compiler: Correct depfile generation when there are no outputs

Prevents the compiler from generating a malformed depfile if an inpout file did not lead to any meaningful generation. This is an edge case that is not exercised often.

PiperOrigin-RevId: 542354842

src/google/protobuf/compiler/command_line_interface.cc

@@ -2520,6 +2520,7 @@ bool CommandLineInterface::GenerateDependencyManifestFile(
     output_filenames.push_back(descriptor_set_out_name_);
   }
 
+  // Create the depfile, even if it will be empty.
   int fd;
   do {
     fd = open(dependency_out_name_.c_str(),
@@ -2531,30 +2532,34 @@ bool CommandLineInterface::GenerateDependencyManifestFile(
     return false;
   }
 
-  io::FileOutputStream out(fd);
-  io::Printer printer(&out, '$');
+  // Only write to the depfile if there is at least one output_filename.
+  // Otherwise, the depfile will be malformed.
+  if (!output_filenames.empty()) {
+    io::FileOutputStream out(fd);
+    io::Printer printer(&out, '$');
 
-  for (int i = 0; i < output_filenames.size(); i++) {
-    printer.Print(output_filenames[i]);
-    if (i == output_filenames.size() - 1) {
-      printer.Print(":");
-    } else {
-      printer.Print(" \\\n");
+    for (size_t i = 0; i < output_filenames.size(); i++) {
+      printer.Print(output_filenames[i]);
+      if (i == output_filenames.size() - 1) {
+        printer.Print(":");
+      } else {
+        printer.Print(" \\\n");
+      }
     }
-  }
 
-  for (int i = 0; i < file_set.file_size(); i++) {
-    const FileDescriptorProto& file = file_set.file(i);
-    const std::string& virtual_file = file.name();
-    std::string disk_file;
-    if (source_tree &&
-        source_tree->VirtualFileToDiskFile(virtual_file, &disk_file)) {
-      printer.Print(" $disk_file$", "disk_file", disk_file);
-      if (i < file_set.file_size() - 1) printer.Print("\\\n");
-    } else {
-      std::cerr << "Unable to identify path for file " << virtual_file
-                << std::endl;
-      return false;
+    for (int i = 0; i < file_set.file_size(); i++) {
+      const FileDescriptorProto& file = file_set.file(i);
+      const std::string& virtual_file = file.name();
+      std::string disk_file;
+      if (source_tree &&
+          source_tree->VirtualFileToDiskFile(virtual_file, &disk_file)) {
+        printer.Print(" $disk_file$", "disk_file", disk_file);
+        if (i < file_set.file_size() - 1) printer.Print("\\\n");
+      } else {
+        std::cerr << "Unable to identify path for file " << virtual_file
+                  << std::endl;
+        return false;
+      }
     }
   }