libos: Add a convenience function for opening a file or standard input

apps/cat.cpp

@@ -6,14 +6,10 @@ using os::File;
 
 static Result<void> do_cat(StringView path)
 {
-    SharedPtr<File> f;
+    SharedPtr<File> f = TRY(File::open_input_file(path));
 
     auto out = File::standard_output();
 
-    if (path == "-") f = File::standard_input();
-    else
-        f = TRY(File::open(path, File::ReadOnly));
-
     auto buf = TRY(Buffer::create_sized(4096));
     while (1)
     {

libos/include/os/File.h

@@ -24,6 +24,14 @@ namespace os
         static Result<SharedPtr<File>> open_or_create(StringView path, OpenMode flags, mode_t mode = 0644);
         static Result<SharedPtr<File>> create(StringView path, OpenMode flags, mode_t mode = 0644);
 
+        /*
+          If path is "-", return standard input (as is common for many CLI apps). Otherwise, open path for reading.
+
+          This function is a convenience function for CLI apps, so that they don't need to check path and open standard
+          input if necessary.
+        */
+        static Result<SharedPtr<File>> open_input_file(StringView path);
+
         static SharedPtr<File> standard_input();
         static SharedPtr<File> standard_output();
         static SharedPtr<File> standard_error();

libos/src/File.cpp

@@ -82,6 +82,13 @@ namespace os
         return construct(path, (int)flags | (O_CREAT | O_EXCL), mode);
     }
 
+    Result<SharedPtr<File>> File::open_input_file(StringView path)
+    {
+        if (path == "-"_sv) return standard_input();
+
+        return construct(path, O_RDONLY, 0);
+    }
+
     Result<usize> File::raw_read(u8* buf, usize length)
     {
         long rc = syscall(SYS_read, m_fd, buf, length);