acceptance: a failed check prints the serial log where it looked (#136)

Run 197's riscv64 failure (#131) could only be read from the run's logs
artifact, which Gitea's API does not list. qemu_session.py's check() now
prints, on a FAIL, the serial lines after the first match of an anchor the
check names, or the log's last lines; control characters as escapes, so a
stray byte or terminal sequence in a line shows instead of only failing a
match. The boot-disk checks anchor on 'Boot successful!'. A passing run
prints as before (riscv64 here, logs/ committed).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

scripts/qemu_session.py

@@ -213,9 +213,33 @@ def wait_for(pat, secs, after=0):
     return False
 
 
-def check(name, cond):
+def excerpt(near=None, after=20, tail=40):
+    """The serial log where a failed check looked (#136): the lines after the first match of
+    `near`, else the log's last lines -- control characters shown as escapes, so a terminal's
+    sequences cannot garble the job's output."""
+    lines = text().splitlines()
+    start = None
+    if near is not None:
+        start = next((i for i, l in enumerate(lines) if re.search(near, l)), None)
+    if start is None:
+        start, what = max(0, len(lines) - tail), f"the log's last {tail} lines"
+        stop = len(lines)
+    else:
+        stop, what = min(len(lines), start + 1 + after), f"after /{near}/, line {start + 1}"
+    print(f"  --- serial log, {what} ({os.path.relpath(log, ROOT)}):", flush=True)
+    for i in range(start, stop):
+        shown = "".join(c if c.isprintable() else repr(c)[1:-1] for c in lines[i])
+        print(f"  {i + 1:6} | {shown}", flush=True)
+    print("  ---", flush=True)
+
+
+def check(name, cond, near=None):
+    """PASS or FAIL; a FAIL also prints the serial log where the check looked: after the first
+    line matching `near` (a regex), when it is given, else the log's end."""
     global ok
     print(f"{'PASS' if cond else 'FAIL'}: {name}", flush=True)
+    if not cond:
+        excerpt(near)
     ok = ok and cond
 
 
@@ -328,7 +352,7 @@ if m == "headless":
 
 elif m == "zuse":
     check("Boot disk: not a StarshipOS stick (an ISO or ESP boot)",
-          re.search(r"Boot disk: not a StarshipOS stick", text()) is not None)
+          re.search(r"Boot disk: not a StarshipOS stick", text()) is not None, near=r"Boot successful!")
     check("Zuse: identity confirmed", wait_for(r"Zuse: identity confirmed from attached thumbdrive", 120))
     check("[zuse@Hera] ok>", wait_for(PROMPT, 60))
     n = count(PROMPT)
@@ -343,7 +367,7 @@ elif m in ("stick_boot", "two_sticks"):
     if m == "stick_boot":
         # the PC stick: Artemis is the rest of 2 GiB (well over 1 million user blocks), its blank
         # identity 500 MiB (about 380000)
-        check("Boot disk: a StarshipOS stick", re.search(r"Boot disk: a StarshipOS stick", text()) is not None)
+        check("Boot disk: a StarshipOS stick", re.search(r"Boot disk: a StarshipOS stick", text()) is not None, near=r"Boot successful!")
         check("System: StarForth v4", re.search(r"System: StarForth v4", text()) is not None)
         check("Artemis: the stick's partition attached",
               wait_for(r"Artemis: the stick's partition attached", 120))
@@ -370,7 +394,7 @@ elif m in ("stick_boot", "two_sticks"):
 
 elif m in ("stick_artemis", "stick_zuse"):
     if m == "stick_zuse":
-        check("Boot disk: a StarshipOS stick", re.search(r"Boot disk: a StarshipOS stick", text()) is not None)
+        check("Boot disk: a StarshipOS stick", re.search(r"Boot disk: a StarshipOS stick", text()) is not None, near=r"Boot successful!")
     check("Artemis: the stick's partition attached", wait_for(r"Artemis: the stick's partition attached", 120))
     check("Zuse: identity confirmed", wait_for(r"Zuse: identity confirmed from attached thumbdrive", 120))
     check("[zuse@Hera] ok>", wait_for(PROMPT, 60))