Function InitializeParallelDSM

src/backend/access/transam/parallel.c:238–517

* Establish the dynamic shared memory segment for a parallel context and * copy state and other bookkeeping information that will be needed by * parallel workers into it. */

 * parallel workers into it.
 */
void
InitializeParallelDSM(ParallelContext *pcxt)
{
	MemoryContext oldcontext;
	Size		library_len = 0;
	Size		guc_len = 0;
	Size		combocidlen = 0;
	Size		tsnaplen = 0;
	Size		asnaplen = 0;
	Size		tstatelen = 0;
	Size		pendingsyncslen = 0;
	Size		reindexlen = 0;
	Size		relmapperlen = 0;
	Size		uncommittedenumslen = 0;
	Size		segsize = 0;
	int			i;
	FixedParallelState *fps;
	dsm_handle	session_dsm_handle = DSM_HANDLE_INVALID;
	Snapshot	transaction_snapshot = GetTransactionSnapshot();
	Snapshot	active_snapshot = GetActiveSnapshot();

	if (gp_select_invisible)
		active_snapshot = transaction_snapshot;

	/* We might be running in a very short-lived memory context. */
	oldcontext = MemoryContextSwitchTo(TopTransactionContext);

	/* Allow space to store the fixed-size parallel state. */
	shm_toc_estimate_chunk(&pcxt->estimator, sizeof(FixedParallelState));
	shm_toc_estimate_keys(&pcxt->estimator, 1);

	/*
	 * Normally, the user will have requested at least one worker process, but
	 * if by chance they have not, we can skip a bunch of things here.
	 */
	if (pcxt->nworkers > 0)
	{
		/* Get (or create) the per-session DSM segment's handle. */
		session_dsm_handle = GetSessionDsmHandle();

		/*
		 * If we weren't able to create a per-session DSM segment, then we can
		 * continue but we can't safely launch any workers because their
		 * record typmods would be incompatible so they couldn't exchange
		 * tuples.
		 */
		if (session_dsm_handle == DSM_HANDLE_INVALID)
			pcxt->nworkers = 0;
	}

	if (pcxt->nworkers > 0)
	{
		/* Estimate space for various kinds of state sharing. */
		library_len = EstimateLibraryStateSpace();
		shm_toc_estimate_chunk(&pcxt->estimator, library_len);
		guc_len = EstimateGUCStateSpace();
		shm_toc_estimate_chunk(&pcxt->estimator, guc_len);
		combocidlen = EstimateComboCIDStateSpace();