Add common interface for TBMIterators

Add and use TBMPrivateIterator, which replaces the current TBMIterator
for serial use cases, and repurpose TBMIterator to be a unified
interface for both the serial ("private") and parallel ("shared") TID
Bitmap iterator interfaces. This encapsulation simplifies call sites for
callers supporting both parallel and serial TID Bitmap access.
TBMIterator is not yet used in this commit.

Author: Melanie Plageman
Reviewed-by: Tomas Vondra, Heikki Linnakangas
Discussion: https://postgr.es/m/063e4eb4-32d9-439e-a0b1-75565a9835a8%40iki.fi

1 files changed, 89 insertions, 25 deletions

diff --git a/src/backend/nodes/tidbitmap.c b/src/backend/nodes/tidbitmap.c
index e8ab5d78fcc..687d6f17636 100644
--- a/src/backend/nodes/tidbitmap.c
+++ b/src/backend/nodes/tidbitmap.c
@@ -170,12 +170,12 @@ struct TIDBitmap
 };
 
 /*
- * When iterating over a bitmap in sorted order, a TBMIterator is used to
- * track our progress.  There can be several iterators scanning the same
- * bitmap concurrently.  Note that the bitmap becomes read-only as soon as
- * any iterator is created.
+ * When iterating over a backend-local bitmap in sorted order, a
+ * TBMPrivateIterator is used to track our progress.  There can be several
+ * iterators scanning the same bitmap concurrently.  Note that the bitmap
+ * becomes read-only as soon as any iterator is created.
  */
-struct TBMIterator
+struct TBMPrivateIterator
 {
 	TIDBitmap  *tbm;			/* TIDBitmap we're iterating over */
 	int			spageptr;		/* next spages index */
@@ -213,8 +213,8 @@ typedef struct PTIterationArray
 } PTIterationArray;
 
 /*
- * same as TBMIterator, but it is used for joint iteration, therefore this
- * also holds a reference to the shared state.
+ * same as TBMPrivateIterator, but it is used for joint iteration, therefore
+ * this also holds a reference to the shared state.
  */
 struct TBMSharedIterator
 {
@@ -673,31 +673,32 @@ tbm_is_empty(const TIDBitmap *tbm)
 }
 
 /*
- * tbm_begin_iterate - prepare to iterate through a TIDBitmap
+ * tbm_begin_private_iterate - prepare to iterate through a TIDBitmap
  *
- * The TBMIterator struct is created in the caller's memory context.
- * For a clean shutdown of the iteration, call tbm_end_iterate; but it's
- * okay to just allow the memory context to be released, too.  It is caller's
- * responsibility not to touch the TBMIterator anymore once the TIDBitmap
- * is freed.
+ * The TBMPrivateIterator struct is created in the caller's memory context.
+ * For a clean shutdown of the iteration, call tbm_end_private_iterate; but
+ * it's okay to just allow the memory context to be released, too.  It is
+ * caller's responsibility not to touch the TBMPrivateIterator anymore once
+ * the TIDBitmap is freed.
  *
  * NB: after this is called, it is no longer allowed to modify the contents
  * of the bitmap.  However, you can call this multiple times to scan the
  * contents repeatedly, including parallel scans.
  */
-TBMIterator *
-tbm_begin_iterate(TIDBitmap *tbm)
+TBMPrivateIterator *
+tbm_begin_private_iterate(TIDBitmap *tbm)
 {
-	TBMIterator *iterator;
+	TBMPrivateIterator *iterator;
 
 	Assert(tbm->iterating != TBM_ITERATING_SHARED);
 
 	/*
-	 * Create the TBMIterator struct, with enough trailing space to serve the
-	 * needs of the TBMIterateResult sub-struct.
+	 * Create the TBMPrivateIterator struct, with enough trailing space to
+	 * serve the needs of the TBMIterateResult sub-struct.
 	 */
-	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-									  MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+	iterator = (TBMPrivateIterator *) palloc(sizeof(TBMPrivateIterator) +
+											 MAX_TUPLES_PER_PAGE *
+											 sizeof(OffsetNumber));
 	iterator->tbm = tbm;
 
 	/*
@@ -878,7 +879,7 @@ tbm_prepare_shared_iterate(TIDBitmap *tbm)
 	ptchunks = dsa_get_address(tbm->dsa, tbm->ptchunks);
 
 	/*
-	 * For every shared iterator, referring to pagetable and iterator array,
+	 * For every shared iterator referring to pagetable and iterator array,
 	 * increase the refcount by 1 so that while freeing the shared iterator we
 	 * don't free pagetable and iterator array until its refcount becomes 0.
 	 */
@@ -956,7 +957,7 @@ tbm_advance_schunkbit(PagetableEntry *chunk, int *schunkbitp)
 }
 
 /*
- * tbm_iterate - scan through next page of a TIDBitmap
+ * tbm_private_iterate - scan through next page of a TIDBitmap
  *
  * Returns a TBMIterateResult representing one page, or NULL if there are
  * no more pages to scan.  Pages are guaranteed to be delivered in numerical
@@ -968,7 +969,7 @@ tbm_advance_schunkbit(PagetableEntry *chunk, int *schunkbitp)
  * testing, recheck is always set true when ntuples < 0.)
  */
 TBMIterateResult *
-tbm_iterate(TBMIterator *iterator)
+tbm_private_iterate(TBMPrivateIterator *iterator)
 {
 	TIDBitmap  *tbm = iterator->tbm;
 	TBMIterateResult *output = &(iterator->output);
@@ -1136,14 +1137,14 @@ tbm_shared_iterate(TBMSharedIterator *iterator)
 }
 
 /*
- * tbm_end_iterate - finish an iteration over a TIDBitmap
+ * tbm_end_private_iterate - finish an iteration over a TIDBitmap
  *
  * Currently this is just a pfree, but it might do more someday.  (For
  * instance, it could be useful to count open iterators and allow the
  * bitmap to return to read/write status when there are no more iterators.)
  */
 void
-tbm_end_iterate(TBMIterator *iterator)
+tbm_end_private_iterate(TBMPrivateIterator *iterator)
 {
 	pfree(iterator);
 }
@@ -1556,3 +1557,66 @@ tbm_calculate_entries(double maxbytes)
 
 	return nbuckets;
 }
+
+/*
+ * Create a shared or private bitmap iterator and start iteration.
+ *
+ * `tbm` is only used to create the private iterator and dsa and dsp are only
+ * used to create the shared iterator.
+ *
+ * Before invoking tbm_begin_iterate() to create a shared iterator, one
+ * process must already have invoked tbm_prepare_shared_iterate() to create
+ * and set up the TBMSharedIteratorState.
+ */
+TBMIterator
+tbm_begin_iterate(TIDBitmap *tbm, dsa_area *dsa, dsa_pointer dsp)
+{
+	TBMIterator iterator = {0};
+
+	/* Allocate a private iterator and attach the shared state to it */
+	if (DsaPointerIsValid(dsp))
+	{
+		iterator.shared = true;
+		iterator.i.shared_iterator = tbm_attach_shared_iterate(dsa, dsp);
+	}
+	else
+	{
+		iterator.shared = false;
+		iterator.i.private_iterator = tbm_begin_private_iterate(tbm);
+	}
+
+	return iterator;
+}
+
+/*
+ * Clean up shared or private bitmap iterator.
+ */
+void
+tbm_end_iterate(TBMIterator *iterator)
+{
+	Assert(iterator);
+
+	if (iterator->shared)
+		tbm_end_shared_iterate(iterator->i.shared_iterator);
+	else
+		tbm_end_private_iterate(iterator->i.private_iterator);
+
+	*iterator = (TBMIterator)
+	{
+		0
+	};
+}
+
+/*
+ * Get the next TBMIterateResult from the shared or private bitmap iterator.
+ */
+TBMIterateResult *
+tbm_iterate(TBMIterator *iterator)
+{
+	Assert(iterator);
+
+	if (iterator->shared)
+		return tbm_shared_iterate(iterator->i.shared_iterator);
+	else
+		return tbm_private_iterate(iterator->i.private_iterator);
+}