[WIP] Implementation of create file

[bartlomieju]

Trying to implement simple logger in deno I found that creating a file is still impossible.

I saw work done in #908, but I turns out we can simply use tokio::fs::OpenOptions to configure what mode we want to open file in.

This is very rough minimal implementation that still needs a lot of work.

TODO:

• settle on OpenModes and implement as enum in files.ts
• factor out setup code of tokio::fs::OpenOptions in ops.ts
• tests

[bartlomieju]

File open modes in Python (taken from https://docs.python.org/3/library/io.html#io.FileIO)

The mode can be 'r', 'w', 'x' or 'a' for reading (default), writing, exclusive creation or appending. The file will be created if it doesn’t exist when opened for writing or appending; it will be truncated when opened for writing. FileExistsError will be raised if it already exists when opened for creating. Opening a file for creating implies writing, so this mode behaves in a similar way to 'w'. Add a '+' to the mode to allow simultaneous reading and writing.

Which translates to roughly this:

enum OpenMode {
    Read = 'r',
    Write = 'w', // <- truncates the file if it has content
    ExclusiveOpen = 'x',
    Append = 'a',
    ReadWrite = 'r+' // 'w+' would also work
    ReadAppend = 'a+'
    ReadExclusiveOpen = 'x+' // <- better name needed
}

Opinions welcome.

CC: @ry @kitsonk @piscisaureus

[kitsonk]

@bartlomieju I would totally defer to Ryan or @piscisaureus on this one. I am not really good at having well informed opinions about OS APIs.

[bartlomieju]

Sure, thanks for letting know.

[bartlomieju]

Go's and Ruby's file mode for reference:
https://golang.org/pkg/os/#pkg-constants

const (
        // Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
        O_RDONLY int = syscall.O_RDONLY // open the file read-only.
        O_WRONLY int = syscall.O_WRONLY // open the file write-only.
        O_RDWR   int = syscall.O_RDWR   // open the file read-write.
        // The remaining values may be or'ed in to control behavior.
        O_APPEND int = syscall.O_APPEND // append data to the file when writing.
        O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
        O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
        O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
        O_TRUNC  int = syscall.O_TRUNC  // if possible, truncate file when opened.
)

https://ruby-doc.org/core-2.2.2/IO.html#method-c-new-label-IO+Open+Mode

"r"  Read-only, starts at beginning of file  (default mode).

"r+" Read-write, starts at beginning of file.

"w"  Write-only, truncates existing file
     to zero length or creates a new file for writing.

"w+" Read-write, truncates existing file to zero length
     or creates a new file for reading and writing.

"a"  Write-only, each write call appends data at end of file.
     Creates a new file for writing if file does not exist.

"a+" Read-write, each write call appends data at end of file.
     Creates a new file for reading and writing if file does
     not exist.

"b"  Binary file mode
     Suppresses EOL <-> CRLF conversion on Windows. And
     sets external encoding to ASCII-8BIT unless explicitly
     specified.

"t"  Text file mode

[ry]

This looks great! r w x a with optional plus is the time tested interface.

Can you add some more tests? Add TODOs for the missing ones.

[bartlomieju]

@ry I added OpenMode enum with explanation for each mode.

Please check how OpenOptions::append behaves. Do we need to care about position of reading?

I will write full tests for each mode tomorrow morning. I expect to find some quirky behavior along the way that will have to be pinpointed.

EDIT: grammar

[bartlomieju]

Should I move this whole operation somewhere else?

Probably some comments explaining why each option is used would be appropriate as well.

[bartlomieju]

I got a bit confused, I found that Deno already has writeFile and readFile implemented. Does it make sense to have separate implementations of them and open in Rust?

cc @ry

[ry]

@bartlomieju Regarding writeFile and readFile - these ops are fully buffered. They're fine for small files, but if you have a 1 gigabyte file, you might not want to load it all into memory just to read the first few bytes from it. This is the purpose of having raw file handles.

[bartlomieju]

@ry I resolved your remarks. Please let know if OpenMode comments are alright.

To finish tests we need to have seek working, but that's for another PR.

[ry]

LGTM - thanks for seeing this through!