xch 0.2.0

A CLI utility and rust crate to atomically swap the content of two paths.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127

extern crate uuid;

use std::io;
use std::fs;
use std::path;

use error::{Result, Error};

/// Exchange files/directories by non-atomic renames.
///
/// This tries to swap files or directories by calling `std::fs::rename` 3 times.
/// 1. Rename path1 to a temporary path
/// 2. Rename path2 to path1
/// 3. Rename the temporary path to path2
///
/// This does not happen atomically. This means there is a time frame were path1 does not exist. Or
/// path1 does exists but path2 does not.
///
/// On error the changes are rolled back, if possible.
pub fn xch<A: AsRef<path::Path>, B: AsRef<path::Path>>(path1: A, path2: B) -> Result<()> {
    let path1 = path1.as_ref();
    let path2 = path2.as_ref();

    let one_parent = path1.parent()
        .ok_or_else::<Error, _>(|| format!("Could not find parent directory for {}", path1.display()).into())
        .or_else(|_|
            path2.parent()
                .ok_or_else::<Error, _>(|| format!("Could not find parent directory for {}", path2.display()).into())
        )?;
    let unique_name = format!("{}", uuid::Uuid::new_v4().hyphenated());
    let temp_name = one_parent.join(unique_name);

    let mut transaction = Transaction::new();
    transaction.record_rename(path1, &temp_name);
    transaction.record_rename(path2, path1);
    transaction.record_rename(&temp_name, path2);
    transaction.commit()
}

/// Structure to hold information about a rename to execute.
struct RenameCmd<'a> {
    from: &'a path::Path,
    to: &'a path::Path,
}

impl<'a> RenameCmd<'a> {
    /// Execute the rename represented by this struct.
    fn exec(&self) -> io::Result<()> {
        fs::rename(self.from, self.to)
    }

    /// Rollback the changes created by this struct.
    fn rollback(self) -> io::Result<()> {
        fs::rename(self.to, self.from)
    }
}

/// Represents a new file system transaction.
struct Transaction<'a> {
    to_exec: Vec<RenameCmd<'a>>,
    successful_exec: Vec<RenameCmd<'a>>,
}

impl<'a> Transaction<'a> {
    /// Create a new transaction.
    fn new() -> Transaction<'a> {
        Transaction {
            to_exec: Vec::new(),
            successful_exec: Vec::new(),
        }
    }

    /// Record a rename that should happen on commit.
    fn record_rename(&mut self, from: &'a path::Path, to: &'a path::Path) {
        self.to_exec.push(RenameCmd {
            from,
            to,
        })
    }

    /// Rollback all changes caused by this transaction.
    fn rollback(mut self) -> Result<()> {
        loop {
            if let Some(cmd) = self.successful_exec.pop() {
                if let Err(e) = cmd.rollback() {
                    return Err(e.into());
                }
            } else {
                return Ok(());
            }
        }
    }

    /// Commit all recorded changes.
    ///
    /// On success, all renames are executed as they were recorded.
    /// On error, this calls `rollback()` and tries to undo all changes.
    fn commit(mut self) -> Result<()> {
        let res = {
            let mut drain = self.to_exec.drain(..);
            loop {
                if let Some(cmd) = drain.next() {
                    let ex = cmd.exec();
                    if let Err(e) = ex {
                        break Err(e);
                    } else {
                        self.successful_exec.push(cmd);
                    }
                } else {
                    break Ok(());
                }
            }
        };

        if let Err(rename_err) = res {
            let rollback_res = self.rollback();
            if let Err(e) = rollback_res {
                let rename_err: Error = rename_err.into();
                Err(rename_err.chain_err(|| e))
            } else {
                Err(rename_err.into())
            }
        } else {
            res.map_err(Into::into)
        }
    }
}