Remove dead code and fix comments in fast-path function handling.

HandleFunctionRequest() is no longer responsible for reading the protocol
message from the client, since commit 2b3a8b20. Fix the outdated
comments.

HandleFunctionRequest() now always returns 0, because the code that used
to return EOF was moved in 2b3a8b20. Therefore, the caller no longer
needs to check the return value.

Reported by Andres Freund. Backpatch to all supported versions, even though
this doesn't have any user-visible effect, to make backporting future
patches in this area easier.

Discussion: https://www.postgresql.org/message-id/20170405010525.rt5azbya5fkbhvrx@alap3.anarazel.de

src/backend/tcop/fastpath.c

@@ -249,24 +249,15 @@ fetch_fp_info(Oid func_id, struct fp_info * fip)
  * This corresponds to the libpq protocol symbol "F".
  *
  * INPUT:
- *		In protocol version 3, postgres.c has already read the message body
- *		and will pass it in msgBuf.
- *		In old protocol, the passed msgBuf is empty and we must read the
- *		message here.
- *
- * RETURNS:
- *		0 if successful completion, EOF if frontend connection lost.
- *
- * Note: All ordinary errors result in ereport(ERROR,...).  However,
- * if we lose the frontend connection there is no one to ereport to,
- * and no use in proceeding...
+ *		postgres.c has already read the message body and will pass it in
+ *		msgBuf.
  *
  * Note: palloc()s done here and in the called function do not need to be
  * cleaned up explicitly.  We are called from PostgresMain() in the
  * MessageContext memory context, which will be automatically reset when
  * control returns to PostgresMain.
  */
-int
+void
 HandleFunctionRequest(StringInfo msgBuf)
 {
 	Oid			fid;
@@ -281,9 +272,8 @@ HandleFunctionRequest(StringInfo msgBuf)
 	char		msec_str[32];
 
 	/*
-	 * Now that we've eaten the input message, check to see if we actually
-	 * want to do the function call or not.  It's now safe to ereport(); we
-	 * won't lose sync with the frontend.
+	 * We only accept COMMIT/ABORT if we are in an aborted transaction, and
+	 * COMMIT/ABORT cannot be executed through the fastpath interface.
 	 */
 	if (IsAbortedTransactionBlockState())
 		ereport(ERROR,
@@ -406,8 +396,6 @@ HandleFunctionRequest(StringInfo msgBuf)
 							msec_str, fip->fname, fid)));
 			break;
 	}
-
-	return 0;
 }
 
 /*

src/backend/tcop/postgres.c

@@ -4164,19 +4164,7 @@ PostgresMain(int argc, char *argv[],
 				/* switch back to message context */
 				MemoryContextSwitchTo(MessageContext);
 
-				if (HandleFunctionRequest(&input_message) == EOF)
-				{
-					/* lost frontend connection during F message input */
-
-					/*
-					 * Reset whereToSendOutput to prevent ereport from
-					 * attempting to send any more messages to client.
-					 */
-					if (whereToSendOutput == DestRemote)
-						whereToSendOutput = DestNone;
-
-					proc_exit(0);
-				}
+				HandleFunctionRequest(&input_message);
 
 				/* commit the function-invocation transaction */
 				finish_xact_command();

src/include/tcop/fastpath.h

@@ -16,6 +16,6 @@
 #include "lib/stringinfo.h"
 
 extern int	GetOldFunctionMessage(StringInfo buf);
-extern int	HandleFunctionRequest(StringInfo msgBuf);
+extern void HandleFunctionRequest(StringInfo msgBuf);
 
 #endif   /* FASTPATH_H */