Log transient network request failures (#3933)

We retry several kinds of network request failures, but it's often
unclear whether a request was retried or not
(https://github.com/astral-sh/uv/issues/3514#issuecomment-2105485773).
This PR adds a small intermediary layer that logs all transient request
failures, adding the `DEBUG Transient request failure` lines:

```
DEBUG Searching for Python interpreter in virtual environments
DEBUG Found CPython 3.12.3 at `/home/konsti/projects/uv/.venv/bin/python3` (active virtual environment)
DEBUG Using Python 3.12.3 environment at .venv/bin/python3
DEBUG Acquired lock for `.venv`
DEBUG At least one requirement is not satisfied: tqdm
DEBUG Using registry request timeout of 30s
DEBUG Solving with target Python version 3.12.3
DEBUG Adding direct dependency: tqdm*
DEBUG No cache entry for: https://pypi.org/simple/tqdm/
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
error: Could not connect, are you offline?
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
```

I decided for multi-line logging to show the complete error trace since
only `Transient request failure for https://pypi.org/simple/tqdm/,
retrying: Request error: error sending request for url
(https://pypi.org/simple/tqdm/)` doesn't tell you the actual problem (a
dns error).

Note that running with `-v` will not show messages about retry backoff
timing, but running with `RUST_LOG=debug` now shows a complete picture:

```
DEBUG starting new connection: https://pypi.org/
DEBUG resolving host="pypi.org"
DEBUG Transient request failure for https://pypi.org/simple/tqdm/, retrying: Request error: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: error sending request for url (https://pypi.org/simple/tqdm/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Name or service not known
  Caused by: failed to lookup address information: Name or service not known
WARN Retry attempt #2. Sleeping 528.728192ms before the next attempt
```

Fixes #3572

Cargo.lock

@@ -4530,6 +4530,7 @@ dependencies = [
  "hyper-util",
  "insta",
  "install-wheel-rs",
+ "itertools 0.13.0",
  "pep440_rs",
  "pep508_rs",
  "platform-tags",

crates/uv-client/Cargo.toml

@@ -29,6 +29,7 @@ fs-err = { workspace = true, features = ["tokio"] }
 futures = { workspace = true }
 html-escape = { workspace = true }
 http = { workspace = true }
+itertools = { workspace = true }
 reqwest = { workspace = true }
 reqwest-middleware = { workspace = true }
 reqwest-retry = { workspace = true }

crates/uv-client/src/base_client.rs

@@ -1,14 +1,20 @@
-use pep508_rs::MarkerEnvironment;
-use platform_tags::Platform;
-use reqwest::{Client, ClientBuilder};
-use reqwest_middleware::ClientWithMiddleware;
-use reqwest_retry::policies::ExponentialBackoff;
-use reqwest_retry::RetryTransientMiddleware;
-use std::env;
+use std::error::Error;
 use std::fmt::Debug;
 use std::ops::Deref;
 use std::path::Path;
+use std::{env, iter};
+
+use itertools::Itertools;
+use reqwest::{Client, ClientBuilder, Response};
+use reqwest_middleware::ClientWithMiddleware;
+use reqwest_retry::policies::ExponentialBackoff;
+use reqwest_retry::{
+    DefaultRetryableStrategy, RetryTransientMiddleware, Retryable, RetryableStrategy,
+};
 use tracing::debug;
+
+use pep508_rs::MarkerEnvironment;
+use platform_tags::Platform;
 use uv_auth::AuthMiddleware;
 use uv_configuration::KeyringProviderType;
 use uv_fs::Simplified;
@@ -166,7 +172,10 @@ impl<'a> BaseClientBuilder<'a> {
                 // Initialize the retry strategy.
                 let retry_policy =
                     ExponentialBackoff::builder().build_with_max_retries(self.retries);
-                let retry_strategy = RetryTransientMiddleware::new_with_policy(retry_policy);
+                let retry_strategy = RetryTransientMiddleware::new_with_policy_and_strategy(
+                    retry_policy,
+                    LoggingRetryableStrategy,
+                );
                 let client = client.with(retry_strategy);
 
                 // Initialize the authentication middleware to set headers.
@@ -225,3 +234,30 @@ impl Deref for BaseClient {
         &self.client
     }
 }
+
+/// The same as [`DefaultRetryableStrategy`], but retry attempts on transient request failures are
+/// logged, so we can tell whether a request was retried before failing or not.
+struct LoggingRetryableStrategy;
+
+impl RetryableStrategy for LoggingRetryableStrategy {
+    fn handle(&self, res: &Result<Response, reqwest_middleware::Error>) -> Option<Retryable> {
+        let retryable = DefaultRetryableStrategy.handle(res);
+        if retryable == Some(Retryable::Transient) {
+            match res {
+                Ok(response) => {
+                    debug!("Transient request failure for: {}", response.url());
+                }
+                Err(err) => {
+                    let context = iter::successors(err.source(), |&err| err.source())
+                        .map(|err| format!("  Caused by: {err}"))
+                        .join("\n");
+                    debug!(
+                        "Transient request failure for {}, retrying: {err}\n{context}",
+                        err.url().map(|url| url.as_str()).unwrap_or("unknown URL")
+                    );
+                }
+            }
+        }
+        retryable
+    }
+}