fix(mine): identify lock holder + exit non-zero on contention

When a `mempalace mine` collided with another writer (live mcp_server,
another mine, anything taking mine_palace_lock), the operator saw a
generic "another `mempalace mine` is already running" message and the
CLI exited 0 — making the contention invisible to nohup or scripts
checking $?. The reporter ran a `nohup mempalace mine ... & disown`
and got a 200-byte log with only the auto-defaults warning, no clue
that an MCP server was holding the store.

palace.py: the lock file now records the holder's PID + first three
argv tokens on acquire. A failed acquire reads the file and surfaces
"palace <path> is held by PID N (mempalace mcp_server); wait for it
to finish or stop the holder before retrying" in the
MineAlreadyRunning message. Open mode changes from "w" to "a+" so the
prior holder's identity survives long enough to be read.

miner.mine() now lets MineAlreadyRunning propagate. cmd_mine catches
it, prints the holder-aware message to stderr, and exits non-zero so
shell wrappers detect the contention.

Note: this is a behavior change for in-process callers that depended
on miner.mine() silently swallowing MineAlreadyRunning. The silent
swallow was the bug.

Closes #1264

mempalace/cli.py

@@ -500,31 +500,41 @@ def cmd_mine(args):
             llm_provider=None,
         )
 
-    if args.mode == "convos":
-        from .convo_miner import mine_convos
+    from .palace import MineAlreadyRunning
 
-        mine_convos(
-            convo_dir=args.dir,
-            palace_path=palace_path,
-            wing=args.wing,
-            agent=args.agent,
-            limit=args.limit,
-            dry_run=args.dry_run,
-            extract_mode=args.extract,
-        )
-    else:
-        from .miner import mine
+    try:
+        if args.mode == "convos":
+            from .convo_miner import mine_convos
 
-        mine(
-            project_dir=args.dir,
-            palace_path=palace_path,
-            wing_override=args.wing,
-            agent=args.agent,
-            limit=args.limit,
-            dry_run=args.dry_run,
-            respect_gitignore=not args.no_gitignore,
-            include_ignored=include_ignored,
-        )
+            mine_convos(
+                convo_dir=args.dir,
+                palace_path=palace_path,
+                wing=args.wing,
+                agent=args.agent,
+                limit=args.limit,
+                dry_run=args.dry_run,
+                extract_mode=args.extract,
+            )
+        else:
+            from .miner import mine
+
+            mine(
+                project_dir=args.dir,
+                palace_path=palace_path,
+                wing_override=args.wing,
+                agent=args.agent,
+                limit=args.limit,
+                dry_run=args.dry_run,
+                respect_gitignore=not args.no_gitignore,
+                include_ignored=include_ignored,
+            )
+    except MineAlreadyRunning as exc:
+        # A live MCP server or another mine is already writing to this
+        # palace. Surface the holder identity so the operator knows what
+        # to wait for (or stop), and exit non-zero so wrappers like
+        # nohup / scripts can detect the contention.
+        print(f"mempalace: {exc}", file=sys.stderr)
+        sys.exit(1)
 
 
 def cmd_sweep(args):

mempalace/miner.py

@@ -21,7 +21,6 @@ from typing import Optional
 from .palace import (
     NORMALIZE_VERSION,
     SKIP_DIRS,
-    MineAlreadyRunning,
     build_closet_lines,
     file_already_mined,
     get_closets_collection,
@@ -1035,26 +1034,21 @@ def mine(
             files=files,
         )
 
-    try:
-        with mine_palace_lock(palace_path):
-            return _mine_impl(
-                project_dir,
-                palace_path,
-                wing_override=wing_override,
-                agent=agent,
-                limit=limit,
-                dry_run=dry_run,
-                respect_gitignore=respect_gitignore,
-                include_ignored=include_ignored,
-                files=files,
-            )
-    except MineAlreadyRunning:
-        print(
-            f"mempalace: another `mine` is already running against "
-            f"{palace_path} — exiting cleanly.",
-            file=sys.stderr,
+    # MineAlreadyRunning propagates so the CLI can render a clear holder-aware
+    # message and exit non-zero. In-process callers (tests, library users) that
+    # expect to coexist with another writer should handle the exception.
+    with mine_palace_lock(palace_path):
+        return _mine_impl(
+            project_dir,
+            palace_path,
+            wing_override=wing_override,
+            agent=agent,
+            limit=limit,
+            dry_run=dry_run,
+            respect_gitignore=respect_gitignore,
+            include_ignored=include_ignored,
+            files=files,
         )
-        return
 
 
 def _mine_impl(

mempalace/palace.py

@@ -9,6 +9,7 @@ import hashlib
 import logging
 import os
 import re
+import sys
 import threading
 from typing import Optional
 
@@ -364,6 +365,41 @@ def _mark_released(lock_key: str) -> None:
     _holder_state().discard(lock_key)
 
 
+def _format_lock_holder(content: str) -> str:
+    """Render a lock-file body as 'PID N (cmdline)' for diagnostic messages."""
+    parts = content.split(maxsplit=1)
+    if not parts or not parts[0].isdigit():
+        return "another writer (identity not recorded)"
+    pid = parts[0]
+    if len(parts) > 1 and parts[1].strip():
+        return f"PID {pid} ({parts[1].strip()})"
+    return f"PID {pid}"
+
+
+def _read_lock_holder(lock_file) -> str:
+    """Read the prior holder's identity from the lock-file body, best-effort."""
+    try:
+        lock_file.seek(0)
+        content = lock_file.read().strip()
+    except OSError:
+        return "another writer (identity not recorded)"
+    if not content:
+        return "another writer (identity not recorded)"
+    return _format_lock_holder(content)
+
+
+def _write_lock_holder(lock_file) -> None:
+    """Record this process's identity in the lock-file body. Best-effort."""
+    try:
+        ident = f"{os.getpid()} {' '.join(sys.argv[:3])}".strip()
+        lock_file.seek(0)
+        lock_file.truncate()
+        lock_file.write(ident)
+        lock_file.flush()
+    except OSError:
+        pass
+
+
 @contextlib.contextmanager
 def mine_palace_lock(palace_path: str):
     """Per-palace non-blocking lock around the full `mine` pipeline.
@@ -407,7 +443,10 @@ def mine_palace_lock(palace_path: str):
         yield
         return
 
-    lf = open(lock_path, "w")
+    # "a+" preserves the prior holder's identity recorded inside the file so
+    # a failed acquire can name who is holding the lock (#1264). "w" mode
+    # would have truncated the file before we could read it.
+    lf = open(lock_path, "a+")
     acquired = False
     try:
         if os.name == "nt":
@@ -417,8 +456,10 @@ def mine_palace_lock(palace_path: str):
                 msvcrt.locking(lf.fileno(), msvcrt.LK_NBLCK, 1)
                 acquired = True
             except OSError as exc:
+                holder = _read_lock_holder(lf)
                 raise MineAlreadyRunning(
-                    f"another `mempalace mine` is already running against {resolved}"
+                    f"palace {resolved} is held by {holder}; "
+                    "wait for it to finish or stop the holder before retrying"
                 ) from exc
         else:
             import fcntl
@@ -427,9 +468,13 @@ def mine_palace_lock(palace_path: str):
                 fcntl.flock(lf, fcntl.LOCK_EX | fcntl.LOCK_NB)
                 acquired = True
             except BlockingIOError as exc:
+                holder = _read_lock_holder(lf)
                 raise MineAlreadyRunning(
-                    f"another `mempalace mine` is already running against {resolved}"
+                    f"palace {resolved} is held by {holder}; "
+                    "wait for it to finish or stop the holder before retrying"
                 ) from exc
+        # Record our own identity for any later contender's diagnostic message.
+        _write_lock_holder(lf)
         _mark_held(palace_key)
         try:
             yield