save: Perform write process safe

[JoeKar]

The target situation shall be, that micro checks if the file to be stored already exists and if so it shall work with a temporary file first, before the target file is overwritten respective the temporary file renamed to the target file.
Possible symlinks pointing to the target file will be resolved, before the save takes place. This shall guarantee that this symlink isn't renamed by the new approach.

TODOs:

• Properly handle all of the irregular file types and character devices etc.pp.
• Simplify the chosen overwriteFile() interface (see: save: Perform write process safe #3273 (comment))
• util.EscapePath(b.AbsPath) does not uniquely encode the file path (see: https://github.com/zyedidia/micro/pull/3273/files#r1599137940)
• b.Backup() executed asynchronously (from backupThread) is accessing the line array without locking. (see: https://github.com/zyedidia/micro/pull/3273/files#r1599137940)
• Decouple the backup step from the target file write (see: https://github.com/zyedidia/micro/pull/3273/files#r1600795305)
• Remove broken backup (see: https://github.com/zyedidia/micro/pull/3273/files#r1599076040)
• Rework save & backup trigger (see: save: Perform write process safe #3273 (comment))

Fixes #1916
Fixes #3148
Fixes #3196

[JoeKar]

Still a lot of rework left, but thank your for your time reviewing this! 👍

[JoeKar]

I did a lot of (commit) refactoring, but still I'm not fully happy with it.
The backup approach is still messy (duplicated code, no single point of truth, etc.), but currently I'm running out of ideas.

[dmaluka]

I did a lot of (commit) refactoring, but still I'm not fully happy with it. The backup approach is still messy (duplicated code, no single point of truth, etc.), but currently I'm running out of ideas.

IIUC, basically, all these troubles are due to the need to avoid circular dependencies between packages?

If so, it seems we could avoid these troubles by implementing overwrite & backup stuff as a part of the config package. OTOH, that sounds like an abuse of the config package: it is supposed to implement stuff related specifically to configuration, not stuff like buffer backups.

So maybe instead, simply provide separate implementations of overwrite & backup stuff for different kinds of files, in both buffer and config packages (especially since, as I noted in another comment, it seems better to have separate implementations of Overwrite() anyway). Maybe make them implement some common interface (if there is a need for that).

A very rough idea:

type FileWriter interface {
	Overwrite(name string) error
	BackupName(name string) string
	KeepBackup() bool
}

and a common Write() implementation using this interface:

func Write(name string, fwriter FileWriter) error

Or maybe even Write() implementations need to be different for different files.

P.S. Side note: maybe let's name it SafeWrite()?

[JoeKar]

Yes, I know dc701a1 doesn't really belong to the PR in the first place, but I touched some WriteFile's anyway.
It was not a big deal to do this cleanup.