Request for creating pipes with fd other than 0,1,2

text/0000-additional-fd.md

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+- Feature Name: additional-fd-command
+- Start Date: 2020-05-31
+- RFC PR: https://github.com/rust-lang/rfcs/pull/2939
+
+# Summary
+
+While spawning a child process, sometimes additional pipes rather than stdin, stdout, stderr are used for IPC. So std::process::Command::new should support it like the nodejs 
+child_process.spawn which has the option to do it.
+
+# Motivation
+* Applications like chrome uses fd 3 and 4 for remote debugging. Tools like puppeteer use the nodejs child_process.spawn function which can do so. 
+* Pipes are often used for processes to communicate with each other. These processes often communicate using pipes other than stdin, stdout, stderr like the above example.
+
+# Design
+Here is an example for how it can be
+
+Parent process:
+```rust
+let (read3, write3) = std::io::pipe();
+let (read4, write4) = std::io::pipe();
+
+let process = Command::new("prog")
+              .stdin(Stdio::null())
+              .stdout(Stdio::null())
+              .stderr(Stdio::null())
+              .pass(&[&read3, &write4])
+              .spawn().unwrap();
+write3.write("Foo").unwrap();
+
+let mut s = String::new();
+read4.read_to_string(&mut s).unwrap();
+```
+
+The pipe() function can return a tuple of PipeReader and PipeWriter
+
+The pass method in std::process::Command can take a slice of &dyn RawFile where RawFile is a trait that specifies
+* The raw fd/HANDLE
+* The flags associated with it
+
+Child process:
+```rust
+let read3 = std::io::PipeReader::from_fd(3);
+let write4 = std::io::PipeWriter::from_fd(4);
+
+let mut s = String::new();
+read3.read_to_string(&mut s).unwrap();
+
+write4.write("Bar").unwrap();
+```
+
+# Implementation 
+In my library I had to use pipes for IPC so I did a basic rust implementation here https://github.com/Srinivasa314/alcro/blob/master/src/chrome/pipe.rs
+## Unix
+For unix `dup2` can be used
+
+## Windows
+Eventhough windows uses handles,the MSVC runtime does have a concept of fd's
+Refer:
+[open_osfhandle](https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/open-osfhandle)
+[get_osfhandle](https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle)
+
+For windows `lpReserved2` of `startupinfo` in `CreateProcess` can be used
+(The method used by libuv:
+https://github.com/nodejs/node/blob/8ae28ff3ac49cbf83f4fc3445d63b9900f3cdcda/deps/uv/src/win/process-stdio.c#L32)
+
+The struct should be packed and crtflags are as in https://github.com/nodejs/node/blob/8ae28ff3ac49cbf83f4fc3445d63b9900f3cdcda/deps/uv/src/win/process-stdio.c#L57
+
+The child process can get the HANDLE from fd even _without_ needing the CRT using GetStartupInfo
+
+# Alternative
+
+Write code using the C API for windows and unix instead of std::process:Command
+
+# Prior art
+NodeJS has child_process.spawn which has options.stdio so that many other pipes can be created and it is frequently used for interprocess communication between node js applications.