From 671e93816a3c638389d5f601c1ea8b1ce0e6d73e Mon Sep 17 00:00:00 2001
From: Andrew Tulloch <andrew@tullo.ch>
Date: Tue, 7 Jan 2025 05:02:33 -0800
Subject: [PATCH] Buffer file reads in `serde_json::from_reader` (#10341)

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## Summary

https://docs.rs/serde_json/latest/serde_json/fn.from_reader.html
suggests that

> When reading from a source against which short reads are not
efficient, such as a
[File](https://doc.rust-lang.org/std/fs/struct.File.html), you will want
to apply your own buffering because serde_json will not buffer the
input. See
[std::io::BufReader](https://doc.rust-lang.org/std/io/struct.BufReader.html).

Without this buffering, we observe a sequence of single byte reads which
can be quite inefficient depending on the underlying filesystem.

This adds buffering with `std::io::BufReader` to resolve this.


<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

Unit tests cover this code.

<!-- How was it tested? -->
---
 crates/uv-distribution-types/src/installed.rs | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/crates/uv-distribution-types/src/installed.rs b/crates/uv-distribution-types/src/installed.rs
index 1d4cf9662..4bd528643 100644
--- a/crates/uv-distribution-types/src/installed.rs
+++ b/crates/uv-distribution-types/src/installed.rs
@@ -1,4 +1,5 @@
 use std::borrow::Cow;
+use std::io::BufReader;
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
 
@@ -309,7 +310,8 @@ impl InstalledDist {
             Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(None),
             Err(err) => return Err(err.into()),
         };
-        let direct_url = serde_json::from_reader::<fs_err::File, DirectUrl>(file)?;
+        let direct_url =
+            serde_json::from_reader::<BufReader<fs_err::File>, DirectUrl>(BufReader::new(file))?;
         Ok(Some(direct_url))
     }
 
@@ -321,7 +323,8 @@ impl InstalledDist {
             Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(None),
             Err(err) => return Err(err.into()),
         };
-        let cache_info = serde_json::from_reader::<fs_err::File, CacheInfo>(file)?;
+        let cache_info =
+            serde_json::from_reader::<BufReader<fs_err::File>, CacheInfo>(BufReader::new(file))?;
         Ok(Some(cache_info))
     }