Move SLRU_PAGES_PER_SEGMENT to pg_config_manual.h

It seems plausible that someone might want to experiment with
different values. The pressing reason though is that I'm reviewing a
patch that requires pg_upgrade to manipulate SLRU files. That patch
needs to access SLRU_PAGES_PER_SEGMENT from pg_upgrade code, and
slru.h, where SLRU_PAGES_PER_SEGMENT is currently defined, cannot be
included from frontend code. Moving it to pg_config_manual.h makes it
accessible.

Now that it's a little more likely that someone might change
SLRU_PAGES_PER_SEGMENT, add a cluster compatibility check for it.

Bump catalog version because of the new field in the control file.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://www.postgresql.org/message-id/c7a4ea90-9f7b-4953-81be-b3fcb47db057@iki.fi

src/backend/access/transam/xlog.c

@@ -4271,6 +4271,7 @@ WriteControlFile(void)
 
 	ControlFile->blcksz = BLCKSZ;
 	ControlFile->relseg_size = RELSEG_SIZE;
+	ControlFile->slru_pages_per_segment = SLRU_PAGES_PER_SEGMENT;
 	ControlFile->xlog_blcksz = XLOG_BLCKSZ;
 	ControlFile->xlog_seg_size = wal_segment_size;
 
@@ -4490,6 +4491,16 @@ ReadControlFile(void)
 						   "RELSEG_SIZE", ControlFile->relseg_size,
 						   "RELSEG_SIZE", RELSEG_SIZE),
 				 errhint("It looks like you need to recompile or initdb.")));
+	if (ControlFile->slru_pages_per_segment != SLRU_PAGES_PER_SEGMENT)
+		ereport(FATAL,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("database files are incompatible with server"),
+		/* translator: %s is a variable name and %d is its value */
+				 errdetail("The database cluster was initialized with %s %d,"
+						   " but the server was compiled with %s %d.",
+						   "SLRU_PAGES_PER_SEGMENT", ControlFile->slru_pages_per_segment,
+						   "SLRU_PAGES_PER_SEGMENT", SLRU_PAGES_PER_SEGMENT),
+				 errhint("It looks like you need to recompile or initdb.")));
 	if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
 		ereport(FATAL,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),

src/include/access/slru.h

@@ -23,21 +23,6 @@
  */
 #define SLRU_MAX_ALLOWED_BUFFERS ((1024 * 1024 * 1024) / BLCKSZ)
 
-/*
- * Define SLRU segment size.  A page is the same BLCKSZ as is used everywhere
- * else in Postgres.  The segment size can be chosen somewhat arbitrarily;
- * we make it 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG
- * or 64K transactions for SUBTRANS.
- *
- * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
- * page numbering also wraps around at 0xFFFFFFFF/xxxx_XACTS_PER_PAGE (where
- * xxxx is CLOG or SUBTRANS, respectively), and segment numbering at
- * 0xFFFFFFFF/xxxx_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need
- * take no explicit notice of that fact in slru.c, except when comparing
- * segment and page numbers in SimpleLruTruncate (see PagePrecedes()).
- */
-#define SLRU_PAGES_PER_SEGMENT	32
-
 /*
  * Page status codes.  Note that these do not include the "dirty" bit.
  * page_dirty can be true only in the VALID or WRITE_IN_PROGRESS states;

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202511071
+#define CATALOG_VERSION_NO	202511101
 
 #endif

src/include/catalog/pg_control.h

@@ -207,6 +207,8 @@ typedef struct ControlFileData
 	uint32		blcksz;			/* data block size for this DB */
 	uint32		relseg_size;	/* blocks per segment of large relation */
 
+	uint32		slru_pages_per_segment; /* size of each SLRU segment */
+
 	uint32		xlog_blcksz;	/* block size within WAL files */
 	uint32		xlog_seg_size;	/* size of each WAL segment */
 

src/include/pg_config_manual.h

@@ -19,6 +19,16 @@
  */
 #define DEFAULT_XLOG_SEG_SIZE	(16*1024*1024)
 
+/*
+ * SLRU segment size.  A page is the same BLCKSZ as is used everywhere else in
+ * Postgres.  The segment size can be chosen somewhat arbitrarily; we make it
+ * 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG or 64K
+ * transactions for SUBTRANS.
+ *
+ * Changing this requires an initdb.
+ */
+#define SLRU_PAGES_PER_SEGMENT	32
+
 /*
  * Maximum length for identifiers (e.g. table names, column names,
  * function names).  Names actually are limited to one fewer byte than this,