journal-upload: verify state file can be saved before uploading

Do our best verify that we can actually write the state file
before upload commences to avoid duplicate messages on the server.

src/journal-remote/journal-upload.c

@@ -31,6 +31,7 @@
 #include "util.h"
 #include "build.h"
 #include "fileio.h"
+#include "mkdir.h"
 #include "conf-parser.h"
 #include "journal-upload.h"
 
@@ -93,6 +94,32 @@ static size_t output_callback(char *buf,
         return size * nmemb;
 }
 
+static int check_cursor_updating(Uploader *u) {
+        _cleanup_free_ char *temp_path = NULL;
+        _cleanup_fclose_ FILE *f = NULL;
+        int r;
+
+        if (!u->state_file)
+                return 0;
+
+        r = mkdir_parents(u->state_file, 0755);
+        if (r < 0) {
+                log_error("Cannot create parent directory of state file %s: %s",
+                          u->state_file, strerror(-r));
+                return r;
+        }
+
+        r = fopen_temporary(u->state_file, &f, &temp_path);
+        if (r < 0) {
+                log_error("Cannot save state to %s: %s",
+                          u->state_file, strerror(-r));
+                return r;
+        }
+        unlink(temp_path);
+
+        return 0;
+}
+
 static int update_cursor_state(Uploader *u) {
         _cleanup_free_ char *temp_path = NULL;
         _cleanup_fclose_ FILE *f = NULL;
@@ -779,6 +806,10 @@ int main(int argc, char **argv) {
 
         sd_event_set_watchdog(u.events, true);
 
+        r = check_cursor_updating(&u);
+        if (r < 0)
+                goto cleanup;
+
         log_debug("%s running as pid "PID_FMT,
                   program_invocation_short_name, getpid());