File::tryOpen

Summary:
Now that we have truthy files, it would be nice to be able to simply
//try// opening files and return a possibly-initialized File.

Test Plan: Unit tests

Reviewed By: chaoyc@fb.com

FB internal diff: D726916

folly/File.cpp

@@ -79,6 +79,16 @@ File::~File() {
   return File(fd, true);
 }
 
+/* static */ File File::tryOpen(const char* name,
+                                int flags,
+                                mode_t mode) {
+  try {
+    return File(name, flags, mode);
+  } catch (const std::system_error&) {
+    return File();
+  }
+}
+
 void File::release() {
   fd_ = -1;
   ownsFd_ = false;

folly/File.h

@@ -54,6 +54,14 @@ class File {
    */
   static File temporary();
 
+  /**
+   * Attempts to open the file at the given path.  Returns an 'closed` File
+   * instance on failure, which will evaluate to false.
+   */
+  static File tryOpen(const char* name,
+                      int flags = O_RDONLY,
+                      mode_t mode = 0644);
+
   /**
    * Return the file descriptor, or -1 if the file was closed.
    */

folly/test/FileTest.cpp

@@ -122,3 +122,8 @@ TEST(File, Truthy) {
     EXPECT_TRUE(false);
   }
 }
+
+TEST(File, TryOpen) {
+  EXPECT_FALSE(!!File::tryOpen("does_not_exist.txt"));
+  EXPECT_TRUE(!!File::tryOpen("/etc/fstab"));
+}