Refactor basebackup.c's _tarWriteDir() function.

Sometimes, we replace a symbolic link that we find in the data
directory with an actual directory within the tarfile that we
create. _tarWriteDir was responsible both for making this
substitution and also for writing the tar header for the
resulting directory into the tar file. Make it do only the first
of those things, and rename to convert_link_to_directory.

Substantially larger refactoring of this source file is planned,
but this little bit seemed to make sense to commit
independently.

Discussion: http://postgr.es/m/CA+Tgmobz6tuv5tr-WxURe5JA1vVcGz85k4kkvoWxcyHvDpEqFA@mail.gmail.com

src/backend/replication/basebackup.c

@@ -71,8 +71,7 @@ static void sendFileWithContent(const char *filename, const char *content,
 								backup_manifest_info *manifest);
 static int64 _tarWriteHeader(const char *filename, const char *linktarget,
 							 struct stat *statbuf, bool sizeonly);
-static int64 _tarWriteDir(const char *pathbuf, int basepathlen, struct stat *statbuf,
-						  bool sizeonly);
+static void convert_link_to_directory(const char *pathbuf, struct stat *statbuf);
 static void send_int8_string(StringInfoData *buf, int64 intval);
 static void SendBackupHeader(List *tablespaces);
 static void perform_base_backup(basebackup_options *opt);
@@ -1381,7 +1380,9 @@ sendDir(const char *path, int basepathlen, bool sizeonly, List *tablespaces,
 			if (strcmp(de->d_name, excludeDirContents[excludeIdx]) == 0)
 			{
 				elog(DEBUG1, "contents of directory \"%s\" excluded from backup", de->d_name);
-				size += _tarWriteDir(pathbuf, basepathlen, &statbuf, sizeonly);
+				convert_link_to_directory(pathbuf, &statbuf);
+				size += _tarWriteHeader(pathbuf + basepathlen + 1, NULL, &statbuf,
+										sizeonly);
 				excludeFound = true;
 				break;
 			}
@@ -1397,7 +1398,9 @@ sendDir(const char *path, int basepathlen, bool sizeonly, List *tablespaces,
 		if (statrelpath != NULL && strcmp(pathbuf, statrelpath) == 0)
 		{
 			elog(DEBUG1, "contents of directory \"%s\" excluded from backup", statrelpath);
-			size += _tarWriteDir(pathbuf, basepathlen, &statbuf, sizeonly);
+			convert_link_to_directory(pathbuf, &statbuf);
+			size += _tarWriteHeader(pathbuf + basepathlen + 1, NULL, &statbuf,
+									sizeonly);
 			continue;
 		}
 
@@ -1409,7 +1412,9 @@ sendDir(const char *path, int basepathlen, bool sizeonly, List *tablespaces,
 		if (strcmp(pathbuf, "./pg_wal") == 0)
 		{
 			/* If pg_wal is a symlink, write it as a directory anyway */
-			size += _tarWriteDir(pathbuf, basepathlen, &statbuf, sizeonly);
+			convert_link_to_directory(pathbuf, &statbuf);
+			size += _tarWriteHeader(pathbuf + basepathlen + 1, NULL, &statbuf,
+									sizeonly);
 
 			/*
 			 * Also send archive_status directory (by hackishly reusing
@@ -1883,12 +1888,11 @@ _tarWriteHeader(const char *filename, const char *linktarget,
 }
 
 /*
- * Write tar header for a directory.  If the entry in statbuf is a link then
- * write it as a directory anyway.
+ * If the entry in statbuf is a link, then adjust statbuf to make it look like a
+ * directory, so that it will be written that way.
  */
-static int64
-_tarWriteDir(const char *pathbuf, int basepathlen, struct stat *statbuf,
-			 bool sizeonly)
+static void
+convert_link_to_directory(const char *pathbuf, struct stat *statbuf)
 {
 	/* If symlink, write it as a directory anyway */
 #ifndef WIN32
@@ -1897,8 +1901,6 @@ _tarWriteDir(const char *pathbuf, int basepathlen, struct stat *statbuf,
 	if (pgwin32_is_junction(pathbuf))
 #endif
 		statbuf->st_mode = S_IFDIR | pg_dir_create_mode;
-
-	return _tarWriteHeader(pathbuf + basepathlen + 1, NULL, statbuf, sizeonly);
 }
 
 /*