Re: WIP/PoC for parallel backup

I had a quick look over these changes and they look good overall.
However, here are my few review comments I caught while glancing the patches
0002 and 0003.


--- 0002 patch

1.
Can lsn option be renamed to start-wal-location? This will be more clear too.

2.
+typedef struct
+{
+    char        name[MAXPGPATH];
+    char        type;
+    int32        size;
+    time_t        mtime;
+} BackupFile;

I think it will be good if we keep this structure in a common place so that
the client can also use it.

3.
+    SEND_FILE_LIST,
+    SEND_FILES_CONTENT,
Can above two commands renamed to SEND_BACKUP_MANIFEST and SEND_BACKUP_FILE
respectively?
The reason behind the first name change is, we are not getting only file lists
here instead we are getting a few more details with that too. And for others,
it will be inline with START_BACKUP/STOP_BACKUP/SEND_BACKUP_MANIFEST.

4.
Typos:
non-exlusive => non-exclusive
retured => returned
optionaly => optionally
nessery => necessary
totoal => total


--- 0003 patch

1.
+static int
+simple_list_length(SimpleStringList *list)
+{
+    int            len = 0;
+    SimpleStringListCell *cell;
+
+    for (cell = list->head; cell; cell = cell->next, len++)
+        ;
+
+    return len;
+}

I think it will be good if it goes to simple_list.c. That will help in other
usages as well.

2.
Please revert these unnecessary changes:

@@ -1475,6 +1575,7 @@ ReceiveAndUnpackTarFile(PGconn *conn, PGresult *res, int rownum)
              */
             snprintf(filename, sizeof(filename), "%s/%s", current_path,
                      copybuf);
+
             if (filename[strlen(filename) - 1] == '/')
             {
                 /*

@@ -1528,8 +1622,8 @@ ReceiveAndUnpackTarFile(PGconn *conn, PGresult *res, int rownum)
                      * can map them too.)
                      */
                     filename[strlen(filename) - 1] = '\0';    /* Remove trailing slash */
-
                     mapped_tblspc_path = get_tablespace_mapping(&copybuf[157]);
+
                     if (symlink(mapped_tblspc_path, filename) != 0)
                     {
                         pg_log_error("could not create symbolic link from \"%s\" to \"%s\": %m",

3.
Typos:
retrive => retrieve
takecare => take care
tablespae => tablespace

4.
ParallelBackupEnd() function does not do anything for parallelism. Will it be
better to just rename it as EndBackup()?

5.
To pass a tablespace path to the server in SEND_FILES_CONTENT, you are reusing
a LABEL option, that seems odd. How about adding a new option for that?

6.
It will be good if we have some comments explaining what the function is
actually doing in its prologue. For functions like:
GetBackupFilesList()
ReceiveFiles()
create_workers_and_fetch()


Thanks