Write output to a temp file and rename it afterwards.

Fixes #20

guetzli/guetzli.cc

@@ -14,11 +14,12 @@
  * limitations under the License.
  */
 
-#include <cstdio>
-#include <cstdlib>
 #include <memory>
 #include <string>
+#include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #include "gflags/gflags.h"
 #include "png.h"
 #include "guetzli/processor.h"
@@ -196,12 +197,30 @@ int main(int argc, char** argv) {
     return 1;
   }
 
-  FILE* fout = fopen(argv[2], "wb");
-  if (!fout) {
-    fprintf(stderr, "Can't open output file for writing\n");
+  if (access(argv[2], W_OK) != 0 && errno != ENOENT) {
+    // This is only relevant if the output directory is sticky, but /tmp often
+    // is.
+    fprintf(stderr, "Can't overwrite the output file\n");
     return 1;
   }
 
+  FILE* fout_tmp;
+  char fout_tmp_name[1024];
+  {
+    snprintf(fout_tmp_name, sizeof(fout_tmp_name), "%sXXXXXX", argv[2]);
+    fout_tmp_name[sizeof(fout_tmp_name) - 1] = '\0';
+    int fd = mkstemp(fout_tmp_name);
+    if (fd < 0) {
+      fprintf(stderr, "Can't create a file in the output directory\n");
+      return 1;
+    }
+    fout_tmp = fdopen(fd, "w");
+    if (!fout_tmp) {
+      perror("fdopen");
+      return 1;
+    }
+  }
+
   std::string in_data = ReadFileOrDie(fin);
   std::string out_data;
 
@@ -237,6 +256,11 @@ int main(int argc, char** argv) {
     }
   }
 
-  WriteFileOrDie(fout, out_data);
+  WriteFileOrDie(fout_tmp, out_data);
+  if (rename(fout_tmp_name, argv[2]) < 0) {
+    fprintf(stderr, "Can't overwrite the output file\n");
+    return 1;
+  }
+
   return 0;
 }