tor_persist/
load_store.rs

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1//! Helper module for loading and storing via serde
2//!
3//! Utilities to load or store a serde-able object,
4//! in JSON format,
5//! to/from a disk file at a caller-specified filename.
6//!
7//! The caller is supposed to do any necessary locking.
8//!
9//! The entrypoints are methods on `[Target]`,
10//! which the caller is supposed to construct.
11
12use std::path::Path;
13
14use fs_mistrust::CheckedDir;
15use serde::{de::DeserializeOwned, Serialize};
16use tor_error::ErrorReport as _;
17use tracing::trace;
18
19use crate::err::ErrorSource;
20
21/// Common arguments to load/store operations
22#[derive(derive_more::Display)]
23#[display("{:?}/{:?}", dir.as_path(), rel_fname)]
24pub(crate) struct Target<'r> {
25    /// Directory
26    pub(crate) dir: &'r CheckedDir,
27
28    /// Filename relative to `dir`
29    ///
30    /// Might be a leafname; must be relative
31    /// Should include the `.json` extension.
32    pub(crate) rel_fname: &'r Path,
33}
34
35impl Target<'_> {
36    /// Load and deserialize a `D` from the file specified by `self`
37    ///
38    /// Returns `None` if the file doesn't exist.
39    pub(crate) fn load<D: DeserializeOwned>(&self) -> Result<Option<D>, ErrorSource> {
40        let string = match self.dir.read_to_string(self.rel_fname) {
41            Ok(string) => string,
42            Err(fs_mistrust::Error::NotFound(_)) => {
43                trace!("loading {self} (not found)");
44                return Ok(None);
45            }
46            Err(e) => {
47                trace!("loading {self}, error {}", e.report());
48                return Err(e.into());
49            }
50        };
51
52        let r = serde_json::from_str(&string)?;
53        trace!("loaded {self}");
54
55        Ok(Some(r))
56    }
57
58    /// Serialise and store an `S` to the file specified by `self`
59    ///
60    /// Concurrent readers (using `load`) will see either the old data,
61    /// or the new data,
62    /// not corruption or a mixture.
63    ///
64    /// Likewise, if something fails, the old data will remain.
65    /// (But, we do *not* use `fsync`.)
66    ///
67    /// It is a serious bug to make several concurrent calls to `store`
68    /// for the same file.
69    /// That might result in corrupted files.
70    ///
71    /// See [`fs_mistrust::CheckedDir::write_and_replace`]
72    /// for more details about the semantics.
73    pub(crate) fn store<S: Serialize>(&self, val: &S) -> Result<(), ErrorSource> {
74        trace!("storing {self}");
75        let output = serde_json::to_string_pretty(val)?;
76
77        self.dir.write_and_replace(self.rel_fname, output)?;
78
79        Ok(())
80    }
81
82    /// Delete the file specified by `self`
83    pub(crate) fn delete(&self) -> Result<(), ErrorSource> {
84        trace!("deleting {self}");
85        self.dir.remove_file(self.rel_fname)?;
86
87        Ok(())
88    }
89}
tor_persist/load_store.rs

tor_persist/
load_store.rs
