Refactor scanner callback interface

This change adds proper result types for the scanner's filter and
tuple handling callbacks. Previously, these callbacks were supposed to
return bool, which was hard to interpret. For instance, for the tuple
handler callback, true meant continue processing the next tuple while
false meant finish the scan. However, this wasn't always clear. Having
proper return types also makes it easier to see from a function's
signature that it is a scanner callback handler, rather than some
other function that can be called directly.

src/bgw/job.c

@@ -96,7 +96,7 @@ typedef struct AccumData
 	size_t		alloc_size;
 } AccumData;
 
-static bool
+static ScanTupleResult
 bgw_job_accum_tuple_found(TupleInfo *ti, void *data)
 {
 	AccumData  *list_data = data;
@@ -106,7 +106,7 @@ bgw_job_accum_tuple_found(TupleInfo *ti, void *data)
 	list_data->list = lappend(list_data->list, job);
 
 	MemoryContextSwitchTo(orig);
-	return true;
+	return SCAN_CONTINUE;
 }
 
 
@@ -166,15 +166,18 @@ bgw_job_scan_job_id(int32 bgw_job_id, tuple_found_func tuple_found, tuple_filter
 					 scankey, 1, tuple_found, tuple_filter, data, mctx, lockmode);
 }
 
-static bool
+static ScanTupleResult
 bgw_job_tuple_found(TupleInfo *ti, void *const data)
 {
 	BgwJob	  **job_pp = data;
 
 	*job_pp = bgw_job_from_tuple(ti->tuple, sizeof(BgwJob), ti->mctx);
 
-	/* return true since used with scan one */
-	return true;
+	/*
+	 * Return SCAN_CONTINUE because we check for multiple tuples as an error
+	 * condition.
+	 */
+	return SCAN_CONTINUE;
 }
 
 BgwJob *
@@ -227,7 +230,7 @@ ts_bgw_job_insert_relation(PG_FUNCTION_ARGS)
 	PG_RETURN_NULL();
 }
 
-static bool
+static ScanTupleResult
 bgw_job_tuple_delete(TupleInfo *ti, void *data)
 {
 	CatalogSecurityContext sec_ctx;
@@ -239,7 +242,7 @@ bgw_job_tuple_delete(TupleInfo *ti, void *data)
 	catalog_delete(ti->scanrel, ti->tuple);
 	catalog_restore_user(&sec_ctx);
 
-	return true;
+	return SCAN_CONTINUE;
 }
 
 static bool

src/bgw/job_stat.c

@@ -36,15 +36,18 @@ bgw_job_stat_next_start_was_set(FormData_bgw_job_stat *fd)
 }
 
 
-static bool
+static ScanTupleResult
 bgw_job_stat_tuple_found(TupleInfo *ti, void *const data)
 {
 	BgwJobStat **job_stat_pp = data;
 
 	*job_stat_pp = bgw_job_stat_from_tuple(ti->tuple, ti->mctx);
 
-	/* return true because used with scan_one */
-	return true;
+	/*
+	 * Return SCAN_CONTINUE because we check for multiple tuples as an error
+	 * condition.
+	 */
+	return SCAN_CONTINUE;
 }
 
 static bool
@@ -89,11 +92,12 @@ bgw_job_stat_find(int32 bgw_job_id)
 	return job_stat;
 }
 
-static bool
+static ScanTupleResult
 bgw_job_stat_tuple_delete(TupleInfo *ti, void *const data)
 {
 	catalog_delete(ti->scanrel, ti->tuple);
-	return true;
+
+	return SCAN_CONTINUE;
 }
 
 void
@@ -109,7 +113,7 @@ bgw_job_stat_delete(int32 bgw_job_id)
 * instance can write once a crash happened in any job. Therefore our only choice is to deduce
 * a crash from the lack of a write (the marked end write in this case).
 */
-static bool
+static ScanTupleResult
 bgw_job_stat_tuple_mark_start(TupleInfo *ti, void *const data)
 {
 	HeapTuple	tuple = heap_copytuple(ti->tuple);
@@ -142,8 +146,7 @@ bgw_job_stat_tuple_mark_start(TupleInfo *ti, void *const data)
 	catalog_update(ti->scanrel, tuple);
 	heap_freetuple(tuple);
 
-	/* scans with catalog_update must return false */
-	return false;
+	return SCAN_DONE;
 }
 
 
@@ -204,7 +207,7 @@ calculate_next_start_on_crash(int consecutive_crashes, BgwJob *job)
 	return failure_calc;
 }
 
-static bool
+static ScanTupleResult
 bgw_job_stat_tuple_mark_end(TupleInfo *ti, void *const data)
 {
 	JobResultCtx *result_ctx = data;
@@ -246,11 +249,10 @@ bgw_job_stat_tuple_mark_end(TupleInfo *ti, void *const data)
 	catalog_update(ti->scanrel, tuple);
 	heap_freetuple(tuple);
 
-	/* scans with catalog_update must return false */
-	return false;
+	return SCAN_DONE;
 }
 
-static bool
+static ScanTupleResult
 bgw_job_stat_tuple_set_next_start(TupleInfo *ti, void *const data)
 {
 	TimestampTz *next_start = data;
@@ -262,8 +264,7 @@ bgw_job_stat_tuple_set_next_start(TupleInfo *ti, void *const data)
 	catalog_update(ti->scanrel, tuple);
 	heap_freetuple(tuple);
 
-	/* scans with catalog_update must return false */
-	return false;
+	return SCAN_DONE;
 }
 
 static bool