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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/201246">201246</a></dd>
<dt>Author</dt> <dd>aestes@apple.com</dd>
<dt>Date</dt> <dd>2016-05-21 16:52:37 -0700 (Sat, 21 May 2016)</dd>
</dl>
<h3>Log Message</h3>
<pre>REGRESSION (<a href="http://trac.webkit.org/projects/webkit/changeset/188642">r188642</a>): All pages are blank when printing a webpage in iOS Safari
https://bugs.webkit.org/show_bug.cgi?id=157924
rdar://problem/22524550
Reviewed by Sam Weinig.
When UIPrintInteractionController asks WKWebView to print a webpage, it does so in several phases. First we're
asked to compute the page count, followed later by a series of messages asking us to draw each page into a
provided CGContext.
When asked for the page count, we send a message to the Web process instructing it to compute and
return the page count synchronously and then immediately start drawing the page for printing. If the drawing has
finished by the time we're asked to print the first page, then we can do so without waiting. But if it hasn't
then we block by calling Connection::waitForMessage(), passing std::chromo::milliseconds::max() as the relative
timeout.
Prior to <a href="http://trac.webkit.org/projects/webkit/changeset/188642">r188642</a>, Connection::waitForMessage() called std::condition_variable::wait_for(), which takes a
relative timeout value. <a href="http://trac.webkit.org/projects/webkit/changeset/188642">r188642</a> replaced this with WTF::Condition::waitUntil(), which takes an absolute timeout
instead. To convert from relative to absolute, this line was added to Connection::waitForMessage():
Condition::Clock::time_point absoluteTimeout = Condition::Clock::now() + timeout;
std::chrono will convert both operands to a common duration type before performing the addition. When timeout
equals something very large, like milliseconds::max(), this conversion results in signed integer overflow,
giving absoluteTimeout a value less than Clock::now() and making waitForMessage time out immediately.
To fix this, compute how many milliseconds remain on our clock, and add the smaller of that and the timeout
value to Clock::now() to arrive at an absolute timeout.
* Platform/IPC/Connection.cpp:
(IPC::Connection::waitForMessage):
* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _printedDocument]): Removed an unnecessary nanoseconds-to-milliseconds conversion.</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceWebKit2ChangeLog">trunk/Source/WebKit2/ChangeLog</a></li>
<li><a href="#trunkSourceWebKit2PlatformIPCConnectioncpp">trunk/Source/WebKit2/Platform/IPC/Connection.cpp</a></li>
<li><a href="#trunkSourceWebKit2UIProcessAPICocoaWKWebViewmm">trunk/Source/WebKit2/UIProcess/API/Cocoa/WKWebView.mm</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceWebKit2ChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebKit2/ChangeLog (201245 => 201246)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebKit2/ChangeLog 2016-05-21 19:21:35 UTC (rev 201245)
+++ trunk/Source/WebKit2/ChangeLog 2016-05-21 23:52:37 UTC (rev 201246)
</span><span class="lines">@@ -1,3 +1,39 @@
</span><ins>+2016-05-21 Andy Estes <aestes@apple.com>
+
+ REGRESSION (r188642): All pages are blank when printing a webpage in iOS Safari
+ https://bugs.webkit.org/show_bug.cgi?id=157924
+ rdar://problem/22524550
+
+ Reviewed by Sam Weinig.
+
+ When UIPrintInteractionController asks WKWebView to print a webpage, it does so in several phases. First we're
+ asked to compute the page count, followed later by a series of messages asking us to draw each page into a
+ provided CGContext.
+
+ When asked for the page count, we send a message to the Web process instructing it to compute and
+ return the page count synchronously and then immediately start drawing the page for printing. If the drawing has
+ finished by the time we're asked to print the first page, then we can do so without waiting. But if it hasn't
+ then we block by calling Connection::waitForMessage(), passing std::chromo::milliseconds::max() as the relative
+ timeout.
+
+ Prior to r188642, Connection::waitForMessage() called std::condition_variable::wait_for(), which takes a
+ relative timeout value. r188642 replaced this with WTF::Condition::waitUntil(), which takes an absolute timeout
+ instead. To convert from relative to absolute, this line was added to Connection::waitForMessage():
+
+ Condition::Clock::time_point absoluteTimeout = Condition::Clock::now() + timeout;
+
+ std::chrono will convert both operands to a common duration type before performing the addition. When timeout
+ equals something very large, like milliseconds::max(), this conversion results in signed integer overflow,
+ giving absoluteTimeout a value less than Clock::now() and making waitForMessage time out immediately.
+
+ To fix this, compute how many milliseconds remain on our clock, and add the smaller of that and the timeout
+ value to Clock::now() to arrive at an absolute timeout.
+
+ * Platform/IPC/Connection.cpp:
+ (IPC::Connection::waitForMessage):
+ * UIProcess/API/Cocoa/WKWebView.mm:
+ (-[WKWebView _printedDocument]): Removed an unnecessary nanoseconds-to-milliseconds conversion.
+
</ins><span class="cx"> 2016-05-20 Enrica Casucci <enrica@apple.com>
</span><span class="cx">
</span><span class="cx"> Drag cannot start if no drag data or custom data is available in the Pasteboard.
</span></span></pre></div>
<a id="trunkSourceWebKit2PlatformIPCConnectioncpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebKit2/Platform/IPC/Connection.cpp (201245 => 201246)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebKit2/Platform/IPC/Connection.cpp 2016-05-21 19:21:35 UTC (rev 201245)
+++ trunk/Source/WebKit2/Platform/IPC/Connection.cpp 2016-05-21 23:52:37 UTC (rev 201246)
</span><span class="lines">@@ -432,8 +432,12 @@
</span><span class="cx"> m_waitingForMessage = &waitingForMessage;
</span><span class="cx"> }
</span><span class="cx">
</span><ins>+ // Clamp the timeout to however much time is remaining on our clock.
+ auto now = Condition::Clock::now();
+ auto remainingClockTime = std::chrono::duration_cast<std::chrono::milliseconds>(Condition::Clock::time_point::max() - now);
+ auto absoluteTimeout = now + std::min(remainingClockTime, timeout);
+
</ins><span class="cx"> // Now wait for it to be set.
</span><del>- Condition::Clock::time_point absoluteTimeout = Condition::Clock::now() + timeout;
</del><span class="cx"> while (true) {
</span><span class="cx"> std::unique_lock<Lock> lock(m_waitForMessageMutex);
</span><span class="cx">
</span></span></pre></div>
<a id="trunkSourceWebKit2UIProcessAPICocoaWKWebViewmm"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebKit2/UIProcess/API/Cocoa/WKWebView.mm (201245 => 201246)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebKit2/UIProcess/API/Cocoa/WKWebView.mm 2016-05-21 19:21:35 UTC (rev 201245)
+++ trunk/Source/WebKit2/UIProcess/API/Cocoa/WKWebView.mm 2016-05-21 23:52:37 UTC (rev 201246)
</span><span class="lines">@@ -4550,11 +4550,6 @@
</span><span class="cx"> _page->send(Messages::WebPage::EndPrinting());
</span><span class="cx"> }
</span><span class="cx">
</span><del>-// FIXME: milliseconds::max() overflows when converted to nanoseconds, causing condition_variable::wait_for() to believe
-// a timeout occurred on any spurious wakeup. Use nanoseconds::max() (converted to ms) to avoid this. We should perhaps
-// change waitForAndDispatchImmediately() to take nanoseconds to avoid this issue.
-static constexpr std::chrono::milliseconds didFinishLoadingTimeout = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::nanoseconds::max());
-
</del><span class="cx"> - (CGPDFDocumentRef)_printedDocument
</span><span class="cx"> {
</span><span class="cx"> if ([self _isDisplayingPDF]) {
</span><span class="lines">@@ -4563,7 +4558,7 @@
</span><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> if (_pageIsPrintingToPDF) {
</span><del>- if (!_page->process().connection()->waitForAndDispatchImmediately<Messages::WebPageProxy::DidFinishDrawingPagesToPDF>(_page->pageID(), didFinishLoadingTimeout)) {
</del><ins>+ if (!_page->process().connection()->waitForAndDispatchImmediately<Messages::WebPageProxy::DidFinishDrawingPagesToPDF>(_page->pageID(), std::chrono::milliseconds::max())) {
</ins><span class="cx"> ASSERT_NOT_REACHED();
</span><span class="cx"> return nullptr;
</span><span class="cx"> }
</span></span></pre>
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