fix(python): preserve historical signed code fields

Keep signed co_stacksize and co_flags for historical magic entries whose constructors accepted them. Preserve magic 3410 with a constructor-revision limitation; reject negative values from 3411 onward.

Fixes #6487

src/python/PythonPycParser.test.ts

@@ -1520,9 +1520,12 @@ it("rejects code CPython treats as corrupt instead of complete coverage", () =>
   expect(() => parse(file("py311", 3531, { flags: 0x80000000 }))).toThrow(
     /co_flags is negative/,
   );
-  expect(() => parse(file("py37", 3401, { flags: 0x80000000 }))).toThrow(
+  expect(() => parse(file("py38", 3411, { flags: 0x80000000 }))).toThrow(
     /co_flags is negative/,
   );
+  expect(() => parse(file("py38", 3411, { stacksize: 0xffffffff }))).toThrow(
+    /co_stacksize is negative/,
+  );
 
   const valid = parse(
     file("py311", 3531, {
@@ -1544,6 +1547,25 @@ it("rejects code CPython treats as corrupt instead of complete coverage", () =>
   const negativePy37Flags = parse(file("py37", 3394, { flags: 0x80000000 }));
   expect(root(negativePy37Flags).flags).toBe(-0x80000000);
   expect(pythonPycSchema.safeParse(negativePy37Flags).success).toBe(true);
+  const negativePy37Stacksize = parse(
+    file("py37", 3394, { stacksize: 0xffffffff }),
+  );
+  expect(root(negativePy37Stacksize).stacksize).toBe(-1);
+  expect(pythonPycSchema.safeParse(negativePy37Stacksize).success).toBe(true);
+  const negativePy38AlphaFlags = parse(
+    file("py37", 3401, { flags: 0x80000000 }),
+  );
+  expect(root(negativePy38AlphaFlags).flags).toBe(-0x80000000);
+  const ambiguousPy38Fields = parse(
+    file("py38", 3410, { stacksize: 0xffffffff, flags: 0x80000000 }),
+  );
+  expect(root(ambiguousPy38Fields)).toMatchObject({
+    stacksize: -1,
+    flags: -0x80000000,
+  });
+  expect(ambiguousPy38Fields.limitations).toEqual(
+    expect.arrayContaining([expect.stringContaining("Magic 3410 is shared")]),
+  );
   const py37 = parse(
     file("py37", 3394, {
       argcount: 1,

src/python/PythonPycParser.ts

@@ -235,7 +235,7 @@ export const parsePythonPyc = (
     marshal,
     mapped.code_layout,
     input,
-    mapped.python_version !== "3.7",
+    mapped.negative_code_fields === "rejected",
   );
   const root = reader.readObject(0);
   if (reader.offset !== marshal.length)
@@ -268,6 +268,12 @@ export const parsePythonPyc = (
         ? "source_hash_hex is the PEP 552 header field; the original .py is not read."
         : "source_mtime and source_size are header fields; they do not prove a matching .py is present.",
       "python_version is the pinned CPython magic table for this header, not a runtime interpreter.",
+      ...(mapped.negative_code_fields === "revision-dependent" &&
+      codeObjects.some((code) => code.stacksize < 0 || code.flags < 0)
+        ? [
+            "Magic 3410 is shared by CPython revisions with different negative co_stacksize/co_flags constructor checks; these raw values are preserved, but this header cannot establish constructor acceptance.",
+          ]
+        : []),
       ...(unknownFlags === 0
         ? []
         : [
@@ -323,7 +329,7 @@ class MarshalReader {
     readonly bytes: Buffer,
     readonly layout: PythonPycCodeLayout,
     readonly input: InspectPythonPycInput,
-    readonly rejectNegativeCodeFlags: boolean,
+    readonly rejectNegativeCodeFields: boolean,
   ) {}
 
   readObject(depth: number): MarshalValue {
@@ -437,9 +443,11 @@ class MarshalReader {
     const kwonlyargcount = this.nonNegativeInt32("co_kwonlyargcount");
     const nlocals =
       this.layout === "py311" ? null : this.nonNegativeInt32("co_nlocals");
-    const stacksize = this.nonNegativeInt32("co_stacksize");
+    const stacksize = this.i32();
+    if (this.rejectNegativeCodeFields && stacksize < 0)
+      malformed("co_stacksize is negative.");
     const flags = this.i32();
-    if (this.rejectNegativeCodeFlags && flags < 0)
+    if (this.rejectNegativeCodeFields && flags < 0)
       malformed("co_flags is negative.");
     const bytecode = asBytes(this.readObject(depth + 1), "co_code");
     // 3.11+ `_PyCode_Validate` rejects a length that is not a whole code unit.
@@ -1501,7 +1509,7 @@ class MarshalReader {
     return value;
   }
 
-  /** CPython `r_long` int fields must not be negative in a code object. */
+  /** CPython `r_long` count fields that its constructors require non-negative. */
   nonNegativeInt32(field: string): number {
     const value = this.i32();
     if (value < 0) malformed(`${field} is negative.`);