Use Enum for top level logical replication message types.

Logical replication protocol uses a single byte character to identify a
message type in logical replication protocol. The code uses string
literals for the same. Use Enum so that

1. All the string literals used can be found at a single place. This
makes it easy to add more types without the risk of conflicts.

2. It's easy to locate the code handling a given message type.

3. When used with switch statements, it is easy to identify the missing
cases using -Wswitch.

Author: Ashutosh Bapat
Reviewed-by: Kyotaro Horiguchi, Andres Freund, Peter Smith and Amit Kapila
Discussion: https://postgr.es/m/CAExHW5uPzQ7L0oAd_ENyvaiYMOPgkrAoJpE+ZY5-obdcVT6NPg@mail.gmail.com

src/backend/replication/logical/proto.c

@@ -44,7 +44,7 @@ static const char *logicalrep_read_namespace(StringInfo in);
 void
 logicalrep_write_begin(StringInfo out, ReorderBufferTXN *txn)
 {
-	pq_sendbyte(out, 'B');		/* BEGIN */
+	pq_sendbyte(out, LOGICAL_REP_MSG_BEGIN);
 
 	/* fixed fields */
 	pq_sendint64(out, txn->final_lsn);
@@ -76,7 +76,7 @@ logicalrep_write_commit(StringInfo out, ReorderBufferTXN *txn,
 {
 	uint8		flags = 0;
 
-	pq_sendbyte(out, 'C');		/* sending COMMIT */
+	pq_sendbyte(out, LOGICAL_REP_MSG_COMMIT);
 
 	/* send the flags field (unused for now) */
 	pq_sendbyte(out, flags);
@@ -112,7 +112,7 @@ void
 logicalrep_write_origin(StringInfo out, const char *origin,
 						XLogRecPtr origin_lsn)
 {
-	pq_sendbyte(out, 'O');		/* ORIGIN */
+	pq_sendbyte(out, LOGICAL_REP_MSG_ORIGIN);
 
 	/* fixed fields */
 	pq_sendint64(out, origin_lsn);
@@ -141,7 +141,7 @@ void
 logicalrep_write_insert(StringInfo out, TransactionId xid, Relation rel,
 						HeapTuple newtuple, bool binary)
 {
-	pq_sendbyte(out, 'I');		/* action INSERT */
+	pq_sendbyte(out, LOGICAL_REP_MSG_INSERT);
 
 	/* transaction ID (if not valid, we're not streaming) */
 	if (TransactionIdIsValid(xid))
@@ -185,7 +185,7 @@ void
 logicalrep_write_update(StringInfo out, TransactionId xid, Relation rel,
 						HeapTuple oldtuple, HeapTuple newtuple, bool binary)
 {
-	pq_sendbyte(out, 'U');		/* action UPDATE */
+	pq_sendbyte(out, LOGICAL_REP_MSG_UPDATE);
 
 	Assert(rel->rd_rel->relreplident == REPLICA_IDENTITY_DEFAULT ||
 		   rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL ||
@@ -263,7 +263,7 @@ logicalrep_write_delete(StringInfo out, TransactionId xid, Relation rel,
 		   rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL ||
 		   rel->rd_rel->relreplident == REPLICA_IDENTITY_INDEX);
 
-	pq_sendbyte(out, 'D');		/* action DELETE */
+	pq_sendbyte(out, LOGICAL_REP_MSG_DELETE);
 
 	/* transaction ID (if not valid, we're not streaming) */
 	if (TransactionIdIsValid(xid))
@@ -317,7 +317,7 @@ logicalrep_write_truncate(StringInfo out,
 	int			i;
 	uint8		flags = 0;
 
-	pq_sendbyte(out, 'T');		/* action TRUNCATE */
+	pq_sendbyte(out, LOGICAL_REP_MSG_TRUNCATE);
 
 	/* transaction ID (if not valid, we're not streaming) */
 	if (TransactionIdIsValid(xid))
@@ -369,7 +369,7 @@ logicalrep_write_rel(StringInfo out, TransactionId xid, Relation rel)
 {
 	char	   *relname;
 
-	pq_sendbyte(out, 'R');		/* sending RELATION */
+	pq_sendbyte(out, LOGICAL_REP_MSG_RELATION);
 
 	/* transaction ID (if not valid, we're not streaming) */
 	if (TransactionIdIsValid(xid))
@@ -425,7 +425,7 @@ logicalrep_write_typ(StringInfo out, TransactionId xid, Oid typoid)
 	HeapTuple	tup;
 	Form_pg_type typtup;
 
-	pq_sendbyte(out, 'Y');		/* sending TYPE */
+	pq_sendbyte(out, LOGICAL_REP_MSG_TYPE);
 
 	/* transaction ID (if not valid, we're not streaming) */
 	if (TransactionIdIsValid(xid))
@@ -755,7 +755,7 @@ void
 logicalrep_write_stream_start(StringInfo out,
 							  TransactionId xid, bool first_segment)
 {
-	pq_sendbyte(out, 'S');		/* action STREAM START */
+	pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_START);
 
 	Assert(TransactionIdIsValid(xid));
 
@@ -788,7 +788,7 @@ logicalrep_read_stream_start(StringInfo in, bool *first_segment)
 void
 logicalrep_write_stream_stop(StringInfo out)
 {
-	pq_sendbyte(out, 'E');		/* action STREAM END */
+	pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_END);
 }
 
 /*
@@ -800,7 +800,7 @@ logicalrep_write_stream_commit(StringInfo out, ReorderBufferTXN *txn,
 {
 	uint8		flags = 0;
 
-	pq_sendbyte(out, 'c');		/* action STREAM COMMIT */
+	pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_COMMIT);
 
 	Assert(TransactionIdIsValid(txn->xid));
 
@@ -849,7 +849,7 @@ void
 logicalrep_write_stream_abort(StringInfo out, TransactionId xid,
 							  TransactionId subxid)
 {
-	pq_sendbyte(out, 'A');		/* action STREAM ABORT */
+	pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_ABORT);
 
 	Assert(TransactionIdIsValid(xid) && TransactionIdIsValid(subxid));
 

src/backend/replication/logical/worker.c

@@ -1896,67 +1896,66 @@ apply_handle_truncate(StringInfo s)
 static void
 apply_dispatch(StringInfo s)
 {
-	char		action = pq_getmsgbyte(s);
+	LogicalRepMsgType action = pq_getmsgbyte(s);
 
 	switch (action)
 	{
-			/* BEGIN */
-		case 'B':
+		case LOGICAL_REP_MSG_BEGIN:
 			apply_handle_begin(s);
-			break;
-			/* COMMIT */
-		case 'C':
+			return;
+
+		case LOGICAL_REP_MSG_COMMIT:
 			apply_handle_commit(s);
-			break;
-			/* INSERT */
-		case 'I':
+			return;
+
+		case LOGICAL_REP_MSG_INSERT:
 			apply_handle_insert(s);
-			break;
-			/* UPDATE */
-		case 'U':
+			return;
+
+		case LOGICAL_REP_MSG_UPDATE:
 			apply_handle_update(s);
-			break;
-			/* DELETE */
-		case 'D':
+			return;
+
+		case LOGICAL_REP_MSG_DELETE:
 			apply_handle_delete(s);
-			break;
-			/* TRUNCATE */
-		case 'T':
+			return;
+
+		case LOGICAL_REP_MSG_TRUNCATE:
 			apply_handle_truncate(s);
-			break;
-			/* RELATION */
-		case 'R':
+			return;
+
+		case LOGICAL_REP_MSG_RELATION:
 			apply_handle_relation(s);
-			break;
-			/* TYPE */
-		case 'Y':
+			return;
+
+		case LOGICAL_REP_MSG_TYPE:
 			apply_handle_type(s);
-			break;
-			/* ORIGIN */
-		case 'O':
+			return;
+
+		case LOGICAL_REP_MSG_ORIGIN:
 			apply_handle_origin(s);
-			break;
-			/* STREAM START */
-		case 'S':
+			return;
+
+		case LOGICAL_REP_MSG_STREAM_START:
 			apply_handle_stream_start(s);
-			break;
-			/* STREAM END */
-		case 'E':
+			return;
+
+		case LOGICAL_REP_MSG_STREAM_END:
 			apply_handle_stream_stop(s);
-			break;
-			/* STREAM ABORT */
-		case 'A':
+			return;
+
+		case LOGICAL_REP_MSG_STREAM_ABORT:
 			apply_handle_stream_abort(s);
-			break;
-			/* STREAM COMMIT */
-		case 'c':
+			return;
+
+		case LOGICAL_REP_MSG_STREAM_COMMIT:
 			apply_handle_stream_commit(s);
-			break;
-		default:
+			return;
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_PROTOCOL_VIOLATION),
 			 errmsg("invalid logical replication message type \"%c\"", action)));
-	}
 }
 
 /*

src/include/replication/logicalproto.h

@@ -33,6 +33,33 @@
 #define LOGICALREP_PROTO_STREAM_VERSION_NUM 2
 #define LOGICALREP_PROTO_MAX_VERSION_NUM LOGICALREP_PROTO_STREAM_VERSION_NUM
 
+/*
+ * Logical message types
+ *
+ * Used by logical replication wire protocol.
+ *
+ * Note: though this is an enum, the values are used to identify message types
+ * in logical replication protocol, which uses a single byte to identify a
+ * message type. Hence the values should be single byte wide and preferrably
+ * human readable characters.
+ */
+typedef enum LogicalRepMsgType
+{
+	LOGICAL_REP_MSG_BEGIN = 'B',
+	LOGICAL_REP_MSG_COMMIT = 'C',
+	LOGICAL_REP_MSG_ORIGIN = 'O',
+	LOGICAL_REP_MSG_INSERT = 'I',
+	LOGICAL_REP_MSG_UPDATE = 'U',
+	LOGICAL_REP_MSG_DELETE = 'D',
+	LOGICAL_REP_MSG_TRUNCATE = 'T',
+	LOGICAL_REP_MSG_RELATION = 'R',
+	LOGICAL_REP_MSG_TYPE = 'Y',
+	LOGICAL_REP_MSG_STREAM_START = 'S',
+	LOGICAL_REP_MSG_STREAM_END = 'E',
+	LOGICAL_REP_MSG_STREAM_COMMIT = 'c',
+	LOGICAL_REP_MSG_STREAM_ABORT = 'A'
+} LogicalRepMsgType;
+
 /*
  * This struct stores a tuple received via logical replication.
  * Keep in mind that the columns correspond to the *remote* table.