Fix error message on short read of pg_control

Instead of saying "error: success", indicate that we got a working read
but it was too short.

src/backend/access/transam/xlog.c

@@ -4486,6 +4486,7 @@ ReadControlFile(void)
 	pg_crc32c	crc;
 	int			fd;
 	static char wal_segsz_str[20];
+	int			r;
 
 	/*
 	 * Read data...
@@ -4499,10 +4500,17 @@ ReadControlFile(void)
 						XLOG_CONTROL_FILE)));
 
 	pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_READ);
-	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
+	r = read(fd, ControlFile, sizeof(ControlFileData));
+	if (r != sizeof(ControlFileData))
+	{
+		if (r < 0)
 			ereport(PANIC,
 					(errcode_for_file_access(),
 					 errmsg("could not read from control file: %m")));
+		else
+			ereport(PANIC,
+					 (errmsg("could not read from control file: read %d bytes, expected %d", r, (int) sizeof(ControlFileData))));
+	}
 	pgstat_report_wait_end();
 
 	close(fd);

src/common/controldata_utils.c

@@ -44,6 +44,7 @@ get_controlfile(const char *DataDir, const char *progname, bool *crc_ok_p)
 	int			fd;
 	char		ControlFilePath[MAXPGPATH];
 	pg_crc32c	crc;
+	int			r;
 
 	AssertArg(crc_ok_p);
 
@@ -64,7 +65,10 @@ get_controlfile(const char *DataDir, const char *progname, bool *crc_ok_p)
 	}
 #endif
 
-	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
+	r = read(fd, ControlFile, sizeof(ControlFileData));
+	if (r != sizeof(ControlFileData))
+	{
+		if (r < 0)
 #ifndef FRONTEND
 			ereport(ERROR,
 					(errcode_for_file_access(),
@@ -76,6 +80,19 @@ get_controlfile(const char *DataDir, const char *progname, bool *crc_ok_p)
 			exit(EXIT_FAILURE);
 		}
 #endif
+		else
+#ifndef FRONTEND
+			ereport(ERROR,
+					(errmsg("could not read file \"%s\": read %d bytes, expected %d",
+							ControlFilePath, r, (int) sizeof(ControlFileData))));
+#else
+		{
+			fprintf(stderr, _("%s: could not read file \"%s\": read %d bytes, expected %d\n"),
+					progname, ControlFilePath, r, (int) sizeof(ControlFileData));
+			exit(EXIT_FAILURE);
+		}
+#endif
+	}
 
 	close(fd);