7 files changed, 59 insertions, 7 deletions
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index d10704360a6..ea1f2d2993c 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -4284,6 +4284,33 @@ WriteControlFile(void)
 
 	ControlFile->float8ByVal = FLOAT8PASSBYVAL;
 
+	/*
+	 * Initialize the default 'char' signedness.
+	 *
+	 * The signedness of the char type is implementation-defined. For instance
+	 * on x86 architecture CPUs, the char data type is typically treated as
+	 * signed by default, whereas on aarch architecture CPUs, it is typically
+	 * treated as unsigned by default. In v17 or earlier, we accidentally let
+	 * C implementation signedness affect persistent data. This led to
+	 * inconsistent results when comparing char data across different
+	 * platforms.
+	 *
+	 * This flag can be used as a hint to ensure consistent behavior for
+	 * pre-v18 data files that store data sorted by the 'char' type on disk,
+	 * especially in cross-platform replication scenarios.
+	 *
+	 * Newly created database clusters unconditionally set the default char
+	 * signedness to true. pg_upgrade changes this flag for clusters that were
+	 * initialized on signedness=false platforms. As a result,
+	 * signedness=false setting will become rare over time. If we had known
+	 * about this problem during the last development cycle that forced initdb
+	 * (v8.3), we would have made all clusters signed or all clusters
+	 * unsigned. Making pg_upgrade the only source of signedness=false will
+	 * cause the population of database clusters to converge toward that
+	 * retrospective ideal.
+	 */
+	ControlFile->default_char_signedness = true;
+
 	/* Contents are protected with a CRC */
 	INIT_CRC32C(ControlFile->crc);
 	COMP_CRC32C(ControlFile->crc,
@@ -4613,6 +4640,19 @@ DataChecksumsEnabled(void)
 }
 
 /*
+ * Return true if the cluster was initialized on a platform where the
+ * default signedness of char is "signed". This function exists for code
+ * that deals with pre-v18 data files that store data sorted by the 'char'
+ * type on disk (e.g., GIN and GiST indexes). See the comments in
+ * WriteControlFile() for details.
+ */
+bool
+GetDefaultCharSignedness(void)
+{
+	return ControlFile->default_char_signedness;
+}
+
+/*
  * Returns a fake LSN for unlogged relations.
  *
  * Each call generates an LSN that is greater than any previous value
diff --git a/src/backend/utils/misc/pg_controldata.c b/src/backend/utils/misc/pg_controldata.c
index 9dfba499c13..6d036e3bf32 100644
--- a/src/backend/utils/misc/pg_controldata.c
+++ b/src/backend/utils/misc/pg_controldata.c
@@ -203,8 +203,8 @@ pg_control_recovery(PG_FUNCTION_ARGS)
 Datum
 pg_control_init(PG_FUNCTION_ARGS)
 {
-	Datum		values[11];
-	bool		nulls[11];
+	Datum		values[12];
+	bool		nulls[12];
 	TupleDesc	tupdesc;
 	HeapTuple	htup;
 	ControlFileData *ControlFile;
@@ -254,6 +254,9 @@ pg_control_init(PG_FUNCTION_ARGS)
 	values[10] = Int32GetDatum(ControlFile->data_checksum_version);
 	nulls[10] = false;
 
+	values[11] = BoolGetDatum(ControlFile->default_char_signedness);
+	nulls[11] = false;
+
 	htup = heap_form_tuple(tupdesc, values, nulls);
 
 	PG_RETURN_DATUM(HeapTupleGetDatum(htup));
diff --git a/src/bin/pg_controldata/pg_controldata.c b/src/bin/pg_controldata/pg_controldata.c
index cf11ab3f2ee..bea779eef94 100644
--- a/src/bin/pg_controldata/pg_controldata.c
+++ b/src/bin/pg_controldata/pg_controldata.c
@@ -336,6 +336,8 @@ main(int argc, char *argv[])
 		   (ControlFile->float8ByVal ? _("by value") : _("by reference")));
 	printf(_("Data page checksum version:           %u\n"),
 		   ControlFile->data_checksum_version);
+	printf(_("Default char data signedness:         %s\n"),
+		   (ControlFile->default_char_signedness ? _("signed") : _("unsigned")));
 	printf(_("Mock authentication nonce:            %s\n"),
 		   mock_auth_nonce_str);
 	return 0;
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 4411c1468ac..d313099c027 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -231,6 +231,7 @@ extern XLogRecPtr GetXLogWriteRecPtr(void);
 extern uint64 GetSystemIdentifier(void);
 extern char *GetMockAuthenticationNonce(void);
 extern bool DataChecksumsEnabled(void);
+extern bool GetDefaultCharSignedness(void);
 extern XLogRecPtr GetFakeLSNForUnloggedRel(void);
 extern Size XLOGShmemSize(void);
 extern void XLOGShmemInit(void);
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index 1d609f1af47..d179b512be9 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202502211
+#define CATALOG_VERSION_NO	202502212
 
 #endif
diff --git a/src/include/catalog/pg_control.h b/src/include/catalog/pg_control.h
index 3797f25b306..63e834a6ce4 100644
--- a/src/include/catalog/pg_control.h
+++ b/src/include/catalog/pg_control.h
@@ -22,7 +22,7 @@
 
 
 /* Version identifier for this pg_control format */
-#define PG_CONTROL_VERSION	1700
+#define PG_CONTROL_VERSION	1800
 
 /* Nonce key length, see below */
 #define MOCK_AUTH_NONCE_LEN		32
@@ -222,6 +222,12 @@ typedef struct ControlFileData
 	uint32		data_checksum_version;
 
 	/*
+	 * True if the default signedness of char is "signed" on a platform where
+	 * the cluster is initialized.
+	 */
+	bool		default_char_signedness;
+
+	/*
 	 * Random nonce, used in authentication requests that need to proceed
 	 * based on values that are cluster-unique, like a SASL exchange that
 	 * failed at an early stage.
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 9e803d610d7..e2d5c0d0886 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -12206,9 +12206,9 @@
   descr => 'pg_controldata init state information as a function',
   proname => 'pg_control_init', provolatile => 'v', prorettype => 'record',
   proargtypes => '',
-  proallargtypes => '{int4,int4,int4,int4,int4,int4,int4,int4,int4,bool,int4}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{max_data_alignment,database_block_size,blocks_per_segment,wal_block_size,bytes_per_wal_segment,max_identifier_length,max_index_columns,max_toast_chunk_size,large_object_chunk_size,float8_pass_by_value,data_page_checksum_version}',
+  proallargtypes => '{int4,int4,int4,int4,int4,int4,int4,int4,int4,bool,int4,bool}',
+  proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{max_data_alignment,database_block_size,blocks_per_segment,wal_block_size,bytes_per_wal_segment,max_identifier_length,max_index_columns,max_toast_chunk_size,large_object_chunk_size,float8_pass_by_value,data_page_checksum_version,default_char_signedness}',
   prosrc => 'pg_control_init' },
 
 # subscripting support for built-in types