Most of these changes were sourced from mv's existing backup control
implementation. A later commit will update the mv utility to use this
new share backup control.
Fails trom time to time with:
```
info: installing component 'rustc'
memory allocation of 16777216 bytes failed
Error: The process 'C:\Rust\.cargo\bin\rustup.exe' failed with exit code 3221226505
```
on Build (windows-latest, i686-pc-windows-gnu, feat_os_windows)
Add some abstractions to simplify the `rbuf_but_last_n_lines()`
function, which implements the "take all but the last `n` lines"
functionality of the `head` program. This commit adds
- `RingBuffer`, a fixed-size ring buffer,
- `ZLines`, an iterator over zero-terminated "lines",
- `TakeAllBut`, an iterator over all but the last `n` elements of an
iterator.
These three together make the implementation of
`rbuf_but_last_n_lines()` concise.
Reorganize the code in `truncate.rs` into three distinct functions
representing the three modes of operation of the `truncate` program. The
three modes are
- `truncate -r RFILE FILE`, which sets the length of `FILE` to match the
length of `RFILE`,
- `truncate -r RFILE -s NUM FILE`, which sets the length of `FILE`
relative to the given `RFILE`,
- `truncate -s NUM FILE`, which sets the length of `FILE` either
absolutely or relative to its curent length.
This organization of the code makes it more concise and easier to
follow.
Create a method that computes the final target size in bytes for the
file to truncate, given the reference file size and the parameter to the
`TruncateMode`.
Add a helper function to contain the code for parsing the size and the
modifier symbol, if any. This commit also changes the `TruncateMode`
enum so that the parameter for each "mode" is stored along with the
enumeration value. This is because the parameter has a different meaning
in each mode.
Remove "read" permissions from the `OpenOptions` when opening a new file
just to truncate it. We will never read from the file, only write to
it. (Specifically, we will only call `File::set_len()`.)
