proposal: os: add ErrOutOfFileDescriptors

[mateusz834]

Currently there is no easy cross-platform way to detect the out of file descriptors error.

I propose adding a new error to the os package (os.ErrOutOfFileDescriptors) and adding support for it in syscall.Errno via the Is method (as already done with other errors like NoSuchFIle). So matching syscall.EMFILE, syscall.ENFILE on unix, syscall.EMFILE on other platforms.

I am still unsure about the name os.ErrOutOfFileDescriptors or os.ErrTooManyOpenFiles.

[ianlancetaylor]

Can you point to some examples of code that would use this facility? Thanks.

[mateusz834]

Mainly as an alternative to the deprecated Temporary() method (#45729), that is used in retry Accept() loops like this in net/http:

go/src/net/http/server.go

Lines 3059 to 3061 in d0c72c2

	if ne, ok := err.(net.Error); ok && ne.Temporary() {
	if tempDelay == 0 {
	tempDelay = 5 * time.Millisecond

errors.Is(err, os.ErrOutOfFileDescriptors) is clearer about the expected behaviour than using the Temporary().