Bugfix #4363

In windows, without temp path set, the 'save buffers' render option crashes.

I've coded a blenlib BLI_is_writable(char *filename) to check for such
cases. This is not much needed in Blender, since the open() command is
checked for. However, file saving happens deep inside the C++ exr lib, and
it throws an exception crash when a file cannot be written.

source/blender/blenlib/BLI_blenlib.h

@@ -256,7 +256,7 @@ int    BLI_exist(char *name);
 void  BLI_recurdir_fileops(char *dirname);
 int BLI_link(char *file, char *to);
 int BLI_backup(char *file, char *from, char *to);
-
+int BLI_is_writable(char *filename);
 
 /**
  * @attention Do not confuse with BLI_exist

source/blender/blenlib/intern/fileops.c

@@ -125,6 +125,21 @@ int BLI_gzip(char *from, char *to) {
 	return 0;
 }
 
+/* return 1 when file can be written */
+int BLI_is_writable(char *filename)
+{
+	int file;
+	
+	file = open(filename, O_BINARY | O_RDWR | O_CREAT | O_TRUNC, 0666);
+	
+	if (file < 0)
+		return 0;
+	else {
+		close(file);
+		return 1;
+	}
+}
+
 #ifdef WIN32
 
 static char str[MAXPATHLEN+12];

source/blender/render/intern/source/pipeline.c

@@ -1102,9 +1102,10 @@ static void threaded_tile_processor(Render *re)
 	
 	if(rr->exrhandle) {
 		char str[FILE_MAXDIR+FILE_MAXFILE];
-		render_unique_exr_name(re, str);
-		printf("write exr tmp file, %dx%d, %s\n", rr->rectx, rr->recty, str);
 		
+		render_unique_exr_name(re, str);
+		
+		printf("write exr tmp file, %dx%d, %s\n", rr->rectx, rr->recty, str);
 		IMB_exrtile_begin_write(rr->exrhandle, str, rr->rectx, rr->recty, rr->rectx/re->xparts, rr->recty/re->yparts);
 	}
 	
@@ -1721,6 +1722,18 @@ static int is_rendering_allowed(Render *re)
 		}
 	}
 	
+	if(re->r.scemode & R_EXR_TILE_FILE) {
+		char str[FILE_MAXDIR+FILE_MAXFILE];
+		
+		render_unique_exr_name(re, str);
+		
+		if (BLI_is_writable(str)==0) {
+			re->error("Can not save render buffers, check the temp default path");
+			return 0;
+		}
+		
+	}
+	
 	if(re->r.scemode & R_DOCOMP) {
 		if(re->scene->use_nodes) {
 			bNodeTree *ntree= re->scene->nodetree;