fix(function): report code-placeholder compiler invariants as server failures (#8402)

* fix(function): report code-placeholder compiler invariants as server failures

Overlapping source edits and exhausted parser-sentinel/Python-marker allocation are broken compiler invariants, not problems in the user's code, but they threw CodePlaceholderCompileError and the Function route answered them with 422 like a user syntax error. They now throw a sibling CodePlaceholderInvariantError, which the route's instanceof check does not match, so it falls through to the existing 500 path. User placeholder and syntax errors keep their 422.

* fix(function): keep an exhausted JavaScript sentinel space a user compile error

User code can occupy every sentinel a short placeholder could take, so running
out of them is the code's fault, not a compiler invariant.

Author: waleedlatif1

apps/sim/lib/execution/code-placeholders/compiler.test.ts

@@ -6,6 +6,11 @@ import {
   type CodePlaceholderRuntimeBinding,
   compileCodePlaceholders,
 } from '@/lib/execution/code-placeholders'
+import {
+  applySourceEdits,
+  CodePlaceholderCompileError,
+  CodePlaceholderInvariantError,
+} from '@/lib/execution/code-placeholders/shared'
 import { CodeLanguage } from '@/lib/execution/languages'
 
 const installedGlobals = new Set<string>()
@@ -1543,3 +1548,29 @@ describe('python true positives survive the dot guard', () => {
     expect(await directReadNames(code, CodeLanguage.Python)).toEqual(['API_KEY'])
   })
 })
+
+describe('compiler failure classification', () => {
+  it('reports overlapping source edits as a compiler invariant, not a user compile error', () => {
+    const applyOverlappingEdits = () =>
+      applySourceEdits('return value', [
+        { start: 0, end: 6, text: 'yield' },
+        { start: 3, end: 9, text: 'x' },
+      ])
+
+    expect(applyOverlappingEdits).toThrow(CodePlaceholderInvariantError)
+    expect(applyOverlappingEdits).not.toThrow(CodePlaceholderCompileError)
+  })
+
+  it('reports an exhausted JavaScript sentinel space as a user compile error', async () => {
+    /** Every `$xyz$` sentinel a five-character `{{a}}` could take, packed under the code cap. */
+    const alphabet = [...'0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ']
+    const payloads = alphabet.flatMap((a) =>
+      alphabet.flatMap((b) => alphabet.map((c) => `${a}${b}${c}`))
+    )
+    const code = `const taken = '$${payloads.join('$')}$'\nreturn {{a}}`
+
+    await expect(
+      compileCodePlaceholders({ code, language: CodeLanguage.JavaScript, params: { a: 1 } })
+    ).rejects.toBeInstanceOf(CodePlaceholderCompileError)
+  })
+})

apps/sim/lib/execution/code-placeholders/python.ts

@@ -2,6 +2,7 @@ import { sha256Hex } from '@sim/security/hash'
 import {
   applySourceEdits,
   CodePlaceholderCompileError,
+  CodePlaceholderInvariantError,
   createCodePlaceholderCompilationContext,
   isOffsetInRanges,
   type SourceEdit,
@@ -243,7 +244,7 @@ function createSentinel(code: string): string {
     const sentinel = `__sim_placeholder_${digest}__`
     if (!code.includes(sentinel)) return sentinel
   }
-  throw new CodePlaceholderCompileError('Unable to allocate a collision-free Python marker')
+  throw new CodePlaceholderInvariantError('Unable to allocate a collision-free Python marker')
 }
 
 function pythonTriviaGap(

apps/sim/lib/execution/code-placeholders/shared.ts

@@ -27,6 +27,18 @@ export class CodePlaceholderCompileError extends Error {
   }
 }
 
+/**
+ * A broken compiler invariant: Sim's own compilation failed, not the user's code.
+ * Deliberately not a {@link CodePlaceholderCompileError}, so callers that answer a
+ * user compile error with a client status treat this as a server failure.
+ */
+export class CodePlaceholderInvariantError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'CodePlaceholderInvariantError'
+  }
+}
+
 /**
  * Scans `{{name}}` placeholders, accepting exactly what `/\{\{([^}]+)\}\}/g` accepts —
  * a name may contain `{`, because parameter keys are arbitrary strings rather than
@@ -270,7 +282,7 @@ export function applySourceEdits(code: string, edits: SourceEdit[]): string {
   let output = ''
   for (const edit of sorted) {
     if (edit.start < cursor || edit.end < edit.start || edit.end > code.length) {
-      throw new CodePlaceholderCompileError('Overlapping code placeholder transformations')
+      throw new CodePlaceholderInvariantError('Overlapping code placeholder transformations')
     }
     output += code.slice(cursor, edit.start)
     output += edit.text