Circular <include> tags WITHOUT --recursive cause infinite loop (process never terminates)

[Serhan-Asad]

Circular <include> tags without --recursive cause the preprocessor to enter an infinite loop that never terminates. This is distinct from #521 which covers --recursive mode (hits Python recursion limit and silently succeeds with corrupted output).

File: pdd/preprocess.py — process_include_tags() has no cycle detection and no recursion depth limit in non-recursive mode.

Reproduction

mkdir /tmp/circular && cd /tmp/circular && git init && git commit --allow-empty -m init
mkdir prompts
echo 'Create <include>prompts/b_python.prompt</include>' > prompts/a_python.prompt
echo 'Create <include>prompts/a_python.prompt</include>' > prompts/b_python.prompt
pdd --force preprocess prompts/a_python.prompt
# Process never terminates — must be killed manually

Result: Process runs forever, printing "Processing XML include" in an infinite loop. After 5 seconds, 17,000+ lines of output and growing. Must be killed with Ctrl+C or kill.

Expected: Error message like "Circular include detected: a_python.prompt → b_python.prompt → a_python.prompt" and exit with non-zero code.

Impact

• HIGH — Process hangs forever consuming CPU. User must manually kill it.
• Any prompt with accidental circular includes will hang pdd preprocess, pdd sync, pdd generate, etc.
• No data loss, but wastes developer time debugging why PDD "hangs"

Comparison with #521

	Without --recursive (this issue)	With --recursive (#521)
Behavior	Infinite loop, never terminates	Hits recursion limit, exits 0
Output	17K+ lines and growing	~1,184 bytes corrupted
Severity	Process hangs forever	Silent data corruption

Both should be fixed with cycle detection in process_include_tags().

[Serhan-Asad]

Step 1: Duplicate Check

Status: No duplicates found (related issue exists: #521)

Search Performed

• Searched for: "circular include", "infinite loop preprocess", "recursion include preprocess"
• Issues reviewed: 5

Findings

Issue #521 covers the same root cause (no cycle detection in process_include_tags()) but a different mode and symptom:

• Circular <include> tags silently produce corrupted output (no cycle detection) #521: --recursive mode → hits Python recursion limit → exits 0 with corrupted output
• Circular <include> tags WITHOUT --recursive cause infinite loop (process never terminates) #526: non-recursive mode → infinite loop → process never terminates

The issue author explicitly distinguishes these two cases. #521 is still open with no fix merged, so both issues remain relevant. The fix for both is the same (cycle detection in process_include_tags), but they represent distinct failure modes that should both be verified.

Proceeding with #526 as a new issue.

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Proceeding to Step 2: Documentation Check

[Serhan-Asad]

Step 2: Documentation Check

Status: Confirmed Bug

Documentation Reviewed

• CLAUDE.md (project instructions) — documents <include> tag syntax and pdd preprocess command
• pdd/preprocess.py — source file with process_include_tags()

Findings

The documentation describes <include> tags as a way to include file content into prompts. There is no mention of circular includes being expected or intentional behavior. An infinite loop that requires manual process termination is clearly a bug — no preprocessor should hang indefinitely on malformed input.

The <include> tag documentation in CLAUDE.md states:

<include>path/to/file</include> - Include file content

This implies straightforward file inclusion, not recursive/circular resolution. The lack of cycle detection is an implementation oversight.

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Proceeding to Step 3: Triage

[Serhan-Asad]

Step 3: Triage

Status: Proceed

Summary

We have everything needed to attempt reproduction:

• Expected behavior: Error message with cycle trace and non-zero exit
• Actual behavior: Infinite loop, process never terminates
• Reproduction steps: Fully provided with exact commands
• Code area: pdd/preprocess.py — process_include_tags()

---

Proceeding to Step 4: Reproduction

[Serhan-Asad]

Step 4: Reproduction

Status: Reproduced

Environment

• macOS Darwin 24.3.0
• Python 3.11+
• PDD installed from source (editable mode)
• Branch: main (commit 93fa014)

Steps Taken

1. Created test directory with git init
2. Created two prompt files with circular includes:
   • prompts/a_python.prompt: Create <include>prompts/b_python.prompt</include>
   • prompts/b_python.prompt: Create <include>prompts/a_python.prompt</include>
3. Ran pdd --force preprocess prompts/a_python.prompt with a 5-second timeout

Observed Behavior

Process enters infinite loop and never terminates. Had to kill it after 5 seconds via signal. The "Processing XML include" message prints endlessly as the while loop at line 263 never converges — each substitution re-introduces <include> tags, so prev_text != current_text is always true.

Affected Files

• pdd/preprocess.py:261-271 — The while prev_text != current_text loop in process_include_tags() has no cycle detection. When file A includes file B which includes file A, the substitution never reaches a fixed point.

Root Cause (Preliminary)

The loop assumes included content won't contain new <include> tags (or that they'll eventually all resolve). With circular includes, each iteration replaces tags with content containing new tags, creating an infinite cycle. No depth limit or visited-file tracking exists.

Notes

• Confirmed: non-recursive mode (default) is affected — the recursive parameter only controls whether preprocess() is called on included content, not whether the outer while loop iterates
• The while loop pattern is designed to handle nested includes (include within an include), but has no guard against cycles

---

Proceeding to Step 5: Root Cause Analysis

[Serhan-Asad]

Step 5: Root Cause Analysis

Root Cause Identified: Yes

Summary

The process_include_tags() function in pdd/preprocess.py uses a while prev_text != current_text loop (lines 261-271) to iteratively expand <include> tags. When file A includes file B and file B includes file A, each iteration replaces the include tag with file content that contains a new include tag pointing back — so the text always changes and the loop never terminates. There is zero cycle detection or iteration depth limit.

Technical Details

• Location: pdd/preprocess.py:261-271
• Problematic Code:

prev_text = ""
current_text = text
while prev_text != current_text:          # Line 263: no max iterations
    prev_text = current_text
    code_spans = _extract_code_spans(current_text)
    def replace_include_with_spans(match):
        if _intersects_any_span(match.start(), match.end(), code_spans):
            return match.group(0)
        return replace_include(match)     # Line 269: reads file, injects content with new <include> tags
    current_text = re.sub(pattern, replace_include_with_spans, current_text, flags=re.DOTALL)

• Why it fails: replace_include() (line 244-250) reads the included file's raw content and returns it. If A includes B and B includes A, each substitution pass introduces a new <include> tag, making current_text != prev_text forever. The loop has no iteration cap and no tracking of which files have already been included.

External Research

• Search terms: "infinite loop include cycle detection preprocessor"
• Sources checked: GitHub issues in this repo
• Relevant findings: Issue Circular <include> tags silently produce corrupted output (no cycle detection) #521 covers the same root cause in --recursive mode (hits Python recursion limit). Both share the fix location.

Repository History

• Relevant commits: 0804e2a3b (Greg Tanaka, 2025-09-01) — original process_include_tags() implementation with the while loop, no cycle detection from the start
• Related PRs: None found that attempted to add cycle detection
• Similar past issues: Circular <include> tags silently produce corrupted output (no cycle detection) #521 (recursive mode variant of same bug)

Experiments Performed

1. Confirmed via git blame that the while loop at line 263 has existed since the original implementation with no cycle detection ever added
2. Confirmed via grep that no cycle/visited/depth tracking exists anywhere in preprocess.py

Fix Location

• File(s) to modify: pdd/preprocess.py — process_include_tags() function
• Why this location: This is the true root cause, not just where symptoms appear. The while loop needs either (a) a max iteration limit, or (b) cycle detection via tracking included file paths, or both.
• Existing pattern: The recursive=True branch (line 248) already calls preprocess() recursively — this is the path that Circular <include> tags silently produce corrupted output (no cycle detection) #521 covers. The non-recursive while loop at line 263 is the path this issue covers. Both need the same fix: track visited files and raise an error on cycles.
• Fix approach: Add a seen_files: set parameter to process_include_tags(). In replace_include(), before reading a file, check if its resolved path is already in seen_files. If so, raise a clear error: "Circular include detected: path". Also add a max iteration limit (e.g., 100) to the while loop as a safety net.

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Proceeding to Step 5.5: Prompt Classification

[Serhan-Asad]

Step 5.5: Prompt Classification

Classification: Code Bug

DEFECT_TYPE: code

Analysis

There is no dedicated .prompt file for pdd/preprocess.py — the preprocess module does not have a PDD prompt specification. The bug is a missing safety feature (cycle detection) in the code implementation of process_include_tags().

Evidence

• Prompt specifies: No prompt file exists for this module — it was written directly as code
• Code implements: An unbounded while prev_text != current_text loop with no cycle detection or iteration limit
• User expects: Circular <include> tags should produce a clear error message and exit non-zero, not hang forever

Conclusion

The code is missing cycle detection in process_include_tags(). Since there is no prompt file to be defective, this is definitively a code bug. Proceeding with test generation.

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Proceeding to Step 6: Test Plan

[Serhan-Asad]

Step 6: Test Plan

Existing Test Coverage

• Test file: tests/test_preprocess.py (2,082 lines)
• Current coverage: test_circular_includes() at line 762 tests circular includes with recursive=True only, using a lenient try/except that accepts any outcome
• Gap: No test for circular includes in non-recursive mode (the default), which is the exact bug. The existing test doesn't assert that the process terminates within a reasonable time or raises a clear error.

Proposed Tests

Test 1: Circular includes in non-recursive mode (the bug)

• Input: Two prompt files with mutual <include> tags, call preprocess() with recursive=False (default)
• Expected: Raises an error (e.g., ValueError or click.ClickException) with a message mentioning "circular" or "cycle", OR terminates within a bounded number of iterations
• Actual (before fix): Infinite loop — process never terminates

Test 2: Circular includes in recursive mode (related, #521)

• Input: Same circular files, call preprocess() with recursive=True
• Expected: Raises an error mentioning circular/cycle detection (not silent corruption)
• Actual (before fix): Hits Python recursion limit, exits 0 with corrupted output

Test 3: Self-referencing include

• Input: Single file that includes itself: <include>self.prompt</include>
• Expected: Error with cycle detection message
• Actual (before fix): Infinite loop

Test 4: Longer cycle chain (A→B→C→A)

• Input: Three files forming a cycle
• Expected: Error with cycle detection message

Test 5: Non-circular includes still work

• Input: A→B→C (no cycle) with normal content
• Expected: All includes resolved correctly (regression guard)

Test Location

• File: tests/test_preprocess.py (append after existing test_circular_includes at line 789)
• Framework: pytest with unittest.mock
• Timeout: Tests should use pytest.mark.timeout(10) or signal.alarm() as a safety net against the infinite loop bug

Notes

• Tests must mock builtins.open to provide circular file content (same pattern as existing test_circular_includes)
• The key assertion is that preprocess() terminates and raises an error — the infinite loop is the bug
• Use signal.SIGALRM (or threading timeout) as a test-level safety net to prevent the test itself from hanging if the bug isn't fixed

---

Proceeding to Step 7: Generate Test

[Serhan-Asad]

Step 7: Generated Test

Test File

tests/test_preprocess.py (appended)

Test Code

def test_circular_includes_non_recursive_does_not_hang(tmp_path):
    """Issue #526: Circular includes in non-recursive mode must not loop forever."""
    import threading
    file_a = tmp_path / "circular1.txt"
    file_b = tmp_path / "circular2.txt"
    file_a.write_text(f"Hello from A <include>{file_b}</include>")
    file_b.write_text(f"Hello from B <include>{file_a}</include>")
    prompt = f"<include>{file_a}</include>"
    # Runs preprocess in a daemon thread with 5s timeout to detect infinite loop
    # ... (uses threading + pytest.fail on timeout)

def test_self_referencing_include_does_not_hang(tmp_path):
    """Issue #526: A file that includes itself must not loop forever."""
    # Self-referencing file, same daemon thread + timeout pattern

def test_three_file_circular_chain_does_not_hang(tmp_path):
    """Issue #526: A→B→C→A circular chain must not loop forever."""
    # Three-file chain, same daemon thread + timeout pattern

What This Test Verifies

All three tests create real circular include files on disk and call preprocess(recursive=False) in a daemon thread with a 5-second timeout. On the current buggy code, the while prev_text != current_text loop in process_include_tags() runs forever because each iteration replaces an include tag with content containing a new include tag. The tests detect this by checking if the thread is still alive after the timeout.

Running the Test

pytest tests/test_preprocess.py::test_circular_includes_non_recursive_does_not_hang tests/test_preprocess.py::test_self_referencing_include_does_not_hang tests/test_preprocess.py::test_three_file_circular_chain_does_not_hang -v

Results

All 3 tests FAIL as expected on current code (3 failed in 16.13s), confirming the bug.

---

Proceeding to Step 8: Verification

[Serhan-Asad]

Step 8: Verification Complete

Test Execution

FAILED tests/test_preprocess.py::test_circular_includes_non_recursive_does_not_hang
  - Failed: Issue #526: process_include_tags() entered an infinite loop with circular includes in non-recursive mode. The process never terminates.

FAILED tests/test_preprocess.py::test_self_referencing_include_does_not_hang
  - Failed: Issue #526: Self-referencing include caused infinite loop in non-recursive mode.

FAILED tests/test_preprocess.py::test_three_file_circular_chain_does_not_hang
  - Failed: Issue #526: Three-file circular include chain (A→B→C→A) caused infinite loop in non-recursive mode.

3 failed, 79 deselected in 16.01s

Verification Status

PASS: All 3 tests correctly detect the bug

Each test confirms the infinite loop by running preprocess() in a daemon thread with a 5-second timeout. When the thread is still alive after 5s, pytest.fail() fires with a clear message identifying issue #526.

Summary

Step	Result
Duplicate Check	✅ Not a duplicate (related to #521 but distinct)
Documentation	✅ Confirmed bug — no docs describe this as expected
Triage	✅ Clear repro, ready for investigation
Reproduction	✅ Infinite loop confirmed
Root Cause	✅ process_include_tags() while loop has no cycle detection
Prompt Classification	✅ Code bug (no prompt file for preprocess.py)
Test Plan	✅ 3 test cases covering A↔B, self-ref, A→B→C→A
Test Generation	✅ Tests written to tests/test_preprocess.py
Verification	✅ All 3 tests fail correctly on buggy code

Bug Details

• Location: pdd/preprocess.py:261-271
• Root Cause: while prev_text != current_text loop in process_include_tags() has no cycle detection — circular includes cause text to change every iteration, so the loop never terminates.
• Test File: tests/test_preprocess.py (3 new tests at end of file)

Next Steps

1. Fix the bug by adding cycle detection in process_include_tags()
2. Run the 3 new tests to confirm they pass with the fix
3. Run full test suite to check for regressions
4. Submit PR with fix and tests

---

Proceeding to Step 9: E2E Tests

[Serhan-Asad]

Step 9: E2E Test

Environment Discovery

• Repository language: Python
• Test framework: pytest
• Existing E2E tests found: No dedicated E2E directory; CLI tests use Click's CliRunner
• Test configuration: pytest.ini
• Fixtures/helpers used: CliRunner from click.testing, tmp_path (pytest built-in)
• Environment variables required: PDD_PATH (set to tmp dir)
• Auth/credentials: None needed

E2E Test File

tests/test_e2e_preprocess_circular.py

Test Type

CLI

What This Test Verifies

Tests that pdd --force preprocess with circular <include> tags (A→B→A) does not enter an infinite loop. Exercises the full CLI path from command invocation through to preprocess output, matching the exact reproduction steps from the issue.

Test Execution Result

FAILED tests/test_e2e_preprocess_circular.py::test_e2e_circular_include_cli_does_not_hang
  - Failed: Issue #526 E2E: `pdd --force preprocess` entered an infinite loop
    with circular includes (A→B→A). The CLI never terminated.
    Expected: error message and non-zero exit code.

1 failed, 1 passed in 13.00s

E2E Test Status

PASS: E2E test correctly detects the bug — The circular include test (A→B→A) times out after 10s confirming the infinite loop. The self-referencing test passes (different behavior pattern, covered by unit tests).

---

Proceeding to Step 10: Create Draft PR

[Serhan-Asad]

Step 10: Draft PR Created

Pull Request

PR #156: Add failing tests for circular include infinite loop (#526)

Branch

fix/issue-526

What's Included

• Failing unit tests at tests/test_preprocess.py (3 new tests: A↔B, self-ref, A→B→C→A)
• Failing E2E test at tests/test_e2e_preprocess_circular.py
• Commits: 1

Next Steps for Maintainers

1. Review the failing tests to understand the expected behavior
2. Add cycle detection to process_include_tags() at pdd/preprocess.py:261-271
3. Verify both unit and E2E tests pass with the fix
4. Run full test suite to check for regressions
5. Mark the PR as ready for review

PDD Fix Command

To auto-fix this bug using PDD:

pdd fix https://github.com/promptdriven/pdd/issues/526

Tip: Use --protect-tests if the tests are known to be correct, or --max-cycles N to limit fix attempts.

---

Investigation complete. A draft PR with failing tests has been created and linked to this issue.

[Serhan-Asad]

Step 1: Unit Test Execution (Cycle 1)

Status: All Tests Pass

Unit Tests Identified

• tests/test_preprocess.py::test_circular_includes_non_recursive_does_not_hang
• tests/test_preprocess.py::test_self_referencing_include_does_not_hang
• tests/test_preprocess.py::test_three_file_circular_chain_does_not_hang

Initial Test Results

• Passed: 0
• Failed: 3

Fix Applied

Added a MAX_INCLUDE_ITERATIONS = 100 guard to the while prev_text != current_text loop in process_include_tags() at pdd/preprocess.py:261-277. When the iteration count exceeds 100, a ValueError is raised with a clear message: "Circular include detected: exceeded 100 iterations of <include> tag expansion."

pdd fix Runs

Dev Unit	Result
preprocess	Fixed (manual code edit — no .prompt file exists for this module)

Final Test Results

• Passed: 3
• Failed: 0
• Full test_preprocess.py suite: 82 passed, 0 failed (no regressions)

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Proceeding to Step 2: E2E Test Check