fix(jobs): validate resume checkpoints before allocation

harness/tht/jobs/models.py

@@ -7,7 +7,7 @@ from datetime import UTC, datetime
 from pathlib import Path
 from typing import Literal, Self
 
-from pydantic import BaseModel, ConfigDict, Field, field_serializer, field_validator
+from pydantic import BaseModel, ConfigDict, Field, field_serializer, field_validator, model_validator
 
 
 _JOB_KEY = re.compile(r"^[a-z][a-z0-9_-]{0,63}$")
@@ -113,6 +113,26 @@ class StageRun(_FrozenModel):
 
     _name_key = field_validator("name")(_validate_job_key)
 
+    @model_validator(mode="after")
+    def state_shape(self) -> "StageRun":
+        if self.status == "pending" and any(
+            value is not None for value in (self.started_at, self.finished_at, self.error)
+        ):
+            raise ValueError("pending stage cannot contain timestamps or error")
+        if self.status == "running" and (
+            self.started_at is None or self.finished_at is not None or self.error is not None
+        ):
+            raise ValueError("running stage requires only started_at")
+        if self.status == "succeeded" and (
+            self.started_at is None or self.finished_at is None or self.error is not None
+        ):
+            raise ValueError("succeeded stage requires timestamps and no error")
+        if self.status == "failed" and (
+            self.started_at is None or self.finished_at is None or self.error is None
+        ):
+            raise ValueError("failed stage requires timestamps and safe error")
+        return self
+
 
 class JobRun(_FrozenModel):
     """Durable checkpoint, persisted after every state transition."""
@@ -144,6 +164,32 @@ class JobRun(_FrozenModel):
     _persisted_versions = field_validator("spec_version", "pipeline_version")(_validate_job_key)
     _resumed_from = field_validator("resumed_from")(_validate_run_id)
 
+    @model_validator(mode="after")
+    def ledger_shape(self) -> "JobRun":
+        names = [stage.name for stage in self.stages]
+        if len(names) != len(set(names)):
+            raise ValueError("stage identifiers must be unique")
+        statuses = [stage.status for stage in self.stages]
+        first_incomplete = next(
+            (index for index, status in enumerate(statuses) if status != "succeeded"),
+            len(statuses),
+        )
+        if any(status != "pending" for status in statuses[first_incomplete + 1 :]):
+            raise ValueError("stage ledger must be an ordered execution prefix")
+        if self.status == "succeeded" and (
+            self.finished_at is None or any(status != "succeeded" for status in statuses)
+        ):
+            raise ValueError("succeeded job requires a complete succeeded ledger")
+        if self.status == "failed" and (
+            self.finished_at is None
+            or first_incomplete == len(statuses)
+            or statuses[first_incomplete] != "failed"
+        ):
+            raise ValueError("failed job requires the first incomplete stage to be failed")
+        if self.status == "running" and self.finished_at is not None:
+            raise ValueError("running job cannot have finished_at")
+        return self
+
 
 class JobReport(JobRun):
     """Public machine-readable terminal report (contains no paths or stage outputs)."""

harness/tht/jobs/runner.py

@@ -94,8 +94,11 @@ def _resume_run(spec: JobSpec, run_id: str, stages: Sequence[Stage], source: Job
     requested_names = list(spec.stage_ids)
     if len(requested_names) != len(stages) or len(requested_names) != len(set(requested_names)):
         raise CorruptCheckpointError("resume checkpoint is incompatible with requested stages")
+    source_fingerprint = _source_compatibility_fingerprint(source)
+    if source.compatibility_fingerprint != source_fingerprint:
+        raise CorruptCheckpointError("resume checkpoint compatibility fingerprint is invalid")
     expected = _compatibility_fingerprint(spec, requested_names)
-    if source.compatibility_fingerprint != expected:
+    if source_fingerprint != expected or [stage.name for stage in source.stages] != requested_names:
         raise CorruptCheckpointError(
             "resume checkpoint is incompatible; start an intentional new run without resume"
         )
@@ -129,12 +132,8 @@ def run_job(spec: JobSpec, stages: Sequence[Stage]) -> JobReport:
     """Run stages once, returning a terminal report instead of leaking stage exceptions."""
     with WorkspaceJobLock(spec.workspace_root, spec.workspace_id, spec.job_type):
         jobs_root = spec.workspace_root / ".tht-jobs" / spec.job_type / "runs"
-        run_id = uuid.uuid4().hex
-        run_dir = jobs_root / run_id
-        _prepare_run_directory(spec.workspace_root, spec.job_type, run_id)
-        checkpoint_path = run_dir / "checkpoint.json"
         if spec.resume_run_id is None:
-            run = _new_run(spec, run_id, stages)
+            source = None
         else:
             source_path = jobs_root / spec.resume_run_id / "checkpoint.json"
             if not source_path.exists():
@@ -146,6 +145,15 @@ def run_job(spec: JobSpec, stages: Sequence[Stage]) -> JobReport:
                 if len(matches) == 1:
                     source_path = matches[0]
             source = _load_checkpoint(source_path)
+            _validate_resume_source(spec, stages, source)
+
+        run_id = uuid.uuid4().hex
+        run_dir = jobs_root / run_id
+        _prepare_run_directory(spec.workspace_root, spec.job_type, run_id)
+        checkpoint_path = run_dir / "checkpoint.json"
+        if source is None:
+            run = _new_run(spec, run_id, stages)
+        else:
             run = _resume_run(spec, run_id, stages, source)
         _persist(checkpoint_path, run)
         context = JobContext(run_id, spec.job_type, spec.dry_run, spec.workspace_root, run_dir)
@@ -213,6 +221,26 @@ def _compatibility_fingerprint(spec: JobSpec, stage_ids: list[str]) -> str:
     return _value_fingerprint(canonical)
 
 
+def _source_compatibility_fingerprint(source: JobRun) -> str:
+    payload = {
+        "schema_version": source.schema_version,
+        "workspace": source.workspace_fingerprint,
+        "job_type": source.job_type,
+        "dry_run": source.dry_run,
+        "spec_version": source.spec_version,
+        "pipeline_version": source.pipeline_version,
+        "config_fingerprint": source.config_fingerprint,
+        "input_fingerprint": source.input_fingerprint,
+        "stage_ids": [stage.name for stage in source.stages],
+    }
+    canonical = json.dumps(payload, sort_keys=True, separators=(",", ":"))
+    return _value_fingerprint(canonical)
+
+
+def _validate_resume_source(spec: JobSpec, stages: Sequence[Stage], source: JobRun) -> None:
+    _resume_run(spec, "0" * 32, stages, source)
+
+
 def _prepare_run_directory(workspace_root: Path, job_type: str, run_id: str) -> None:
     parent_fd = os.open(workspace_root, os.O_RDONLY | os.O_DIRECTORY | os.O_NOFOLLOW)
     try: