From 0207ea0cf1350b7db12441c7073b81995ebc27e3 Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Wed, 12 Mar 2014 13:25:40 -0400
Subject: [PATCH] Add FreePageManagerInquireLargest.

---
 contrib/test_freepage/test_freepage--1.0.sql |  2 +
 contrib/test_freepage/test_freepage.c        | 11 +++++
 src/backend/utils/mmgr/freepage.c            | 44 ++++++++++++++++++++
 src/include/utils/freepage.h                 |  1 +
 4 files changed, 58 insertions(+)

diff --git a/contrib/test_freepage/test_freepage--1.0.sql b/contrib/test_freepage/test_freepage--1.0.sql
index 490c662dca..5d3191e88b 100644
--- a/contrib/test_freepage/test_freepage--1.0.sql
+++ b/contrib/test_freepage/test_freepage--1.0.sql
@@ -7,6 +7,8 @@ CREATE FUNCTION init(size pg_catalog.int8) RETURNS pg_catalog.void
 	AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
 CREATE FUNCTION get(pages pg_catalog.int8) RETURNS pg_catalog.int8
 	AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
+CREATE FUNCTION inquire_largest() RETURNS pg_catalog.int8
+	AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
 CREATE FUNCTION put(first_page pg_catalog.int8, npages pg_catalog.int8)
 	RETURNS pg_catalog.void AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
 CREATE FUNCTION dump() RETURNS pg_catalog.text
diff --git a/contrib/test_freepage/test_freepage.c b/contrib/test_freepage/test_freepage.c
index 9aea03a813..074cf56891 100644
--- a/contrib/test_freepage/test_freepage.c
+++ b/contrib/test_freepage/test_freepage.c
@@ -21,11 +21,13 @@
 PG_MODULE_MAGIC;
 PG_FUNCTION_INFO_V1(init);
 PG_FUNCTION_INFO_V1(get);
+PG_FUNCTION_INFO_V1(inquire_largest);
 PG_FUNCTION_INFO_V1(put);
 PG_FUNCTION_INFO_V1(dump);
 
 Datum		init(PG_FUNCTION_ARGS);
 Datum		get(PG_FUNCTION_ARGS);
+Datum		inquire_largest(PG_FUNCTION_ARGS);
 Datum		put(PG_FUNCTION_ARGS);
 Datum		dump(PG_FUNCTION_ARGS);
 
@@ -81,6 +83,15 @@ get(PG_FUNCTION_ARGS)
 	PG_RETURN_INT64(first_page);
 }
 
+Datum
+inquire_largest(PG_FUNCTION_ARGS)
+{
+	if (fpm == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(FreePageManagerInquireLargest(fpm));
+}
+
 Datum
 put(PG_FUNCTION_ARGS)
 {
diff --git a/src/backend/utils/mmgr/freepage.c b/src/backend/utils/mmgr/freepage.c
index 2c1845a79b..87fbfb8841 100644
--- a/src/backend/utils/mmgr/freepage.c
+++ b/src/backend/utils/mmgr/freepage.c
@@ -183,6 +183,50 @@ FreePageManagerGet(FreePageManager *fpm, Size npages, Size *first_page)
 	return result;
 }
 
+/*
+ * Return the size of the largest run of pages that the user could
+ * succesfully get.
+ */
+Size
+FreePageManagerInquireLargest(FreePageManager *fpm)
+{
+	LWLock *lock = fpm_lock(fpm);
+	char   *base = fpm_segment_base(fpm);
+	Size	largest = 0;
+
+	if (lock != NULL)
+		LWLockAcquire(lock, LW_EXCLUSIVE);
+
+	if (!relptr_is_null(fpm->freelist[FPM_NUM_FREELISTS - 1]))
+	{
+		FreePageSpanLeader *candidate;
+
+		candidate = relptr_access(base, fpm->freelist[FPM_NUM_FREELISTS - 1]);
+		do
+		{
+			if (candidate->npages > largest)
+				largest = candidate->npages;
+			candidate = relptr_access(base, candidate->next);
+		} while (candidate != NULL);
+	}
+	else
+	{
+		Size	f = FPM_NUM_FREELISTS - 1;
+
+		do
+		{
+			--f;
+			if (!relptr_is_null(fpm->freelist[f]))
+				largest = f + 1;
+		} while (f > 0);
+	}
+
+	if (lock != NULL)
+		LWLockRelease(lock);
+
+	return largest;
+}
+
 /*
  * Transfer a run of pages to the free page manager.
  */
diff --git a/src/include/utils/freepage.h b/src/include/utils/freepage.h
index 51ba40a84d..df25464c1e 100644
--- a/src/include/utils/freepage.h
+++ b/src/include/utils/freepage.h
@@ -90,6 +90,7 @@ extern bool FreePageManagerGet(FreePageManager *fpm, Size npages,
 						Size *first_page);
 extern void FreePageManagerPut(FreePageManager *fpm, Size first_page,
 						Size npages);
+extern Size FreePageManagerInquireLargest(FreePageManager *fpm);
 extern char *FreePageManagerDump(FreePageManager *fpm);
 
 #endif   /* FREEPAGE_H */
-- 
2.39.5