[ty] add cycle handling for FunctionType::signature query (#17833)

This fixes cycle panics in several ecosystem projects (moved to
`good.txt` in a following PR
https://github.com/astral-sh/ruff/pull/17834 because our mypy-primer job
doesn't handle it well if we move projects to `good.txt` in the same PR
that fixes `ty` to handle them), as well as in the minimal case in the
added mdtest. It also fixes a number of panicking fuzzer seeds. It
doesn't appear to cause any regression in any ecosystem project or any
fuzzer seed.

crates/ty_python_semantic/src/types.rs

@@ -6495,6 +6495,11 @@ impl<'db> FunctionSignature<'db> {
     pub(crate) fn iter(&self) -> Iter<Signature<'db>> {
         self.as_slice().iter()
     }
+
+    /// Returns the "bottom" signature (subtype of all fully-static signatures.)
+    pub(crate) fn bottom(db: &'db dyn Db) -> Self {
+        Self::Single(Signature::bottom(db))
+    }
 }
 
 impl<'db> IntoIterator for &'db FunctionSignature<'db> {
@@ -6639,7 +6644,7 @@ impl<'db> FunctionType<'db> {
     ///
     /// Were this not a salsa query, then the calling query
     /// would depend on the function's AST and rerun for every change in that file.
-    #[salsa::tracked(return_ref)]
+    #[salsa::tracked(return_ref, cycle_fn=signature_cycle_recover, cycle_initial=signature_cycle_initial)]
     pub(crate) fn signature(self, db: &'db dyn Db) -> FunctionSignature<'db> {
         if let Some(overloaded) = self.to_overloaded(db) {
             FunctionSignature::Overloaded(
@@ -6849,6 +6854,22 @@ impl<'db> FunctionType<'db> {
     }
 }
 
+fn signature_cycle_recover<'db>(
+    _db: &'db dyn Db,
+    _value: &FunctionSignature<'db>,
+    _count: u32,
+    _function: FunctionType<'db>,
+) -> salsa::CycleRecoveryAction<FunctionSignature<'db>> {
+    salsa::CycleRecoveryAction::Iterate
+}
+
+fn signature_cycle_initial<'db>(
+    db: &'db dyn Db,
+    _function: FunctionType<'db>,
+) -> FunctionSignature<'db> {
+    FunctionSignature::bottom(db)
+}
+
 /// Non-exhaustive enumeration of known functions (e.g. `builtins.reveal_type`, ...) that might
 /// have special behavior.
 #[derive(
@@ -7060,10 +7081,7 @@ impl<'db> CallableType<'db> {
     /// `(*args: object, **kwargs: object) -> Never`.
     #[cfg(test)]
     pub(crate) fn bottom(db: &'db dyn Db) -> Type<'db> {
-        Type::Callable(CallableType::single(
-            db,
-            Signature::new(Parameters::object(db), Some(Type::Never)),
-        ))
+        Type::Callable(CallableType::single(db, Signature::bottom(db)))
     }
 
     /// Return a "normalized" version of this `Callable` type.

crates/ty_python_semantic/src/types/signatures.rs

@@ -291,6 +291,11 @@ impl<'db> Signature<'db> {
         }
     }
 
+    /// Return the "bottom" signature, subtype of all other fully-static signatures.
+    pub(crate) fn bottom(db: &'db dyn Db) -> Self {
+        Self::new(Parameters::object(db), Some(Type::Never))
+    }
+
     pub(crate) fn normalized(&self, db: &'db dyn Db) -> Self {
         Self {
             generic_context: self.generic_context,
@@ -957,7 +962,6 @@ impl<'db> Parameters<'db> {
     }
 
     /// Return parameters that represents `(*args: object, **kwargs: object)`.
-    #[cfg(test)]
     pub(crate) fn object(db: &'db dyn Db) -> Self {
         Self {
             value: vec![