[idd] capture: keep preparing during publication

Do not freeze the spare retained candidate while a frame is copied to
IVSHMEM. Allow newer source frames to replace that candidate and retain
any frame prepared after the published sequence for the next deadline.

Take the pipeline lock before selecting the publication candidate so its
transport copy is submitted before any newly admitted preparation.

idd/LGIdd/CSwapChainProcessor.cpp

@@ -950,13 +950,20 @@ int CSwapChainProcessor::AcquireCandidate(bool exclusiveSample)
   uint64_t oldest     = UINT64_MAX;
   bool     superseded = false;
   bool     idle       = true;
+  bool     publishing = false;
 
   AcquireSRWLockExclusive(&m_candidateLock);
   for (unsigned i = 0; i < ARRAYSIZE(m_candidates); ++i)
+  {
     if (m_candidates[i].state != CANDIDATE_FREE)
+    {
       idle = false;
+      if (m_candidates[i].state == CANDIDATE_PUBLISHING)
+        publishing = true;
+    }
     else if (selected < 0)
       selected = static_cast<int>(i);
+  }
 
   // Effect timing samples must not queue behind work which can later be
   // superseded, otherwise that discarded work contaminates the sample.
@@ -968,7 +975,11 @@ int CSwapChainProcessor::AcquireCandidate(bool exclusiveSample)
     if (candidate.state == CANDIDATE_READY)
       ++readyCount;
 
-  if (!exclusiveSample && selected < 0 && readyCount > 1)
+  // Preserve one completed fallback unless another candidate is already
+  // publishing. In that case its peer must remain available for new source
+  // frames instead of being frozen for the duration of the transport copy.
+  if (!exclusiveSample && selected < 0 &&
+      readyCount > (publishing ? 0U : 1U))
     for (unsigned i = 0; i < ARRAYSIZE(m_candidates); ++i)
       if (m_candidates[i].state == CANDIDATE_READY &&
           m_candidates[i].sequence < oldest)
@@ -1094,6 +1105,10 @@ bool CSwapChainProcessor::PublishNewestCandidate(
   const CFrameScheduler::Schedule& schedule, bool periodic,
   uint64_t publishStart)
 {
+  // Once a deadline is due, submit transport work before allowing another
+  // preparation to enqueue on the same physical copy queue.
+  CSRWExclusiveLock pipelineLock(&m_pipelineLock);
+
   int      selectedCandidate = -1;
   uint64_t newestSequence    = 0;
 
@@ -1108,11 +1123,8 @@ bool CSwapChainProcessor::PublishNewestCandidate(
     }
 
   if (selectedCandidate >= 0)
-    for (unsigned i = 0; i < ARRAYSIZE(m_candidates); ++i)
-      if (static_cast<int>(i) == selectedCandidate)
-        m_candidates[i].state = CANDIDATE_PUBLISHING;
-      else if (m_candidates[i].state == CANDIDATE_READY)
-        m_candidates[i].state = CANDIDATE_HELD;
+    m_candidates[static_cast<unsigned>(selectedCandidate)].state =
+      CANDIDATE_PUBLISHING;
   ReleaseSRWLockExclusive(&m_candidateLock);
 
   if (selectedCandidate < 0)
@@ -1125,14 +1137,10 @@ bool CSwapChainProcessor::PublishNewestCandidate(
     AcquireSRWLockExclusive(&m_candidateLock);
     if (m_candidates[candidateIndex].state == CANDIDATE_PUBLISHING)
       m_candidates[candidateIndex].state = CANDIDATE_READY;
-    for (FrameCandidate& candidate : m_candidates)
-      if (candidate.state == CANDIDATE_HELD)
-        candidate.state = CANDIDATE_READY;
     ReleaseSRWLockExclusive(&m_candidateLock);
     SignalCandidateState();
   };
 
-  CSRWExclusiveLock pipelineLock(&m_pipelineLock);
   AcquireSRWLockShared(&m_candidateLock);
   const bool candidateValid =
     m_candidates[candidateIndex].state == CANDIDATE_PUBLISHING &&
@@ -1290,11 +1298,6 @@ bool CSwapChainProcessor::PublishNewestCandidate(
     }
     m_devContext->ForceFrame();
 
-    AcquireSRWLockExclusive(&m_candidateLock);
-    for (FrameCandidate& held : m_candidates)
-      if (held.state == CANDIDATE_HELD)
-        held.state = CANDIDATE_READY;
-    ReleaseSRWLockExclusive(&m_candidateLock);
     SignalCandidateState();
     return false;
   }
@@ -1304,10 +1307,11 @@ bool CSwapChainProcessor::PublishNewestCandidate(
 
   unsigned superseded = 0;
   AcquireSRWLockExclusive(&m_candidateLock);
-  for (FrameCandidate& held : m_candidates)
-    if (held.state == CANDIDATE_HELD)
+  for (FrameCandidate& ready : m_candidates)
+    if (ready.state == CANDIDATE_READY &&
+        ready.sequence < candidateSequence)
     {
-      held.state = CANDIDATE_FREE;
+      ready.state = CANDIDATE_FREE;
       ++superseded;
     }
   ReleaseSRWLockExclusive(&m_candidateLock);

idd/LGIdd/CSwapChainProcessor.h

@@ -62,7 +62,6 @@ private:
     CANDIDATE_PREPARING,
     CANDIDATE_READY,
     CANDIDATE_PUBLISHING,
-    CANDIDATE_HELD,
   };
 
   struct FrameCandidate