journal-remote: show error message if output file name does not end with .journal

`journalctl -o export | systemd-journal-remote -o /tmp/dir -`
gives the following error messages.
```
Failed to open output journal /tmp/dir: Invalid argument
Failed to get writer for source stdin: Invalid argument
Failed to create source for fd:0 (stdin): Invalid argument
```
And these are hard to understand what is the problem.
This commit makes journal-remote check whether the output file name
ends with .journal suffix or not, and if not, output error message.

1 files changed, 9 insertions, 4 deletions

diff --git a/src/journal-remote/journal-remote.c b/src/journal-remote/journal-remote.c
index 1ed350649c..7201421d91 100644
--- a/src/journal-remote/journal-remote.c
+++ b/src/journal-remote/journal-remote.c
@@ -1495,10 +1495,15 @@ static int parse_argv(int argc, char *argv[]) {
                 arg_split_mode = JOURNAL_WRITE_SPLIT_NONE;
         }
 
-        if (arg_split_mode == JOURNAL_WRITE_SPLIT_NONE
-            && arg_output && is_dir(arg_output, true) > 0) {
-                log_error("For SplitMode=none, output must be a file.");
-                return -EINVAL;
+        if (arg_split_mode == JOURNAL_WRITE_SPLIT_NONE && arg_output) {
+                if (is_dir(arg_output, true) > 0) {
+                        log_error("For SplitMode=none, output must be a file.");
+                        return -EINVAL;
+                }
+                if (!endswith(arg_output, ".journal")) {
+                        log_error("For SplitMode=none, output file name must end with .journal.");
+                        return -EINVAL;
+                }
         }
 
         if (arg_split_mode == JOURNAL_WRITE_SPLIT_HOST