Fix removal of files in pgstats directories

Instead of deleting all files in stats_temp_directory and the permanent
directory on a crash, only remove those files that match the pattern of
files we actually write in them, to avoid possibly clobbering existing
unrelated contents of the temporary directory.  Per complaint from Jeff
Janes, and subsequent discussion, starting at message
CAMkU=1z9+7RsDODnT4=cDFBRBp8wYQbd_qsLcMtKEf-oFwuOdQ@mail.gmail.com

Also, fix a bug in the same routine to avoid removing files from the
permanent directory twice (instead of once from that directory and then
from the temporary directory), also per report from Jeff Janes, in
message
CAMkU=1wbk947=-pAosDMX5VC+sQw9W4ttq6RM9rXu=MjNeEQKA@mail.gmail.com

src/backend/postmaster/pgstat.c

@@ -563,15 +563,37 @@ pgstat_reset_remove_files(const char *directory)
 	struct dirent *entry;
 	char		fname[MAXPGPATH];
 
-	dir = AllocateDir(pgstat_stat_directory);
-	while ((entry = ReadDir(dir, pgstat_stat_directory)) != NULL)
+	dir = AllocateDir(directory);
+	while ((entry = ReadDir(dir, directory)) != NULL)
 	{
-		if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
+		int		nitems;
+		Oid		tmp_oid;
+		char	tmp_type[8];
+		char	tmp_rest[2];
+
+		if (strncmp(entry->d_name, ".", 2) == 0 ||
+			strncmp(entry->d_name, "..", 3) == 0)
+			continue;
+
+		/*
+		 * Skip directory entries that don't match the file names we write.
+		 * See get_dbstat_filename for the database-specific pattern.
+		 */
+		nitems = sscanf(entry->d_name, "db_%u.%5s%1s",
+						&tmp_oid, tmp_type, tmp_rest);
+		if (nitems != 2)
+		{
+			nitems = sscanf(entry->d_name, "global.%5s%1s",
+							tmp_type, tmp_rest);
+			if (nitems != 1)
 				continue;
+		}
 
-		/* XXX should we try to ignore files other than the ones we write? */
+		if (strncmp(tmp_type, "tmp", 4) != 0 &&
+			strncmp(tmp_type, "stat", 5) != 0)
+			continue;
 
-		snprintf(fname, MAXPGPATH, "%s/%s", pgstat_stat_directory,
+		snprintf(fname, MAXPGPATH, "%s/%s", directory,
 				 entry->d_name);
 		unlink(fname);
 	}
@@ -3631,6 +3653,7 @@ get_dbstat_filename(bool permanent, bool tempname, Oid databaseid,
 {
 	int			printed;
 
+	/* NB -- pgstat_reset_remove_files knows about the pattern this uses */
 	printed = snprintf(filename, len, "%s/db_%u.%s",
 					   permanent ? PGSTAT_STAT_PERMANENT_DIRECTORY :
 					   pgstat_stat_directory,