Discard insufficient fork markers (#10682)

In #10669, a pyproject.toml with requires-python but no environment had
a lockfile covering only a subset of the requires-python space:

```toml
resolution-markers = [
    "python_full_version >= '3.10' and platform_python_implementation == 'CPython'",
    "python_full_version == '3.9.*'",
    "python_full_version < '3.9'",
]
```

This marker set is invalid, we have to reject the lockfile. (We can
still use the versions though, to avoid churn).

Part 1/2 of #10669

crates/uv-resolver/src/lock/mod.rs

@@ -723,8 +723,43 @@ impl Lock {
         self.fork_markers.as_slice()
     }
 
+    /// Checks whether the fork markers cover the entire supported marker space.
+    ///
+    /// Returns the actually covered and the expected marker space on validation error.
+    pub fn check_marker_coverage(&self) -> Result<(), (MarkerTree, MarkerTree)> {
+        let fork_markers_union = if self.fork_markers().is_empty() {
+            self.requires_python.to_marker_tree()
+        } else {
+            let mut fork_markers_union = MarkerTree::FALSE;
+            for fork_marker in self.fork_markers() {
+                fork_markers_union.or(fork_marker.pep508());
+            }
+            fork_markers_union
+        };
+        let mut environments_union = if !self.supported_environments.is_empty() {
+            let mut environments_union = MarkerTree::FALSE;
+            for fork_marker in &self.supported_environments {
+                environments_union.or(*fork_marker);
+            }
+            environments_union
+        } else {
+            MarkerTree::TRUE
+        };
+        // When a user defines environments, they are implicitly constrained by requires-python.
+        environments_union.and(self.requires_python.to_marker_tree());
+        if fork_markers_union.negate().is_disjoint(environments_union) {
+            Ok(())
+        } else {
+            Err((fork_markers_union, environments_union))
+        }
+    }
+
     /// Returns the TOML representation of this lockfile.
     pub fn to_toml(&self) -> Result<String, toml_edit::ser::Error> {
+        // Catch a lockfile where the union of fork markers doesn't cover the supported
+        // environments.
+        debug_assert!(self.check_marker_coverage().is_ok());
+
         // We construct a TOML document manually instead of going through Serde to enable
         // the use of inline tables.
         let mut doc = toml_edit::DocumentMut::new();

crates/uv/src/commands/project/lock.rs

@@ -965,6 +965,15 @@ impl ValidatedLock {
             return Ok(Self::Versions(lock));
         }
 
+        if let Err((fork_markers_union, environments_union)) = lock.check_marker_coverage() {
+            warn_user!(
+                "Ignoring existing lockfile due to fork markers not covering the supported environments: `{}` vs `{}`",
+                fork_markers_union.try_to_string().unwrap_or("true".to_string()),
+                environments_union.try_to_string().unwrap_or("true".to_string()),
+            );
+            return Ok(Self::Versions(lock));
+        }
+
         // If the set of required platforms has changed, we have to perform a clean resolution.
         let expected = lock.simplified_required_environments();
         let actual = required_environments

crates/uv/tests/it/lock.rs

@@ -26372,3 +26372,84 @@ fn lock_empty_extra() -> Result<()> {
 
     Ok(())
 }
+
+/// The fork markers in the lockfile don't cover the supported environments (here: universal). We
+/// need to discard the lockfile.
+#[test]
+fn lock_invalid_fork_markers() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    context.temp_dir.child("pyproject.toml").write_str(
+        r#"
+        [project]
+        name = "attrs"
+        requires-python = ">=3.8"
+        version = "1.0.0"
+
+        [dependency-groups]
+        dev = ["idna"]
+        "#,
+    )?;
+
+    context.temp_dir.child("uv.lock").write_str(
+        r#"
+        version = 1
+        requires-python = ">=3.8"
+        resolution-markers = [
+            "python_full_version >= '3.10' and platform_python_implementation == 'CPython'",
+            "python_full_version == '3.9.*'",
+            "python_full_version < '3.9'",
+        ]
+
+        [options]
+        exclude-newer = "2024-03-25T00:00:00Z"
+
+        [[package]]
+        name = "attrs"
+        version = "1.0.0"
+        source = { editable = "." }
+
+        [package.dev-dependencies]
+        dev = [
+            { name = "idna", marker = "python_full_version < '3.10' or platform_python_implementation == 'CPython'" },
+        ]
+
+        [package.metadata]
+
+        [package.metadata.requires-dev]
+        dev = [{ name = "idna" }]
+
+        [[package]]
+        name = "idna"
+        version = "3.10"
+        source = { registry = "https://pypi.org/simple" }
+        sdist = { url = "https://files.pythonhosted.org/packages/f1/70/7703c29685631f5a7590aa73f1f1d3fa9a380e654b86af429e0934a32f7d/idna-3.10.tar.gz", hash = "sha256:12f65c9b470abda6dc35cf8e63cc574b1c52b11df2c86030af0ac09b01b13ea9", size = 190490 }
+        wheels = [
+            { url = "https://files.pythonhosted.org/packages/76/c6/c88e154df9c4e1a2a66ccf0005a88dfb2650c1dffb6f5ce603dfbd452ce3/idna-3.10-py3-none-any.whl", hash = "sha256:946d195a0d259cbba61165e88e65941f16e9b36ea6ddb97f00452bae8b1287d3", size = 70442 },
+        ]
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Ignoring existing lockfile due to fork markers not covering the supported environments: `(python_full_version >= '3.8' and python_full_version < '3.10') or (python_full_version >= '3.8' and platform_python_implementation == 'CPython')` vs `python_full_version >= '3.8'`
+    Resolved 2 packages in [TIME]
+    Updated idna v3.10 -> v3.6
+    "###);
+
+    // Check that the lockfile got updated and we don't show the warning anymore.
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    "###);
+
+    Ok(())
+}