fix(application): unlink temp file coming from stdin

Stop mutating `file_str`, use a dedicated `temp_file_str` to store the
location of the temporary file when using swappy with `-f -` option.

Closes #80

include/swappy.h

@@ -160,6 +160,8 @@ struct swappy_state {
   char *file_str;
   char *output_file;
 
+  char *temp_file_str;
+
   struct swappy_box *window;
   struct swappy_box *geometry;
 

src/application.c

@@ -1,5 +1,6 @@
 #include <gdk/gdk.h>
 #include <glib-2.0/glib.h>
+#include <glib/gstdio.h>
 #include <gtk/gtk.h>
 #include <stdio.h>
 #include <time.h>
@@ -235,6 +236,13 @@ void application_finish(struct swappy_state *state) {
   cairo_surface_destroy(state->rendered_surface);
   cairo_surface_destroy(state->original_image_surface);
   cairo_surface_destroy(state->scaled_image_surface);
+  if (state->temp_file_str) {
+    g_info("deleting temporary file: %s", state->temp_file_str);
+    if (g_unlink(state->temp_file_str) != 0) {
+      g_warning("unable to delete temporary file: %s", state->temp_file_str);
+    }
+    g_free(state->temp_file_str);
+  }
   g_free(state->file_str);
   g_free(state->geometry);
   g_free(state->window);
@@ -713,9 +721,8 @@ static gint command_line_handler(GtkApplication *app,
 
   if (has_option_file(state)) {
     if (is_file_from_stdin(state->file_str)) {
-      char *new_file_str = file_dump_stdin_into_a_temp_file();
-      g_free(state->file_str);
-      state->file_str = new_file_str;
+      char *temp_file_str = file_dump_stdin_into_a_temp_file();
+      state->temp_file_str = temp_file_str;
     }
 
     if (!buffer_init_from_file(state)) {

src/buffer.c

@@ -6,7 +6,10 @@
 #include "swappy.h"
 
 bool buffer_init_from_file(struct swappy_state *state) {
-  char *file = state->file_str;
+  char *file =
+      state->temp_file_str != NULL ? state->temp_file_str : state->file_str;
+
+  g_info("creating cairo image surface from file: %s", file);
 
   cairo_surface_t *surface = cairo_image_surface_create_from_png(file);
   cairo_status_t status = cairo_surface_status(surface);