bench-spec-ab: absolute launcher paths

A relative serve-script path plus a cd into its own directory double-counts the
prefix, so every configuration failed to start ('scripts/serve-frank-medium.sh:
No such file or directory'). The safety net did its job -- the unit was restored
-- but the run measured nothing.

scripts/bench-spec-ab.py

@@ -39,6 +39,7 @@ constraint — it is draftless and is added to the chain unconditionally.
 """
 import argparse
 import json
+import os
 import re
 import subprocess
 import sys
@@ -197,16 +198,20 @@ def main():
     args = ap.parse_args()
 
     configs = parse_configs(args.configs) if args.configs else DEFAULT_CONFIGS
+    # Absolute paths: the launcher must not depend on the caller's cwd, and a
+    # relative path plus a `cd` to its own directory double-counts the prefix.
+    env_file = os.path.abspath(args.env_file)
+    serve_script = os.path.abspath(args.serve_script)
     base, logfile = args.base_url, args.log
     results = {}
     log("spec A/B start: " + ", ".join(n for n, _ in configs) +
         " (unit " + UNIT + " stopped for the duration)", logfile)
     stop_unit()
     for name, over in configs:
-        env_path = write_env(args.env_file, name, over)
+        env_path = write_env(env_file, name, over)
         logf = "/tmp/spec-ab-" + name.replace("/", "_") + ".log"
-        sh("cd " + sh("dirname " + args.serve_script) + " && FRANK_MEDIUM_ENV_FILE=" + env_path +
+        sh("FRANK_MEDIUM_ENV_FILE=" + env_path + " setsid bash " + serve_script +
-           " setsid bash " + args.serve_script + " > " + logf + " 2>&1 < /dev/null &")
+           " > " + logf + " 2>&1 < /dev/null &")
         if not wait_health(base):
             log(name + ": FAILED to become healthy -- " + sh("tail -3 " + logf), logfile)
             results[name] = {"error": "not healthy", "tail": sh("tail -6 " + logf)}