Detect musl and error for musl pbs builds (#6643)

As described in #4242, we're currently incorrectly downloading glibc
python-build-standalone on musl target, but we also can't fix this by
using musl python-build-standalone on musl targets since the musl builds
are effectively broken.

We reintroduce the libc detection previously removed in #2381, using it
to detect which libc is the current one before we have a python
interpreter. I changed the strategy a big to support an empty `PATH`
which we use in the tests.

For simplicity, i've decided to just filter out the musl
python-build-standalone archives from the list of available archive,
given this is temporary. This means we show the same error message as if
we don't have a build for the platform. We could also add a dedicated
error message for musl.

Fixes #4242

## Test Plan

Tested manually.

On my ubuntu host, python downloads continue to pass:
```
target/x86_64-unknown-linux-musl/debug/uv python install
```

On alpine, we fail:
```
$ docker run -it --rm -v .:/io alpine /io/target/x86_64-unknown-linux-musl/debug/uv python install
  Searching for Python installations
  error: No download found for request: cpython-any-linux-x86_64-musl
```

Cargo.toml

@@ -90,6 +90,7 @@ fs-err = { version = "2.11.0" }
 fs2 = { version = "0.4.3" }
 futures = { version = "0.3.30" }
 glob = { version = "0.3.1" }
+goblin = { version = "0.8.2", default-features = false, features = ["std", "elf32", "elf64", "endian_fd"] }
 hex = { version = "0.4.3" }
 home = { version = "0.5.9" }
 html-escape = { version = "0.2.13" }

crates/uv-python/Cargo.toml

@@ -32,6 +32,7 @@ clap = { workspace = true, optional = true }
 configparser = { workspace = true }
 fs-err = { workspace = true, features = ["tokio"] }
 futures = { workspace = true }
+goblin = { workspace = true }
 itertools = { workspace = true }
 owo-colors = { workspace = true }
 regex = { workspace = true }

crates/uv-python/src/downloads.rs

@@ -22,6 +22,7 @@ use crate::implementation::{
     Error as ImplementationError, ImplementationName, LenientImplementationName,
 };
 use crate::installation::PythonInstallationKey;
+use crate::libc::LibcDetectionError;
 use crate::platform::{self, Arch, Libc, Os};
 use crate::{Interpreter, PythonRequest, PythonVersion, VersionRequest};
 
@@ -75,6 +76,8 @@ pub enum Error {
         "A mirror was provided via `{0}`, but the URL does not match the expected format: {0}"
     )]
     Mirror(&'static str, &'static str),
+    #[error(transparent)]
+    LibcDetection(#[from] LibcDetectionError),
 }
 
 #[derive(Debug, PartialEq)]
@@ -167,8 +170,7 @@ impl PythonDownloadRequest {
     /// Fill empty entries with default values.
     ///
     /// Platform information is pulled from the environment.
-    #[must_use]
-    pub fn fill(mut self) -> Self {
+    pub fn fill(mut self) -> Result<Self, Error> {
         if self.implementation.is_none() {
             self.implementation = Some(ImplementationName::CPython);
         }
@@ -179,9 +181,9 @@ impl PythonDownloadRequest {
             self.os = Some(Os::from_env());
         }
         if self.libc.is_none() {
-            self.libc = Some(Libc::from_env());
+            self.libc = Some(Libc::from_env()?);
         }
-        self
+        Ok(self)
     }
 
     /// Construct a new [`PythonDownloadRequest`] with platform information from the environment.
@@ -191,7 +193,7 @@ impl PythonDownloadRequest {
             None,
             Some(Arch::from_env()),
             Some(Os::from_env()),
-            Some(Libc::from_env()),
+            Some(Libc::from_env()?),
         ))
     }
 
@@ -387,7 +389,11 @@ impl ManagedPythonDownload {
 
     /// Iterate over all [`PythonDownload`]'s.
     pub fn iter_all() -> impl Iterator<Item = &'static ManagedPythonDownload> {
-        PYTHON_DOWNLOADS.iter()
+        PYTHON_DOWNLOADS
+            .iter()
+            // TODO(konsti): musl python-build-standalone builds are currently broken (statically
+            // linked), so we pretend they don't exist. https://github.com/astral-sh/uv/issues/4242
+            .filter(|download| download.key.libc != Libc::Some(target_lexicon::Environment::Musl))
     }
 
     pub fn url(&self) -> &str {

crates/uv-python/src/installation.rs

@@ -99,7 +99,7 @@ impl PythonInstallation {
             {
                 if let Some(request) = PythonDownloadRequest::from_request(&request) {
                     debug!("Requested Python not found, checking for available download...");
-                    match Self::fetch(request.fill(), client_builder, cache, reporter).await {
+                    match Self::fetch(request.fill()?, client_builder, cache, reporter).await {
                         Ok(installation) => Ok(installation),
                         Err(Error::Download(downloads::Error::NoDownloadFound(_))) => {
                             Err(Error::MissingPython(err))

crates/uv-python/src/lib.rs

@@ -24,6 +24,7 @@ mod environment;
 mod implementation;
 mod installation;
 mod interpreter;
+mod libc;
 pub mod managed;
 pub mod platform;
 mod pointer_size;