Fix relative and absolute path handling in lockfiles (#4266)

Previously, `b` in the test case would have been incorrectly locked to
the path of `a`. I've moved `relative_to` into uv-fs since it's now used
in two different places.

Previously failing lockfile when `a/pyproject.toml` and
`a/b/pyproject.toml` exist (not in a workspace) and `a` was depending on
`b`:

```toml
version = 1
requires-python = ">=3.11, <3.13"

[[distribution]]
name = "b"
version = "0.1.0"
source = "directory+/home/konsti/projects/uv/a"
sdist = { path = "/home/konsti/projects/uv/a" }

[[distribution]]
name = "black"
version = "0.1.0"
source = "editable+."
sdist = { path = "." }

[[distribution.dependencies]]
name = "b"
version = "0.1.0"
source = "directory+/home/konsti/projects/uv/a"
```

crates/install-wheel-rs/src/wheel.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::io::{BufRead, BufReader, Cursor, Read, Write};
 use std::path::{Path, PathBuf};
-use std::{env, io, iter};
+use std::{env, io};
 
 use data_encoding::BASE64URL_NOPAD;
 use fs_err as fs;
@@ -15,7 +15,7 @@ use zip::write::FileOptions;
 use zip::ZipWriter;
 
 use pypi_types::DirectUrl;
-use uv_fs::Simplified;
+use uv_fs::{relative_to, Simplified};
 
 use crate::record::RecordEntry;
 use crate::script::Script;
@@ -368,38 +368,6 @@ pub(crate) fn parse_wheel_file(wheel_text: &str) -> Result<LibKind, Error> {
     Ok(lib_kind)
 }
 
-/// Give the path relative to the base directory
-///
-/// lib/python/site-packages/foo/__init__.py and lib/python/site-packages -> foo/__init__.py
-/// lib/marker.txt and lib/python/site-packages -> ../../marker.txt
-/// `bin/foo_launcher` and lib/python/site-packages -> ../../../`bin/foo_launcher`
-pub(crate) fn relative_to(path: &Path, base: &Path) -> Result<PathBuf, Error> {
-    // Find the longest common prefix, and also return the path stripped from that prefix
-    let (stripped, common_prefix) = base
-        .ancestors()
-        .find_map(|ancestor| {
-            path.strip_prefix(ancestor)
-                .ok()
-                .map(|stripped| (stripped, ancestor))
-        })
-        .ok_or_else(|| {
-            Error::Io(io::Error::new(
-                io::ErrorKind::Other,
-                format!(
-                    "Trivial strip failed: {} vs. {}",
-                    path.simplified_display(),
-                    base.simplified_display()
-                ),
-            ))
-        })?;
-
-    // go as many levels up as required
-    let levels_up = base.components().count() - common_prefix.components().count();
-    let up = iter::repeat("..").take(levels_up).collect::<PathBuf>();
-
-    Ok(up.join(stripped))
-}
-
 /// Moves the files and folders in src to dest, updating the RECORD in the process
 pub(crate) fn move_folder_recorded(
     src_dir: &Path,
@@ -759,7 +727,7 @@ mod test {
     use crate::wheel::format_shebang;
     use crate::Error;
 
-    use super::{parse_key_value_file, parse_wheel_file, read_record_file, relative_to, Script};
+    use super::{parse_key_value_file, parse_wheel_file, read_record_file, Script};
 
     #[test]
     fn test_parse_key_value_file() {
@@ -816,34 +784,6 @@ mod test {
         assert_eq!(expected, actual);
     }
 
-    #[test]
-    fn test_relative_to() {
-        assert_eq!(
-            relative_to(
-                Path::new("/home/ferris/carcinization/lib/python/site-packages/foo/__init__.py"),
-                Path::new("/home/ferris/carcinization/lib/python/site-packages"),
-            )
-            .unwrap(),
-            Path::new("foo/__init__.py")
-        );
-        assert_eq!(
-            relative_to(
-                Path::new("/home/ferris/carcinization/lib/marker.txt"),
-                Path::new("/home/ferris/carcinization/lib/python/site-packages"),
-            )
-            .unwrap(),
-            Path::new("../../marker.txt")
-        );
-        assert_eq!(
-            relative_to(
-                Path::new("/home/ferris/carcinization/bin/foo_launcher"),
-                Path::new("/home/ferris/carcinization/lib/python/site-packages"),
-            )
-            .unwrap(),
-            Path::new("../../../bin/foo_launcher")
-        );
-    }
-
     #[test]
     fn test_script_from_value() {
         assert_eq!(

crates/uv-distribution/src/metadata/lowering.rs

@@ -10,7 +10,7 @@ use pep440_rs::VersionSpecifiers;
 use pep508_rs::{VerbatimUrl, VersionOrUrl};
 use pypi_types::{Requirement, RequirementSource, VerbatimParsedUrl};
 use uv_configuration::PreviewMode;
-use uv_fs::Simplified;
+use uv_fs::{relative_to, Simplified};
 use uv_git::GitReference;
 use uv_normalize::PackageName;
 use uv_warnings::warn_user_once;
@@ -44,6 +44,8 @@ pub enum LoweringError {
     MissingPreview,
     #[error("Editable must refer to a local directory, not a file: `{0}`")]
     EditableFile(String),
+    #[error(transparent)] // Function attaches the context
+    RelativeTo(io::Error),
 }
 
 /// Combine `project.dependencies` or `project.optional-dependencies` with `tool.uv.sources`.
@@ -234,25 +236,30 @@ fn path_source(
     workspace_root: &Path,
     editable: bool,
 ) -> Result<RequirementSource, LoweringError> {
-    let url = VerbatimUrl::parse_path(path.as_ref(), project_dir)?
-        .with_given(path.as_ref().to_string_lossy());
-    let path_buf = path.as_ref().to_path_buf();
-    let path_buf = path_buf
+    let path = path.as_ref();
+    let url = VerbatimUrl::parse_path(path, project_dir)?.with_given(path.to_string_lossy());
+    let absolute_path = path
+        .to_path_buf()
         .absolutize_from(project_dir)
-        .map_err(|err| LoweringError::Absolutize(path.as_ref().to_path_buf(), err))?
+        .map_err(|err| LoweringError::Absolutize(path.to_path_buf(), err))?
         .to_path_buf();
-    let ascend_to_workspace = project_dir
-        .strip_prefix(workspace_root)
-        .expect("Project must be below workspace root");
-    let is_dir = if let Ok(metadata) = path_buf.metadata() {
+    let relative_to_workspace = if path.is_relative() {
+        // Relative paths in a project are relative to the project root, but the lockfile is
+        // relative to the workspace root.
+        relative_to(&absolute_path, workspace_root).map_err(LoweringError::RelativeTo)?
+    } else {
+        // If the user gave us an absolute path, we respect that.
+        path.to_path_buf()
+    };
+    let is_dir = if let Ok(metadata) = absolute_path.metadata() {
         metadata.is_dir()
     } else {
-        path_buf.extension().is_none()
+        absolute_path.extension().is_none()
     };
     if is_dir {
         Ok(RequirementSource::Directory {
-            install_path: path_buf,
-            lock_path: ascend_to_workspace.join(project_dir),
+            install_path: absolute_path,
+            lock_path: relative_to_workspace,
             url,
             editable,
         })
@@ -261,8 +268,8 @@ fn path_source(
             return Err(LoweringError::EditableFile(url.to_string()));
         }
         Ok(RequirementSource::Path {
-            install_path: path_buf,
-            lock_path: ascend_to_workspace.join(project_dir),
+            install_path: absolute_path,
+            lock_path: relative_to_workspace,
             url,
         })
     }
@@ -275,19 +282,24 @@ fn directory_source(
     workspace_root: &Path,
     editable: bool,
 ) -> Result<RequirementSource, LoweringError> {
-    let url = VerbatimUrl::parse_path(path.as_ref(), project_dir)?
-        .with_given(path.as_ref().to_string_lossy());
-    let path_buf = path.as_ref().to_path_buf();
-    let path_buf = path_buf
+    let path = path.as_ref();
+    let url = VerbatimUrl::parse_path(path, project_dir)?.with_given(path.to_string_lossy());
+    let absolute_path = path
+        .to_path_buf()
         .absolutize_from(project_dir)
-        .map_err(|err| LoweringError::Absolutize(path.as_ref().to_path_buf(), err))?
+        .map_err(|err| LoweringError::Absolutize(path.to_path_buf(), err))?
         .to_path_buf();
-    let ascend_to_workspace = project_dir
-        .strip_prefix(workspace_root)
-        .expect("Project must be below workspace root");
+    let relative_to_workspace = if path.is_relative() {
+        // Relative paths in a project are relative to the project root, but the lockfile is
+        // relative to the workspace root.
+        relative_to(&absolute_path, workspace_root).map_err(LoweringError::RelativeTo)?
+    } else {
+        // If the user gave us an absolute path, we respect that.
+        path.to_path_buf()
+    };
     Ok(RequirementSource::Directory {
-        install_path: path_buf,
-        lock_path: ascend_to_workspace.join(project_dir),
+        install_path: absolute_path,
+        lock_path: relative_to_workspace,
         url,
         editable,
     })

crates/uv-fs/src/path.rs

@@ -1,6 +1,7 @@
 use either::Either;
 use std::borrow::Cow;
 use std::path::{Component, Path, PathBuf};
+use std::{io, iter};
 
 use once_cell::sync::Lazy;
 use path_slash::PathExt;
@@ -320,6 +321,40 @@ fn is_windows_store_python(path: &Path) -> bool {
     is_windows_store_python_shim(path) || is_windows_store_python_executable(path)
 }
 
+/// Compute a path describing `path` relative to `base`.
+///
+/// `lib/python/site-packages/foo/__init__.py` and `lib/python/site-packages` -> `foo/__init__.py`
+/// `lib/marker.txt` and `lib/python/site-packages` -> `../../marker.txt`
+/// `bin/foo_launcher` and `lib/python/site-packages` -> `../../../bin/foo_launcher`
+pub fn relative_to(path: impl AsRef<Path>, base: impl AsRef<Path>) -> Result<PathBuf, io::Error> {
+    // Find the longest common prefix, and also return the path stripped from that prefix
+    let (stripped, common_prefix) = base
+        .as_ref()
+        .ancestors()
+        .find_map(|ancestor| {
+            path.as_ref()
+                .strip_prefix(ancestor)
+                .ok()
+                .map(|stripped| (stripped, ancestor))
+        })
+        .ok_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::Other,
+                format!(
+                    "Trivial strip failed: {} vs. {}",
+                    path.simplified_display(),
+                    base.simplified_display()
+                ),
+            )
+        })?;
+
+    // go as many levels up as required
+    let levels_up = base.as_ref().components().count() - common_prefix.components().count();
+    let up = iter::repeat("..").take(levels_up).collect::<PathBuf>();
+
+    Ok(up.join(stripped))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -378,4 +413,32 @@ mod tests {
         let err = normalize_path(path).unwrap_err();
         assert_eq!(err.kind(), std::io::ErrorKind::InvalidInput);
     }
+
+    #[test]
+    fn test_relative_to() {
+        assert_eq!(
+            relative_to(
+                Path::new("/home/ferris/carcinization/lib/python/site-packages/foo/__init__.py"),
+                Path::new("/home/ferris/carcinization/lib/python/site-packages"),
+            )
+            .unwrap(),
+            Path::new("foo/__init__.py")
+        );
+        assert_eq!(
+            relative_to(
+                Path::new("/home/ferris/carcinization/lib/marker.txt"),
+                Path::new("/home/ferris/carcinization/lib/python/site-packages"),
+            )
+            .unwrap(),
+            Path::new("../../marker.txt")
+        );
+        assert_eq!(
+            relative_to(
+                Path::new("/home/ferris/carcinization/bin/foo_launcher"),
+                Path::new("/home/ferris/carcinization/lib/python/site-packages"),
+            )
+            .unwrap(),
+            Path::new("../../../bin/foo_launcher")
+        );
+    }
 }