system/FileDescriptor: add method IsValid()

src/system/FileDescriptor.cxx

@@ -57,6 +57,16 @@
 #define O_CLOEXEC 0
 #endif
 
+#ifndef _WIN32
+
+bool
+FileDescriptor::IsValid() const noexcept
+{
+	return IsDefined() && fcntl(fd, F_GETFL) >= 0;
+}
+
+#endif
+
 bool
 FileDescriptor::Open(const char *pathname, int flags, mode_t mode) noexcept
 {

src/system/FileDescriptor.hxx

@@ -64,6 +64,14 @@ public:
 		return fd >= 0;
 	}
 
+#ifndef _WIN32
+	/**
+	 * Ask the kernel whether this is a valid file descriptor.
+	 */
+	gcc_pure
+	bool IsValid() const noexcept;
+#endif
+
 	/**
 	 * Returns the file descriptor.  This may only be called if
 	 * IsDefined() returns true.