vcl: fix hangs in SvpSalInstance

Since commit cb8bfa9a32799bcde4e960fa56e388d5f7b2a928 the main thread
will read from the timer pipe until it is empty. But evidently this
introduces the problem that the poll() in another thread will not
return, as the file descriptor will no longer be readable; see
https://paste.debian.net/1011306/ for a reproducer of that rather
under-documented poll behaviour.  So other threads can get stuck
forever in poll, and then the main thread can block in poll too with
no other thread to wake it up.  This is the problem that plagues
UITest_writerperfect_epubexport.

The timer pipe is difficult to fix, since the main thread can block on
either the poll or the subsequent AcquireYieldMutex().

So replace the timer pipe with a condition etc. that is mostly
copied from the OSX AquaSalInstance/SalYieldMutex implementation.

The main thread now does something different than the other threads,
and blocks on a condition_variable with a timeout, while other
threads still block on acquiring the mutex.

Non-main threads can poke the main thread to do a DoYield()
on their behalf, and then get the result back with a blocking
read from a pipe, all while holding the YieldMutex.  This
requires some fudging of the YieldMutex so that the main
thread can borrow the ownership temporarily.

Unfortunately SvpSalInstance, in addition to being directly
instantiable for --headless, has a whole bunch of subclasses:

* HeadlessSalInstance
* AndroidSalInstance
* IosSalInstance
* GtkInstance (in the gtk3 case)
* KDE5SalInstance

Of these GtkInstance overrides everything related to the
DoYield/SalYieldMutex implementation, but the others will be
affected by the change.

This commit will probably break IOS due to me not understanding the
point of the undocumented random #ifdef IOS in svpinst.cxx.

Change-Id: I1bbb143952dda89579e97ac32cd147e5b987573c
Reviewed-on: https://gerrit.libreoffice.org/50237
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

1 files changed, 43 insertions, 5 deletions

diff --git a/vcl/inc/headless/svpinst.hxx b/vcl/inc/headless/svpinst.hxx
index ba80960326dc..6027b2b4d683 100644
--- a/vcl/inc/headless/svpinst.hxx
+++ b/vcl/inc/headless/svpinst.hxx
@@ -22,6 +22,7 @@
 
 #include <osl/mutex.hxx>
 #include <osl/thread.hxx>
+#include <osl/conditn.hxx>
 #include <salinst.hxx>
 #include <salwtype.hxx>
 #include <saltimer.hxx>
@@ -30,6 +31,7 @@
 #include <unx/genprn.h>
 
 #include <list>
+#include <condition_variable>
 
 #include <time.h>
 
@@ -56,19 +58,55 @@ public:
 class SvpSalFrame;
 class GenPspGraphics;
 
+enum class SvpRequest
+{
+    NONE,
+    MainThreadDispatchOneEvent,
+    MainThreadDispatchAllEvents,
+};
+
+class SvpSalYieldMutex : public SalYieldMutex
+{
+private:
+    // note: these members might as well live in SvpSalInstance, but there is
+    // at least one subclass of SvpSalInstance (GTK3) that doesn't use them.
+    friend class SvpSalInstance;
+    // members for communication from main thread to non-main thread
+    int                     m_FeedbackFDs[2];
+    osl::Condition          m_NonMainWaitingYieldCond;
+    // members for communication from non-main thread to main thread
+    bool                    m_bNoYieldLock = false; // accessed only on main thread
+    std::mutex              m_WakeUpMainMutex; // guard m_wakeUpMain & m_Request
+    std::condition_variable m_WakeUpMainCond;
+    bool                    m_wakeUpMain = false;
+    SvpRequest              m_Request = SvpRequest::NONE;
+
+protected:
+    virtual void            doAcquire( sal_uInt32 nLockCount ) override;
+    virtual sal_uInt32      doRelease( bool bUnlockAll ) override;
+
+public:
+    SvpSalYieldMutex();
+    virtual ~SvpSalYieldMutex() override;
+
+    virtual bool IsCurrentThread() const override;
+
+};
+
 SalInstance* svp_create_SalInstance();
 
+// NOTE: the functions IsMainThread, DoYield and Wakeup *require* the use of
+// SvpSalYieldMutex; if a subclass uses something else it must override these
+// (Wakeup is only called by SvpSalTimer and SvpSalFrame)
 class VCL_DLLPUBLIC SvpSalInstance : public SalGenericInstance, public SalUserEventList
 {
     timeval                 m_aTimeout;
     sal_uLong               m_nTimeoutMS;
-    int                     m_pTimeoutFDS[2];
-
-    void                    DoReleaseYield( int nTimeoutMS );
+    oslThreadIdentifier     m_MainThread;
 
     virtual void            TriggerUserEventProcessing() override;
     virtual void            ProcessEvent( SalUserEvent aEvent ) override;
-    void                    Wakeup();
+    void                    Wakeup(SvpRequest request = SvpRequest::NONE);
 
 public:
     static SvpSalInstance*  s_pDefaultInstance;
@@ -132,7 +170,7 @@ public:
     // and timer
     virtual bool            DoYield(bool bWait, bool bHandleAllCurrentEvents) override;
     virtual bool            AnyInput( VclInputFlags nType ) override;
-    virtual bool            IsMainThread() const override { return true; }
+    virtual bool            IsMainThread() const override;
 
     // may return NULL to disable session management
     virtual SalSession*     CreateSalSession() override;