Fix stack trace frame names to show enclosing function instead of callee (#652)

## Summary

- Rework `Error.formatError` to label each stack frame with the
**enclosing function** (the function whose body contains that position)
instead of the **callee** (the function being called). This matches the
convention used by Java/Python stack traces.
- The fix scans frames outermost-to-innermost to compute the enclosing
scope for each position. Builtin calls (`std.*`), comprehensions, and
default parameter evaluations are transparent and don't create new
scopes. Consecutive same-scope frames are deduplicated.
- Add regression test for cross-file max-stack-frames error
(`error.max_stack_cross_file`).

### Before

```
sjsonnet.Error: [search] Max stack frames exceeded.
    at [search].(b.jsonnet:5:37)
    at [std.objectHas].(b.jsonnet:5:25)
    at [method].(a.jsonnet:5:23)
    at [m].(a.jsonnet:9:2)
    at [<root>].(a.jsonnet:1:7)
```

### After

```
sjsonnet.Error: [search] Max stack frames exceeded.
    at [method].(b.jsonnet:5:37)
    at [m].(a.jsonnet:5:23)
    at [<root>].(a.jsonnet:9:2)
```

Each `[name]` now reflects the function you are **in**, not the function
you **called**.

## Test plan

- [x] All existing JVM tests pass (140/140)
- [x] Updated 6 golden files to match corrected trace format
- [x] Added new regression test `error.max_stack_cross_file` covering
cross-file recursive max-stack error

Author: stephenamar-db

sjsonnet/src/sjsonnet/Error.scala

@@ -105,40 +105,51 @@ object Error {
 
       val allFrames = err.stack.reverse
       val namedFrames = allFrames.filter(_.exprErrorString != null)
-      val frames = if (namedFrames.nonEmpty) namedFrames else allFrames
+      val rawFrames = if (namedFrames.nonEmpty) namedFrames else allFrames
+
+      // Compute enclosing function scope for each frame by scanning from
+      // outermost to innermost. Each frame's callee name tells us what function
+      // was entered; builtin calls (std.*) are transparent and don't change scope.
+      val scopeNames = new Array[String](rawFrames.length)
+      var currentScope: String = rootName
+      var si = rawFrames.length - 1
+      while (si >= 0) {
+        scopeNames(si) = currentScope
+        val frameName = rawFrames(si).exprErrorString
+        if (frameName != null && !isBuiltinName(frameName)) {
+          currentScope = frameName
+        }
+        si -= 1
+      }
+      if (rawFrames.nonEmpty) {
+        scopeNames(0) = Option(rawFrames(0).exprErrorString).getOrElse(rootName)
+      }
+
+      // Deduplicate consecutive same-scope frames, keeping the innermost (first)
+      val frames = new java.util.ArrayList[(Frame, String)](rawFrames.length)
+      si = 0
+      while (si < rawFrames.length) {
+        val name = scopeNames(si)
+        if (frames.isEmpty || frames.get(frames.size - 1)._2 != name) {
+          frames.add((rawFrames(si), name))
+        }
+        si += 1
+      }
 
       var msg = err.getMessage
       var frameStart = 0
-      while (
-        frameStart < frames.length - 1 &&
-        (frames(frameStart).collapseIntoMessage ||
-        frames(frameStart).pos == frames(frameStart + 1).pos)
-      ) {
-        val name = Option(frames(frameStart).exprErrorString).getOrElse(rootName)
-        msg = "[" + name + "] " + msg
-        frameStart += 1
+      // Collapse innermost frame name into message when flagged, hiding it from "at" lines
+      if (frames.size > 0 && rawFrames(0).collapseIntoMessage) {
+        msg = "[" + frames.get(0)._2 + "] " + msg
+        frameStart = 1
       }
 
-      if (frameStart < frames.length && frames(frameStart).collapseIntoMessage) {
-        val f = frames(frameStart)
-        val name = Option(f.exprErrorString).getOrElse(rootName)
-        msg = "[" + name + "] " + msg
-        sb.append(msg)
-        appendFrame(sb, f, rootName)
-      } else {
-        sb.append(msg)
-        var i = frameStart
-        while (i < frames.length) {
-          val f = frames(i)
-          val name =
-            if (i == frames.length - 1) {
-              val n = f.exprErrorString
-              if (n == null || n == rootName) rootName else n
-            } else
-              Option(f.exprErrorString).getOrElse(rootName)
-          appendFrame(sb, f, name)
-          i += 1
-        }
+      sb.append(msg)
+      var i = frameStart
+      while (i < frames.size) {
+        val (f, name) = frames.get(i)
+        appendFrame(sb, f, name)
+        i += 1
       }
       if (err.getCause != null) {
         sb.append("\nCaused by: ")
@@ -166,6 +177,10 @@ object Error {
       }
   }
 
+  private def isBuiltinName(name: String): Boolean =
+    name.startsWith("std.") || name == "default" ||
+    name == "object comprehension" || name == "array comprehension"
+
   private def appendFrame(sb: StringBuilder, f: Frame, name: String): Unit = {
     sb.append("\n    at [").append(name).append("]")
     if (f.pos != null) {

sjsonnet/test/resources/go_test_suite/recursive_thunk.jsonnet.golden

@@ -1,9 +1,7 @@
 sjsonnet.Error: xxx
     at [bar].(recursive_thunk.jsonnet:1:35)
-    at [foo].(recursive_thunk.jsonnet:2:23)
-    at [bar].(recursive_thunk.jsonnet:2:19)
-    at [foo].(recursive_thunk.jsonnet:2:23)
-    at [bar].(recursive_thunk.jsonnet:2:19)
-    at [foo].(recursive_thunk.jsonnet:3:4)
-    at [<root>].(recursive_thunk.jsonnet:1:7)
+    at [foo].(recursive_thunk.jsonnet:2:19)
+    at [bar].(recursive_thunk.jsonnet:2:23)
+    at [foo].(recursive_thunk.jsonnet:2:19)
+    at [<root>].(recursive_thunk.jsonnet:3:4)
 

sjsonnet/test/resources/go_test_suite/std.flatmap5.jsonnet.golden

@@ -1,5 +1,4 @@
 sjsonnet.Error: a
     at [std.flatMap].(std.flatmap5.jsonnet:1:21)
-    at [std.type].(std.flatmap5.jsonnet:2:9)
-    at [<root>].(std.flatmap5.jsonnet:1:7)
+    at [<root>].(std.flatmap5.jsonnet:2:9)
 

sjsonnet/test/resources/go_test_suite/tailstrict2.jsonnet.golden

@@ -1,5 +1,4 @@
 sjsonnet.Error: xxx
     at [e].(tailstrict2.jsonnet:1:13)
-    at [anonymous].(tailstrict2.jsonnet:2:19)
-    at [<root>].(tailstrict2.jsonnet:1:7)
+    at [<root>].(tailstrict2.jsonnet:2:19)
 

sjsonnet/test/resources/new_test_suite/error.max_stack_cross_file.jsonnet

@@ -0,0 +1,9 @@
+local b = import "error.max_stack_cross_file_helper.libsonnet";
+
+local m = function() (
+  local foo = { bar: "hi" };
+  local bar = b.method(foo);
+  std.toString(foo) + " " + bar
+);
+
+m()

sjsonnet/test/resources/new_test_suite/error.max_stack_cross_file.jsonnet.golden

@@ -0,0 +1,5 @@
+sjsonnet.Error: [search] Max stack frames exceeded.
+    at [method].(error.max_stack_cross_file_helper.libsonnet:5:37)
+    at [m].(error.max_stack_cross_file.jsonnet:5:23)
+    at [<root>].(error.max_stack_cross_file.jsonnet:9:2)
+

sjsonnet/test/resources/new_test_suite/error.max_stack_cross_file_helper.libsonnet

@@ -0,0 +1,8 @@
+local search(s) = search(s);
+
+{
+  method: function(arg) (
+    assert std.objectHas(arg, search("foo"));
+    arg
+  )
+}

sjsonnet/test/resources/test_suite/error.01.jsonnet.golden

@@ -1,6 +1,5 @@
 sjsonnet.Error: foo
     at [bananas].(error.01.jsonnet:17:29)
-    at [oranges].(error.01.jsonnet:19:35)
-    at [apples].(error.01.jsonnet:20:7)
-    at [<root>].(error.01.jsonnet:17:7)
+    at [apples].(error.01.jsonnet:19:35)
+    at [<root>].(error.01.jsonnet:20:7)
 

sjsonnet/test/resources/test_suite/error.recursive_function_nonterm.jsonnet.golden

@@ -1,3 +1,3 @@
 sjsonnet.Error: [f] Max stack frames exceeded.
-    at [f].(error.recursive_function_nonterm.jsonnet:20:2)
-    at [<root>].(error.recursive_function_nonterm.jsonnet:17:7)
+    at [<root>].(error.recursive_function_nonterm.jsonnet:20:2)
+

sjsonnet/test/resources/test_suite/error.tailstrict_stack.jsonnet.golden

@@ -1,6 +1,5 @@
 sjsonnet.Error: n is 0
     at [sum_from_one_to_n].(error.tailstrict_stack.jsonnet:10:9)
-    at [sum_from_one_to_n].(error.tailstrict_stack.jsonnet:14:37)
-    at [foo].(error.tailstrict_stack.jsonnet:19:18)
-    at [<root>].(error.tailstrict_stack.jsonnet:1:7)
+    at [foo].(error.tailstrict_stack.jsonnet:14:37)
+    at [<root>].(error.tailstrict_stack.jsonnet:19:18)