# P4LF Session Log — 2026-06-25 (Session 2: Implementation)

## Session Goal

Begin implementation of p4lf following the design decided in Session 1.
No code existed at the start of this session.

## Key Decision Carried Forward

Use **Go + custom file reader** (not go-libtail, not Bash + p4 logtail).
See session_log_2026-06-25.md for the full rationale.

## Development Environment Note

Development is on macOS (Apple Silicon, Go 1.24.5). Target runtime is Linux
(amd64). Go cross-compiles trivially; all code uses only portable stdlib plus
`fsnotify`. Integration/system testing (systemd, real p4d) will be done on
Linux in a later session.

## What Was Built

### Go module

- Module name: `github.com/rcowham/p4lf`
- Single external dependency: `github.com/fsnotify/fsnotify v1.10.1`
  (for inotify-backed file watching; abstracts OS differences)

### Directory structure

```
p4lf/
  cmd/p4lf/main.go                  — entry point, signal handling, main loop
  internal/config/config.go         — config parser (KEY=VALUE format)
  internal/config/config_test.go    — unit tests for config
  internal/tailer/tailer.go         — file reader, rotation detection, state file
  internal/chunker/chunker.go       — chunk assembly, gzip, disk guards
  internal/chunker/chunker_test.go  — unit tests for chunker
  internal/logger/logger.go         — p4lf's own rotating log writer
  go.mod / go.sum
  p4lf.cfg.example                  — documented example config
  p4lf.service                      — systemd unit file
```

### Config settings implemented

| Setting | Notes |
|---|---|
| `P4LogFile` | Path to P4LOG; also read from `$P4LOG` env var |
| `LogTailDelay` | Chunk flush interval; format: 60s / 3m / 1h |
| `LogChunksDir` | Output directory for log.*.gz files |
| `MaxLogChunks` | Max un-consumed chunks before pausing |
| `MinLogSpace` | Min free space (percent or absolute) before pausing |
| `MaxLogSize` | p4lf's own log rotation threshold |
| `MaxRotationRecoverySize` | Max new P4LOG size before giving up on reading from beginning post-rotation |
| `StateFile` | Checkpoint: inode + byte offset, JSON, atomic write |
| `ReadFromStart` | First-run behaviour: BOF (true) or EOF (false) |
| `Debug` | Verbosity: 0/1/2 |

### Rotation detection approach

- On every `Flush()` call, stat the P4LOG **path** (not the open fd)
- Compare returned inode to saved state inode
- If changed → rotation detected; capture trailing bytes from old fd, close it, open new file
- Also detect truncation: if `saved_offset > current_file_size` (handles `copytruncate`)
- State file written atomically (write to `.tmp`, then `os.Rename`)
- State file format: `{"inode": 12345, "offset": 6789}`

### Chunk file naming

```
log.<startOffset>.<endOffset>.gz
```
Written via:
1. Open temp file `P4LOG_chunk.<o1>.<o2>.gz.tmp`
2. Write gzip content
3. `os.Rename` to `log.<o1>.<o2>.gz`  (atomic)

### Signal handling (main.go)

- `SIGTERM` / `SIGINT`: final flush, save state, clean exit
- `SIGHUP`: reload config + rotate p4lf's own log

### Service controls

- Detects config file modtime change each tick (in addition to SIGHUP)
- `WaitForFile()` blocks at startup until P4LOG exists

## Test Results

```
ok  github.com/rcowham/p4lf/internal/config   0.430s
ok  github.com/rcowham/p4lf/internal/chunker  0.265s
```

Config tests cover: defaults, all settings, duration formats (s/m/h/plain),
size formats (K/M/G/T/None/0), percent space spec, absolute space spec,
missing P4LogFile, unknown key, comments, missing config file.

Chunker tests cover: gzip file creation, content correctness, MaxLogChunks guard.

## Linux Build

Cross-compiled successfully: `GOOS=linux GOARCH=amd64` → 3.8MB static binary.

## Perforce Status

All new files opened for add in the default changelist. Not yet submitted.
Pending explicit permission to submit/shelve.

## What's Next (for the next session)

1. **Linux integration testing**: deploy to an SDP environment, run against
   real p4d, verify rotation handling with actual log rotations.
2. **Tailer unit tests**: test rotation detection, state file checkpoint/resume,
   truncation handling (requires temp file manipulation).
3. **Logger unit tests**: test rotation trigger at MaxLogSize, gzip of rotated log.
4. **Install/deploy script**: shell script or Makefile to install binary +
   systemd unit to SDP paths.
5. **README update**: update README.md to reflect Go implementation and revised
   config settings.
6. **Consider**: whether `WaitUntilGuardsPassed` in main loop should have a
   maximum wait timeout before giving up and logging an error.
