@@ -2518,13 +2518,40 @@ _dispatch_operation_perform(dispatch_operation_t op)
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NTSTATUS status = _dispatch_NtQueryInformationFile (hFile ,
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& iosb , & fpli , sizeof (fpli ), FilePipeLocalInformation );
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if (NT_SUCCESS (status )) {
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- // WriteQuotaAvailable is unreliable in the presence
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- // of a blocking reader, when it can return zero, so only
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- // account for it otherwise
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- if (fpli .WriteQuotaAvailable > 0 ) {
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- len = MIN (len , fpli .WriteQuotaAvailable );
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+ // WriteQuotaAvailable is the free space in the output buffer
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+ // that has not already been reserved for reading. In other words,
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+ // WriteQuotaAvailable =
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+ // OutboundQuota - WriteQuotaUsed - QueuedReadSize.
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+ // It is not documented that QueuedReadSize is part of this
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+ // calculation, but this behavior has been observed experimentally.
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+ // Unfortunately, this means that it is not possible to distinguish
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+ // between a full output buffer and a reader blocked waiting for a
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+ // full buffer's worth of data. This is a problem because if the
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+ // output buffer is full and no reader is waiting for data, then
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+ // attempting to write to the buffer of a PIPE_WAIT, non-
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+ // overlapped I/O pipe will block the dispatch queue thread.
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+ //
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+ // In order to work around this idiosyncrasy, we bound the size of
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+ // the write to be OutboundQuota - 1. This affords us a sentinel value
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+ // in WriteQuotaAvailable that can be used to detect if a reader is
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+ // making progress or not.
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+ // WriteQuotaAvailable = 0 => a reader is blocked waiting for data.
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+ // WriteQuotaAvailable = 1 => the pipe has been written to, but no
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+ // reader is making progress.
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+ // When we detect that WriteQuotaAvailable == 1, we write 0 bytes to
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+ // avoid blocking the dispatch queue thread.
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+ if (fpli .WriteQuotaAvailable == 0 ) {
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+ // This condition can only occur when we have a reader blocked
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+ // waiting for data on the pipe. In this case, write a full
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+ // buffer's worth of data (less one byte to preserve this
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+ // sentinel value of WriteQuotaAvailable == 0).
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+ len = MIN (len , fpli .OutboundQuota - 1 );
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+ } else {
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+ // Subtract 1 from WriteQuotaAvailable to ensure we do not fill
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+ // the pipe and preserve the sentinel value of
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+ // WriteQuotaAvailable == 1.
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+ len = MIN (len , fpli .WriteQuotaAvailable - 1 );
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}
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- len = MIN (len , fpli .OutboundQuota );
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}
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OVERLAPPED ovlOverlapped = {};
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