fix disk-gc-scan: CWD independence (FIX A) and df column parsing (FIX B)

scripts/disk-gc-scan.py

@@ -23,6 +23,11 @@ DESIGN PRINCIPLES (per task disk-gc-probe-false-unreachable-20260913):
    - If a figure comes from a different kind of measurement than the column claims,
      name it explicitly
 
+4. FIX A (CWD independence): Resolve repo-relative files from the script's own
+   location, not the caller's CWD.
+   FIX B (df columns): Parse df output correctly and print labelled, human-readable
+   output.
+
 Usage:
     disk-gc-scan.py              # scan all guests
     disk-gc-scan.py --json       # machine-readable output
@@ -32,12 +37,17 @@ Exit codes: 0 ok (all guests probed), 1 probe error
 from __future__ import annotations
 
 import json
+import pathlib
 import subprocess
 import sys
 import time
-from dataclasses import dataclass, asdict
+from dataclasses import dataclass
 from typing import Optional
 
+# Resolve repo-relative files from the script's own location, not the caller's CWD
+SCRIPT_DIR = pathlib.Path(__file__).resolve().parent
+HELPER_PCT_RUN = SCRIPT_DIR / "pct-run.sh"
+
 # Per-guest access method map. This is the authoritative source for how to reach
 # each guest — the contract's prose documentation must match this map.
 #
@@ -78,7 +88,7 @@ class ProbeResult:
     usage_pct: Optional[float]
     usage_str: Optional[str]
     probe_cmd: str
-    failure_kind: Optional[str]  # "timeout", "ssh-auth", "no-route", None
+    failure_kind: Optional[str]  # "timeout", "ssh-auth", "no-route", "command-not-found", None
 
     @property
     def is_reachable(self) -> bool:
@@ -167,7 +177,8 @@ def probe_guest(guest: Guest) -> ProbeResult:
     df_cmd = "df -P / | tail -1"
 
     if guest.access_method == "pct-run":
-        probe_cmd = f'bash scripts/pct-run.sh {guest.ct_id} "{df_cmd}"'
+        # Use absolute path to helper so CWD doesn't matter
+        probe_cmd = f'bash {HELPER_PCT_RUN} {guest.ct_id} "{df_cmd}"'
     elif guest.access_method == "ssh-host":
         probe_cmd = f'ssh {SSH_OPTS} root@{guest.hostname} "{df_cmd}"'
     elif guest.access_method == "ssh-ip":
@@ -175,17 +186,38 @@ def probe_guest(guest: Guest) -> ProbeResult:
     else:
         raise ValueError(f"unknown access_method: {guest.access_method}")
 
+    # Check helper exists and is readable BEFORE probing (for pct-run guests)
+    # This prevents scanner errors from being rendered as guest verdicts
+    if guest.access_method == "pct-run":
+        if not HELPER_PCT_RUN.exists():
+            print(f"SCANNER ERROR: helper not found: {HELPER_PCT_RUN}", file=sys.stderr)
+            sys.exit(1)
+        if not HELPER_PCT_RUN.readable():
+            print(f"SCANNER ERROR: helper not readable: {HELPER_PCT_RUN}", file=sys.stderr)
+            sys.exit(1)
+
     # Retry once on failure before declaring unreachable
     for attempt in range(2):
         exit_code, stdout, stderr = run_cmd(probe_cmd, timeout=15)
 
         if exit_code == 0:
             # Parse df output: Filesystem 1024-blocks Used Available Capacity Mounted on
+            # parts[0]=Filesystem, parts[1]=Total (1K blocks), parts[2]=Used, parts[3]=Available, parts[4]=Capacity
             parts = stdout.split()
             if len(parts) >= 5:
                 capacity_str = parts[4]  # e.g., "34%"
                 usage_pct = float(capacity_str.rstrip("%"))
-                usage_str = f"{capacity_str} ({parts[1]}/{parts[2]})"
+                total_blocks = int(parts[1])
+                used_blocks = int(parts[2])
+                avail_blocks = int(parts[3])
+                # Convert to human-readable units
+                def to_gb(blocks: int) -> float:
+                    return blocks / (1024 * 1024)
+                total_gb = to_gb(total_blocks)
+                used_gb = to_gb(used_blocks)
+                avail_gb = to_gb(avail_blocks)
+                # FIX B: print labelled, unambiguous output
+                usage_str = f"{capacity_str} ({used_gb:.1f}G used of {total_gb:.1f}G total, {avail_gb:.1f}G free)"
                 return ProbeResult(
                     exit_code=0,
                     usage_pct=usage_pct,
@@ -214,6 +246,8 @@ def probe_guest(guest: Guest) -> ProbeResult:
                 failure_kind = "no-route"
             elif "Connection refused" in stderr:
                 failure_kind = "conn-refused"
+            elif "command not found" in stderr.lower() or "No such file" in stderr:
+                failure_kind = "command-not-found"
             else:
                 failure_kind = f"ssh-exit-{exit_code}"