fix(tvix): distinguish search- and relative path resolution errors

Failures to resolve a nix search path lookup in angle brackets can be
caught using tryEval (if it reaches the runtime). Resolving relative
paths (either to the current directory or the current user's home) can
never be caught, even if they happen inside a thunk at runtime (which is
currently the case for home-relative paths).

Change-Id: I7f73221df66d82a381dd4063358906257826995a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7025
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler/mod.rs

@@ -115,7 +115,7 @@ impl<'observer> Compiler<'observer> {
             Some(dir) if cfg!(target_arch = "wasm32") || dir.is_absolute() => Ok(dir),
             _ => {
                 let current_dir = std::env::current_dir().map_err(|e| Error {
-                    kind: ErrorKind::PathResolution(format!(
+                    kind: ErrorKind::RelativePathResolution(format!(
                         "could not determine current directory: {}",
                         e
                     )),
@@ -290,7 +290,7 @@ impl Compiler<'_> {
             if raw_path.len() == 2 {
                 return self.emit_error(
                     node,
-                    ErrorKind::PathResolution("Empty <> path not allowed".into()),
+                    ErrorKind::NixPathResolution("Empty <> path not allowed".into()),
                 );
             }
             let path = &raw_path[1..(raw_path.len() - 1)];

tvix/eval/src/errors.rs

@@ -50,8 +50,11 @@ pub enum ErrorKind {
         rhs: &'static str,
     },
 
-    /// Resolving a user-supplied path literal failed in some way.
+    /// Resolving a user-supplied angle brackets path literal failed in some way.
-    PathResolution(String),
+    NixPathResolution(String),
+
+    /// Resolving a user-supplied relative or home-relative path literal failed in some way.
+    RelativePathResolution(String),
 
     /// Dynamic keys are not allowed in some scopes.
     DynamicKeyInScope(&'static str),
@@ -178,7 +181,7 @@ impl ErrorKind {
     /// Returns `true` if this error can be caught by `builtins.tryEval`
     pub fn is_catchable(&self) -> bool {
         match self {
-            Self::Throw(_) | Self::AssertionFailed | Self::PathResolution(_) => true,
+            Self::Throw(_) | Self::AssertionFailed | Self::NixPathResolution(_) => true,
             Self::ThunkForce(err) => err.kind.is_catchable(),
             _ => false,
         }
@@ -238,7 +241,9 @@ impl Display for Error {
                 write!(f, "can not compare a {} with a {}", lhs, rhs)
             }
 
-            ErrorKind::PathResolution(err) => write!(f, "could not resolve path: {}", err),
+            ErrorKind::NixPathResolution(err) | ErrorKind::RelativePathResolution(err) => {
+                write!(f, "could not resolve path: {}", err)
+            }
 
             ErrorKind::DynamicKeyInScope(scope) => {
                 write!(f, "dynamically evaluated keys are not allowed in {}", scope)
@@ -619,7 +624,9 @@ impl Error {
         let label = match &self.kind {
             ErrorKind::DuplicateAttrsKey { .. } => "in this attribute set",
             ErrorKind::InvalidAttributeName(_) => "in this attribute set",
-            ErrorKind::PathResolution(_) => "in this path literal",
+            ErrorKind::NixPathResolution(_) | ErrorKind::RelativePathResolution(_) => {
+                "in this path literal"
+            }
             ErrorKind::UnexpectedArgument { .. } => "in this function call",
 
             // The spans for some errors don't have any more descriptive stuff
@@ -668,7 +675,7 @@ impl Error {
             ErrorKind::AttributeNotFound { .. } => "E005",
             ErrorKind::TypeError { .. } => "E006",
             ErrorKind::Incomparable { .. } => "E007",
-            ErrorKind::PathResolution(_) => "E008",
+            ErrorKind::NixPathResolution(_) => "E008",
             ErrorKind::DynamicKeyInScope(_) => "E009",
             ErrorKind::UnknownStaticVariable => "E010",
             ErrorKind::UnknownDynamicVariable(_) => "E011",
@@ -691,6 +698,7 @@ impl Error {
             ErrorKind::IO { .. } => "E029",
             ErrorKind::FromJsonError { .. } => "E030",
             ErrorKind::UnexpectedArgument { .. } => "E031",
+            ErrorKind::RelativePathResolution(_) => "E032",
 
             // Placeholder error while Tvix is under construction.
             ErrorKind::NotImplemented(_) => "E999",

tvix/eval/src/nix_search_path.rs

@@ -103,7 +103,7 @@ impl NixSearchPath {
                 return Ok(p);
             }
         }
-        Err(ErrorKind::PathResolution(format!(
+        Err(ErrorKind::NixPathResolution(format!(
             "path '{}' was not found in the Nix search path",
             path.display()
         )))
@@ -178,7 +178,7 @@ mod tests {
             let nix_search_path = NixSearchPath::from_str("./.").unwrap();
             let err = nix_search_path.resolve("nope").unwrap_err();
             assert!(
-                matches!(err, ErrorKind::PathResolution(..)),
+                matches!(err, ErrorKind::NixPathResolution(..)),
                 "err = {err:?}"
             );
         }

tvix/eval/src/vm.rs

@@ -555,7 +555,7 @@ impl<'o> VM<'o> {
                 Value::UnresolvedPath(path) => {
                     match dirs::home_dir() {
                         None => {
-                            return Err(self.error(ErrorKind::PathResolution(
+                            return Err(self.error(ErrorKind::RelativePathResolution(
                                 "failed to determine home directory".into(),
                             )));
                         }