Verify that output file can be written before rendering

src/fart/Engine.cpp

@@ -1,4 +1,7 @@
 #include <fart/Engine.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
 
 Engine::Engine(const RenderParams & params)
     : params(params)
@@ -28,5 +31,23 @@ int Engine::run(string_view input_file, opt<string_view> output_file)
         return 1;
     }
 
+    /* Verify the output file can be written before rendering. */
+    FILE * fh = fopen(output_path.c_str(), "wb");
+    if (fh == nullptr)
+    {
+        fprintf(stderr, "Error opening output file '%s': %s\n", output_path.c_str(), strerror(errno));
+        return 1;
+    }
+    if (fclose(fh) != 0)
+    {
+        fprintf(stderr, "Error closing output file '%s': %s\n", output_path.c_str(), strerror(errno));
+        return 1;
+    }
+    if (remove(output_path.c_str()) != 0)
+    {
+        fprintf(stderr, "Error removing output file '%s': %s\n", output_path.c_str(), strerror(errno));
+        return 1;
+    }
+
     return 0;
 }