fix(erlang): select the last repeated BEAM chunk

Fixes #6518

docs/erlang-beam.md

@@ -24,6 +24,9 @@ requires an absolute filesystem path and preserves that exact selection:
 
 The inline Evidence result contains the artifact path, SHA-256, and byte length.
 `chunks` lists every IFF chunk name, file range, and payload SHA-256.
+Repeated names retain separate chunk occurrences. Decoded tables and the Code
+header use the last occurrence of each name, matching OTP's loader; the chunk
+inventory still exposes earlier payloads and their source ranges.
 
 When `AtU8` or `Atom` is present, `atoms` is the 1-based name table and
 `module` is atom 1. Both the pre-OTP 28 table and the OTP 28+ compact table
@@ -43,4 +46,6 @@ import, and lambda rows together. Malformed `FOR1`/`BEAM` headers, truncated
 chunks, and `AtU8` names that are not valid UTF-8 fail without partial success.
 
 This workflow does not execute bytecode, inflate `LitT` literals, or decode
-`Attr`/`CInf` external-term payloads.
+`Attr`/`CInf` external-term payloads. Complete coverage describes this static
+chunk and table report; it does not establish that an Erlang runtime can load
+or execute the module.

src/erlang/BeamParser.test.ts

@@ -79,6 +79,52 @@ it("reads atoms, exports, imports, lambdas and the Code header", () => {
   ]);
 });
 
+it("selects the last table and Code payload for every repeated chunk name", () => {
+  const original = erlangBeamFixture("selected");
+  const decoy = erlangBeamFixture("decoy");
+  const changedFields = new Map([
+    ["Code", 16],
+    ["ExpT", 8],
+    ["ImpT", 12],
+    ["FunT", 8],
+  ]);
+  for (let at = 12; at < decoy.length;) {
+    const name = decoy.toString("latin1", at, at + 4);
+    const size = decoy.readUInt32BE(at + 4);
+    const field = changedFields.get(name);
+    if (field !== undefined) decoy.writeUInt32BE(9, at + 8 + field);
+    at += 8 + size + ((4 - (size % 4)) % 4);
+  }
+  const repeated = Buffer.concat([
+    original.subarray(0, 12),
+    decoy.subarray(12),
+    original.subarray(12),
+  ]);
+  repeated.writeUInt32BE(repeated.length - 8, 4);
+  const report = parse(repeated);
+  const control = parse(original);
+  expect(report.atoms).toEqual(control.atoms);
+  expect(report.exports).toEqual(control.exports);
+  expect(report.imports).toEqual(control.imports);
+  expect(report.lambdas).toEqual(control.lambdas);
+  expect(report.code).toEqual({
+    ...control.code,
+    instructions: {
+      ...control.code!.instructions,
+      offset: control.code!.instructions.offset + decoy.length - 12,
+    },
+  });
+  expect(report.chunks).toHaveLength(control.chunks.length * 2);
+});
+
+it("rejects a malformed selected atom table instead of using an earlier table", () => {
+  const original = erlangBeamFixture();
+  const invalid = singleChunkBeam("AtU8", Buffer.alloc(0));
+  const repeated = Buffer.concat([original, invalid.subarray(12)]);
+  repeated.writeUInt32BE(repeated.length - 8, 4);
+  expect(() => parse(repeated)).toThrow(/truncated/);
+});
+
 // Expected values come from beam_lib:chunks/2 on the same file under OTP 29.
 it("reads a compiler-produced OTP 29 BEAM with the compact atom table", () => {
   const report = parse(erlangBeamOtp29Fixture());

src/erlang/BeamParser.ts

@@ -256,7 +256,9 @@ export const parseErlangBeam = (
         sha256: sha256(payload),
       },
     });
-    if (!payloads.has(name)) payloads.set(name, { payload, offset: at });
+    // OTP's loader replaces the selected payload on each matching chunk.
+    // Keep every occurrence above; decode the last occurrence of each name.
+    payloads.set(name, { payload, offset: at });
     at = padded;
   }
   const atomChunk = payloads.get("AtU8") ?? payloads.get("Atom");

tests/acceptance/erlangBeamCli.test.ts

@@ -72,6 +72,45 @@ cliTest("reports BEAM exports through the public CLI", async ({ cli }) => {
   ).toBe(true);
 });
 
+cliTest(
+  "decodes the last atom table while retaining every chunk occurrence",
+  async ({ cli }) => {
+    const root = await createTestTempDirectory("rea-beam-repeated-atoms-");
+    const original = erlangBeamOtp29Fixture();
+    const decoy = oneAtomBeam(0).subarray(12);
+    const repeated = Buffer.concat([
+      original.subarray(0, 12),
+      decoy,
+      original.subarray(12),
+    ]);
+    repeated.writeUInt32BE(repeated.length - 8, 4);
+    const path = join(root, "repeated.beam");
+    await writeFile(path, repeated);
+    const result = await cli.run({
+      arguments: ["inspect-erlang-beam", path, "--json"],
+    });
+    expect(result.exitCode, result.stderr).toBe(0);
+    const report = toolContract("inspect_erlang_beam").outputSchema.parse(
+      result.json,
+    ).normalized_result;
+    expect(report.module).toBe("rea_beam_fixture");
+    expect(report.exports).toContainEqual({
+      function_index: 2,
+      function: "hello",
+      arity: 1,
+      label: 2,
+    });
+    const atomChunks = report.chunks.filter(({ name }) => name === "AtU8");
+    expect(atomChunks).toHaveLength(2);
+    expect(atomChunks[0]?.payload.sha256).not.toBe(
+      atomChunks[1]?.payload.sha256,
+    );
+    expect(atomChunks[0]?.payload.location.offset).toBe(20);
+    expect(atomChunks[1]?.payload.location.offset).toBe(20 + decoy.length);
+    expect(report.coverage.status).toBe("complete");
+  },
+);
+
 cliTest(
   "inspects an OTP 29 BEAM with an extended compact atom length and preserves source identity",
   async ({ cli }) => {