockets this value might wrap. When a process gets
@@ -1117,6 +1140,14 @@ static CURLMcode Curl_multi_wait(struct Curl_multi *multi,
     ++nfds;
   }
 
+#ifdef ENABLE_WAKEUP
+  if(use_wakeup && multi->wakeup_pair[0] != CURL_SOCKET_BAD) {
+    ufds[nfds].fd = multi->wakeup_pair[0];
+    ufds[nfds].events = POLLIN;
+    ++nfds;
+  }
+#endif
+
   if(nfds) {
     int pollrc;
     /* wait... */
@@ -1140,6 +1171,29 @@ static CURLMcode Curl_multi_wait(struct Curl_multi *multi,
 
         extra_fds[i].revents = mask;
       }
+
+#ifdef ENABLE_WAKEUP
+      if(use_wakeup && multi->wakeup_pair[0] != CURL_SOCKET_BAD) {
+        if(ufds[curlfds + extra_nfds].revents & POLLIN) {
+          char buf[64];
+          while(1) {
+            /* the reading socket is non-blocking, try to read
+               data from it until it receives an error (except EINTR).
+               In normal cases it will get EAGAIN or EWOULDBLOCK
+               when there is no more data, breaking the loop. */
+            if(sread(multi->wakeup_pair[0], buf, sizeof(buf)) < 0) {
+#ifndef USE_WINSOCK
+              if(EINTR == SOCKERRNO)
+                continue;
+#endif
+              break;
+            }
+          }
+          /* do not count the wakeup socket into the returned value */
+          retcode--;
+        }
+      }
+#endif
     }
   }
 
@@ -1174,7 +1228,8 @@ 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, FALSE);
+  return Curl_multi_wait(multi, extra_fds, extra_nfds, timeout_ms, ret, FALSE,
+                         FALSE);
 }
 
 CURLMcode curl_multi_poll(struct Curl_multi *multi,
@@ -1183,7 +1238,55 @@ CURLMcode curl_multi_poll(struct Curl_multi *multi,
                           int timeout_ms,
                           int *ret)
 {
-  return Curl_multi_wait(multi, extra_fds, extra_nfds, timeout_ms, ret, TRUE);
+  return Curl_multi_wait(multi, extra_fds, extra_nfds, timeout_ms, ret, TRUE,
+                         TRUE);
+}
+
+CURLMcode curl_multi_wakeup(struct Curl_multi *multi)
+{
+  /* this function is usually called from another thread,
+     it has to be careful only to access parts of the
+     Curl_multi struct that are constant */
+
+  /* GOOD_MULTI_HANDLE can be safely called */
+  if(!GOOD_MULTI_HANDLE(multi))
+    return CURLM_BAD_HANDLE;
+
+#ifdef ENABLE_WAKEUP
+  /* the wakeup_pair variable is only written during init and cleanup,
+     making it safe to access from another thread after the init part
+     and before cleanup */
+  if(multi->wakeup_pair[1] != CURL_SOCKET_BAD) {
+    char buf[1];
+    buf[0] = 1;
+    while(1) {
+      /* swrite() is not thread-safe in general, because concurrent calls
+         can have their messages interleaved, but in this case the content
+         of the messages does not matter, which makes it ok to call.
+
+         The write socket is set to non-blocking, this way this function
+         cannot block, making it safe to call even from the same thread
+         that will call Curl_multi_wait(). If swrite() returns that it
+         would block, it's considered successful because it means that
+         previous calls to this function will wake up the poll(). */
+      if(swrite(multi->wakeup_pair[1], buf, sizeof(buf)) < 0) {
+        int err = SOCKERRNO;
+        int return_success;
+#ifdef USE_WINSOCK
+        return_success = WSAEWOULDBLOCK == err;
+#else
+        if(EINTR == err)
+          continue;
+        return_success = EWOULDBLOCK == err || EAGAIN == err;
+#endif
+        if(!return_success)
+          return CURLM_WAKEUP_FAILURE;
+      }
+      return CURLM_OK;
+    }
+  }
+#endif
+  return CURLM_WAKEUP_FAILURE;
 }
 
 /*
@@ -2309,6 +2412,11 @@ CURLMcode curl_multi_cleanup(struct Curl_multi *multi)
 
     Curl_hash_destroy(&multi->hostcache);
     Curl_psl_destroy(&multi->psl);
+
+#ifdef ENABLE_WAKEUP
+    sclose(multi->wakeup_pair[0]);
+    sclose(multi->wakeup_pair[1]);
+#endif
     free(multi);
 
     return CURLM_OK;

lib/multihandle.h

@@ -24,6 +24,7 @@
 
 #include "conncache.h"
 #include "psl.h"
+#include "socketpair.h"
 
 struct Curl_message {
   struct curl_llist_element list;
@@ -66,6 +67,10 @@ typedef enum {
 
 #define CURLPIPE_ANY (CURLPIPE_MULTIPLEX)
 
+#if defined(USE_SOCKETPAIR) && !defined(USE_BLOCKING_SOCKETS)
+#define ENABLE_WAKEUP
+#endif
+
 /* This is the struct known as CURLM on the outside */
 struct Curl_multi {
   /* First a simple identifier to easier detect if a user mix up
@@ -134,6 +139,11 @@ struct Curl_multi {
                                     previous callback */
   bool in_callback;            /* true while executing a callback */
   long max_concurrent_streams; /* max concurrent streams client to support */
+
+#ifdef ENABLE_WAKEUP
+  curl_socket_t wakeup_pair[2]; /* socketpair() used for wakeup
+                                   0 is used for read, 1 is used for write */
+#endif
 };
 
 #endif /* HEADER_CURL_MULTIHANDLE_H */

lib/strerror.c

@@ -389,6 +389,9 @@ curl_multi_strerror(CURLMcode error)
   case CURLM_RECURSIVE_API_CALL:
     return "API function called from within callback";
 
+  case CURLM_WAKEUP_FAILURE:
+    return "Wakeup is unavailable or failed";
+
   case CURLM_LAST:
     break;
   }

packages/OS400/curl.inc.in

@@ -1804,7 +1804,11 @@
      d                 c                   6
      d  CURLM_ADDED_ALREADY...
      d                 c                   7
-     d  CURLM_LAST     c                   8
+     d  CURLM_RECURSIVE_API_CALL...
+     d                 c                   8
+     d  CURLM_WAKEUP_FAILURE...
+     d                 c                   9
+     d  CURLM_LAST     c                   10
       *
      d CURLMSG         s             10i 0 based(######ptr######)               Enum
      d  CURLMSG_NONE   c                   0

tests/data/Makefile.inc

@@ -179,7 +179,7 @@ test1525 test1526 test1527 test1528 test1529 test1530 test1531 test1532 \
 test1533 test1534 test1535 test1536 test1537 test1538 \
 test1540 test1541 \
 test1550 test1551 test1552 test1553 test1554 test1555 test1556 test1557 \
-test1558 test1559 test1560 test1561 test1562 test1563 \
+test1558 test1559 test1560 test1561 test1562 test1563 test1564 test1565 \
 \
 test1590 test1591 test1592 test1593 test1594 test1595 test1596 \
 \

tests/data/test1135

@@ -92,6 +92,7 @@ CURL_EXTERN CURLMcode curl_multi_remove_handle(CURLM *multi_handle,
 CURL_EXTERN CURLMcode curl_multi_fdset(CURLM *multi_handle,
 CURL_EXTERN CURLMcode curl_multi_wait(CURLM *multi_handle,
 CURL_EXTERN CURLMcode curl_multi_poll(CURLM *multi_handle,
+CURL_EXTERN CURLMcode curl_multi_wakeup(CURLM *multi_handle);
 CURL_EXTERN CURLMcode curl_multi_perform(CURLM *multi_handle,
 CURL_EXTERN CURLMcode curl_multi_cleanup(CURLM *multi_handle);
 CURL_EXTERN CURLMsg *curl_multi_info_read(CURLM *multi_handle,