src/client/client_cli.c

@@ -1376,9 +1376,11 @@ static bool cli_handle_io_options(CliParseCtx* ctx) {
       return true;
     }
     if (config->log_file) {
+      /* Detach the logger before closing: log I/O may be in flight and must
+         never touch a freed FILE*. */
+      log_set_file(NULL);
       fclose(config->log_file);
       config->log_file = NULL;
-      log_set_file(NULL);
     }
     FILE* lf = fopen(ctx->argv[++ctx->i], "a");
     if (!lf) {

src/client/scanner.c

@@ -587,12 +587,16 @@ void directory_scanner_destroy(DirectoryScanner* scanner) {
 
 static Chunk* chunk_data_to_chunk(ArrayList* chunk_data) {
   void** chunk_items = array_list_to_array(chunk_data);
-  if (!chunk_items)
+  if (!chunk_items) {
+    array_list_delete(chunk_data);
     return NULL;
+  }
   Chunk* chunk = chunk_create((File**)chunk_items, chunk_data->size);
   free(chunk_items);
-  if (!chunk)
+  if (!chunk) {
+    array_list_delete(chunk_data);
     return NULL;
+  }
   chunk_data->item_destroyer = NULL;
   array_list_delete(chunk_data);
   return chunk;

src/server/server.c

@@ -23,6 +23,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <errno.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
 #include <openssl/x509.h>
 
@@ -502,12 +503,16 @@ void handler(int file_descriptor) {
   gate_ctx.ssl = ssl;
   gate_ctx.fd = file_descriptor;
   gate_ctx.super_mode_override = -1;
-  Config* config = config_receive_with_validate(file_descriptor, server_module_gate, &gate_ctx);
+  /* All teardown state starts empty so the single `done` epilogue is safe to
+   * reach from any error path (including before the config frame arrives). */
+  Config* config = NULL;
+  PipelineContextReceiver* context = NULL;
+  char* joined_destination = NULL;
+  bool charset_ready = false;
+  config = config_receive_with_validate(file_descriptor, server_module_gate, &gate_ctx);
   if (config == NULL) {
     log_message(LOG_LEVEL_ERROR, "Failed to receive config");
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   /* Apply the super-mode veto the gate decided on (operator --no-super, or a
    * daemon module without the `client owner = yes` opt-in) exactly once, so
@@ -519,23 +524,15 @@ void handler(int file_descriptor) {
   protocol_set_8_bit_output(config->eight_bit_output);
   if (!authorized_root) {
     log_message(LOG_LEVEL_ERROR, "No server-side destination root configured");
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   if (!allow_unauthenticated && ssl == NULL) {
     log_message(LOG_LEVEL_ERROR, "Rejected unauthenticated plaintext connection");
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   if (ssl && required_client_cn && !tls_client_identity_allowed(ssl)) {
     log_message(LOG_LEVEL_ERROR, "Rejected TLS client with unauthorized identity");
-    config_delete(config);
-    close(file_descriptor);
-    return;
+    goto done;
   }
   /* Daemon mode: the module's root is the authorized root (installed by
      server_module_gate), and the client's destination is a MODULE-RELATIVE
@@ -545,13 +542,9 @@ void handler(int file_descriptor) {
   if (g_daemon_conf && config->receive_root_directory && config->receive_root_directory[0] == '/') {
     log_message(LOG_LEVEL_ERROR, "Rejected absolute daemon destination (must be relative to the "
                                  "selected module root)");
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   char* destination = config->receive_root_directory;
-  char* joined_destination = NULL;
   if (destination && destination[0] != '/')
     joined_destination = path_cat(authorized_root, destination);
   if (joined_destination)
@@ -560,19 +553,16 @@ void handler(int file_descriptor) {
       !path_is_within(authorized_root, destination)) {
     log_message(LOG_LEVEL_ERROR, "Rejected destination outside authorized root");
     free(joined_destination);
-    config_delete(config);
-    close(file_descriptor);
-    return;
+    joined_destination = NULL;
+    goto done;
   }
   if (joined_destination) {
     free(config->receive_root_directory);
     config->receive_root_directory = joined_destination;
+    joined_destination = NULL;
   }
   if (!config->receive_root_directory) {
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   config->use_delete = config->use_delete && allow_delete;
   /* --iconv (protocol 2.16.0): install the receiver-side wire->local conversion
@@ -581,13 +571,13 @@ void handler(int file_descriptor) {
      any) may override the local charset; a spec the client is known to have
      validated cannot fail here unless the server's override names an
      unsupported charset. */
-  if (config->iconv_spec && !charset_wire_init_receiver(config->iconv_spec, server_iconv_spec)) {
+  if (config->iconv_spec) {
+    if (!charset_wire_init_receiver(config->iconv_spec, server_iconv_spec)) {
       log_message(LOG_LEVEL_ERROR,
                   "--iconv: unsupported charset conversion requested (LOCAL[,REMOTE])");
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+      goto done;
+    }
+    charset_ready = true;
   }
   /* --delete-missing-args deletes destination mirrors receiver-side, so it is
      deletion and stays gated by the same --allow-delete server policy.  When
@@ -602,10 +592,7 @@ void handler(int file_descriptor) {
     log_message(LOG_LEVEL_ERROR, "destination root is not available: %s",
                 escaped_root ? escaped_root : "<allocation failed>");
     free(escaped_root);
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   /* A --delay-updates transfer stages under a private 0700 directory inside
      the receive root.  Create it up front (wiping leftovers of any previously
@@ -614,11 +601,7 @@ void handler(int file_descriptor) {
     config->delay_context = delay_updates_context_create(config->receive_root_directory);
     if (!config->delay_context || !delay_updates_prepare(config->delay_context)) {
       log_message(LOG_LEVEL_ERROR, "Failed to initialize --delay-updates staging area");
-      delay_updates_cleanup(config->delay_context);
-      config_delete(config);
-      close(file_descriptor);
-      protocol_session_unbind();
-      return;
+      goto done;
     }
   }
   /* Preserve the negotiated identity policy for the fd-relative ownership
@@ -628,10 +611,7 @@ void handler(int file_descriptor) {
      rather than silently applying the wrong ownership policy. */
   if (!identity_set_active(config)) {
     log_message(LOG_LEVEL_ERROR, "Failed to activate identity policy");
-    config_delete(config);
-    close(file_descriptor);
-    protocol_session_unbind();
-    return;
+    goto done;
   }
   /* Persist the negotiated --keep-dirlinks policy once, here at config-accept,
      before any multithreaded receiver/writer threads are spawned, so the
@@ -662,38 +642,25 @@ void handler(int file_descriptor) {
     if (!motd_send(file_descriptor, motd ? motd : "")) {
       free(motd);
       log_message(LOG_LEVEL_ERROR, "Failed to send daemon MOTD");
-      config_delete(config);
-      close(file_descriptor);
-      protocol_session_unbind();
-      identity_clear_active();
-      return;
+      goto done;
     }
     free(motd);
   }
   if (config->use_multithreading) {
     Queue* q = queue_create(100, file_destroy);
-    if (q == NULL) {
-      config_delete(config);
-      close(file_descriptor);
-      protocol_session_unbind();
-      identity_clear_active();
-      return;
-    }
-    PipelineContextReceiver* context =
-        pipeline_context_receiver_create(config, q, file_descriptor, ssl);
+    if (q == NULL)
+      goto done;
+    context = pipeline_context_receiver_create(config, q, file_descriptor, ssl);
     if (context == NULL) {
       queue_destroy(q);
-      config_delete(config);
-      close(file_descriptor);
-      protocol_session_unbind();
-      identity_clear_active();
-      return;
+      goto done;
     }
     protocol_session_set_max_alloc(&context->session, config->max_alloc);
     atomic_store(&context->session.total_allocated_bytes,
                  atomic_load(&session.total_allocated_bytes));
     pipeline_context_receiver_set_queue_byte_limit(context, RECEIVER_QUEUE_MAX_BYTES);
-    thrd_t receiver, writer;
+    thrd_t receiver = {0};
+    thrd_t writer = {0};
     bool receiver_created = thrd_create(&receiver, receive_thread, context) == thrd_success;
     bool writer_created = false;
     if (receiver_created)
@@ -706,17 +673,19 @@ void handler(int file_descriptor) {
         cnd_broadcast(&context->condition_not_full);
         cnd_broadcast(&context->condition_not_empty);
         mtx_unlock(&context->mutex);
-        close(file_descriptor);
+        /* Unblock a worker parked in socket I/O without closing the fd (the
+         * child owns the single close).  shutdown() only affects sockets; for
+         * the --stdio pipe the receiver's per-message poll timeout still
+         * bounds the join, so do nothing there rather than close a descriptor
+         * another thread may still be using. */
+        struct stat fd_stat;
+        if (fstat(file_descriptor, &fd_stat) == 0 && S_ISSOCK(fd_stat.st_mode))
+          shutdown(file_descriptor, SHUT_RDWR);
         thrd_join(receiver, NULL);
-      } else {
-        close(file_descriptor);
       }
       if (writer_created)
         thrd_join(writer, NULL);
-      pipeline_context_receiver_destroy(context);
-      protocol_session_unbind();
-      identity_clear_active();
-      return;
+      goto done;
     }
     int receiver_result;
     int writer_result;
@@ -760,21 +729,36 @@ void handler(int file_descriptor) {
     } else {
       send_status(file_descriptor, STATUS_ERROR);
     }
-    if (!transfer_ok) {
+    if (!transfer_ok)
       log_message(LOG_LEVEL_ERROR, "Transfer failed");
-      if (config->delay_updates && config->delay_context)
-        delay_updates_cleanup(config->delay_context);
-    }
-    pipeline_context_receiver_destroy(context);
   } else {
     if (receiver_receive_files(config, file_descriptor) != 0)
       log_message(LOG_LEVEL_ERROR, "Transfer failed");
-    config_delete(config);
   }
-  protocol_session_unbind();
-  identity_clear_active();
+
+done:
+  /* Single cleanup epilogue: every error path jumps here, so the iconv
+   * receiver conversion is released, the identity snapshot cleared, the
+   * protocol session unbound and the config freed exactly once.  The
+   * connection fd is deliberately NOT closed here -- the child functions own
+   * its single close (plain_child_fn / tls_child_fn), and the --stdio call
+   * site must leave stdin/stdout open. */
+  if (charset_ready)
     charset_wire_free();
-  close(file_descriptor);
+  /* The delay-updates staging tree is released by config_delete (which the
+     branch below always reaches), so it is cleaned exactly once. */
+  identity_clear_active();
+  protocol_session_unbind();
+  if (context != NULL) {
+    /* context owns both the config and the queue it was created with. */
+    pipeline_context_receiver_destroy(context);
+    context = NULL;
+    config = NULL;
+  } else {
+    config_delete(config);
+    config = NULL;
+  }
+  free(joined_destination);
 }
 
 #ifndef FASTSYNC_SERVER_AS_LIB
@@ -969,6 +953,9 @@ int main(int argc, char* argv[]) {
       return 1;
     }
     io_set_fds(STDIN_FILENO, STDOUT_FILENO);
+    /* handler() does not own the stdio fds: it never closes its descriptor
+     * argument, so STDIN/STDOUT stay open for this (single-shot) SSH session
+     * and are released by process exit. */
     handler(STDIN_FILENO);
     release_authorization();
     server_cli_options_free(&opts);

src/shared/config.c

@@ -693,6 +693,9 @@ void config_delete(Config* config) {
   if (config == NULL)
     return;
   if (config->log_file) {
+    /* The logging subsystem borrows this FILE*; detach it before closing so a
+     * concurrent log call can never touch the freed handle. */
+    log_set_file(NULL);
     fclose(config->log_file);
     config->log_file = NULL;
   }
@@ -1428,7 +1431,7 @@ Config* config_receive_with_validate(int file_descriptor, ConfigValidateFunc val
     goto error;
   if (strcmp(config->version, PROTOCOL_VERSION) != 0) {
     char* escaped_version = output_escape(config->version, false);
-    fprintf(stderr, "Protocol version mismatch: client=%s, server=%s\n",
+    log_message(LOG_LEVEL_ERROR, "Protocol version mismatch: client=%s, server=%s",
                 escaped_version ? escaped_version : "<allocation failed>", PROTOCOL_VERSION);
     free(escaped_version);
     send_status(file_descriptor, STATUS_ERROR);
@@ -1455,14 +1458,14 @@ Config* config_receive_with_validate(int file_descriptor, ConfigValidateFunc val
   if (config->compress_choice[0] != '\0' && strcmp(config->compress_choice, "zstd") != 0 &&
       strcmp(config->compress_choice, "none") != 0) {
     char* escaped_choice = output_escape(config->compress_choice, config->eight_bit_output);
-    fprintf(stderr, "Unsupported compression choice: %s\n",
+    log_message(LOG_LEVEL_ERROR, "Unsupported compression choice: %s",
                 escaped_choice ? escaped_choice : "<allocation failed>");
     free(escaped_choice);
     send_status(file_descriptor, STATUS_ERROR);
     goto error;
   }
   if (!validate_received_config(config)) {
-    fprintf(stderr, "Invalid configuration received from client\n");
+    log_message(LOG_LEVEL_ERROR, "Invalid configuration received from client");
     send_status(file_descriptor, STATUS_ERROR);
     goto error;
   }
@@ -1476,7 +1479,7 @@ Config* config_receive_with_validate(int file_descriptor, ConfigValidateFunc val
        * the CONFIG_VALIDATE_ALREADY_TERMINATED sentinel, so no second status is
        * written. */
       if (rejection != CONFIG_VALIDATE_ALREADY_TERMINATED) {
-        fprintf(stderr, "%s\n", rejection);
+        log_message(LOG_LEVEL_ERROR, "%s", rejection);
         send_status(file_descriptor, STATUS_ERROR);
       }
       goto error;

src/shared/log.c

@@ -3,7 +3,9 @@
 #include <stdbool.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
+#include <threads.h>
 #include <time.h>
 
 static const char* log_level_strings[] = {"DEBUG", "INFO", "WARN", "ERROR"};
@@ -15,6 +17,18 @@ static FILE* log_fp = NULL;
 static _Thread_local bool eight_bit_output;
 static LogStderrMode stderr_mode = LOG_STDERR_ERRORS;
 
+/* Serializes access to log_fp and makes each emitted line atomic: the
+ * timestamp prefix, formatted body, and trailing newline are written as one
+ * critical section so concurrent threads cannot interleave partial lines.
+ * Initialized lazily (matching the protocol.c bw_mutex idiom) because logging
+ * can happen before main() installs any synchronization. */
+static mtx_t log_mutex;
+static once_flag log_mutex_once = ONCE_FLAG_INIT;
+
+static void log_mutex_init(void) {
+  mtx_init(&log_mutex, mtx_plain);
+}
+
 void set_log_level(LogLevel level) {
   current_log_level = level;
 }
@@ -41,7 +55,10 @@ uint32_t get_log_info_flags(void) {
 }
 
 void log_set_file(FILE* fp) {
+  call_once(&log_mutex_once, log_mutex_init);
+  mtx_lock(&log_mutex);
   log_fp = fp;
+  mtx_unlock(&log_mutex);
 }
 
 void log_set_8_bit_output(bool enabled) {
@@ -60,13 +77,48 @@ LogStderrMode log_get_stderr_mode(void) {
   return stderr_mode;
 }
 
-static inline void write_message(FILE* dest_io, LogLevel log_level, struct tm t, const char* format,
+/* Format one complete log line (timestamp prefix + body + newline) into a
+ * freshly allocated buffer.  This is pure CPU/malloc work and must happen
+ * OUTSIDE the log mutex: the mutex only guards the log_fp pointer, so a
+ * stalled stderr/stdout pipe cannot block every logging thread.  Returns NULL
+ * on allocation/formatting failure. */
+static char* format_log_line(LogLevel log_level, const struct tm* t, const char* format,
                              va_list args) {
-  fprintf(dest_io, "%04d-%02d-%02d %02d:%02d:%02d [%s]: ", t.tm_year + 1900, t.tm_mon + 1,
-          t.tm_mday, t.tm_hour, t.tm_min, t.tm_sec, log_level_strings[log_level]);
+  char prefix[64];
+  int prefix_len = snprintf(
+      prefix, sizeof(prefix), "%04d-%02d-%02d %02d:%02d:%02d [%s]: ", t->tm_year + 1900,
+      t->tm_mon + 1, t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec, log_level_strings[log_level]);
+  if (prefix_len < 0 || prefix_len >= (int)sizeof(prefix))
+    return NULL;
+  va_list copy;
+  va_copy(copy, args);
+  int body_len = vsnprintf(NULL, 0, format, copy);
+  va_end(copy);
+  if (body_len < 0)
+    return NULL;
+  size_t total = (size_t)prefix_len + (size_t)body_len;
+  char* line = malloc(total + 2); /* body bytes + '\n' + NUL */
+  if (!line)
+    return NULL;
+  memcpy(line, prefix, (size_t)prefix_len);
+  vsnprintf(line + prefix_len, (size_t)body_len + 1, format, args);
+  line[total] = '\n';
+  line[total + 1] = '\0';
+  return line;
+}
 
-  vfprintf(dest_io, format, args);
-  fprintf(dest_io, "\n");
+/* Write an already-formatted line to the console and, if configured, the log
+ * file.  Only the log_fp pointer is read under the mutex (so log_set_file /
+ * config_delete cannot free it while it is in use); the single console fputs
+ * runs unlocked but is internally atomic per stdio stream. */
+static void emit_log_line(FILE* console, const char* line) {
+  fputs(line, console);
+  call_once(&log_mutex_once, log_mutex_init);
+  mtx_lock(&log_mutex);
+  FILE* file = log_fp;
+  if (file)
+    fputs(line, file);
+  mtx_unlock(&log_mutex);
 }
 
 void log_message(LogLevel log_level, const char* format, ...) {
@@ -86,14 +138,12 @@ void log_message(LogLevel log_level, const char* format, ...) {
 
   va_list args;
   va_start(args, format);
-  write_message(dest_io, log_level, t, format, args);
+  char* line = format_log_line(log_level, &t, format, args);
   va_end(args);
-
-  if (log_fp) {
-    va_start(args, format);
-    write_message(log_fp, log_level, t, format, args);
-    va_end(args);
-  }
+  if (!line)
+    return;
+  emit_log_line(dest_io, line);
+  free(line);
 }
 
 void log_debug_message(LogDebugFlag flag, const char* format, ...) {
@@ -107,14 +157,12 @@ void log_debug_message(LogDebugFlag flag, const char* format, ...) {
 
   va_list args;
   va_start(args, format);
-  write_message(stdout, LOG_LEVEL_DEBUG, t, format, args);
+  char* line = format_log_line(LOG_LEVEL_DEBUG, &t, format, args);
   va_end(args);
-
-  if (log_fp) {
-    va_start(args, format);
-    write_message(log_fp, LOG_LEVEL_DEBUG, t, format, args);
-    va_end(args);
-  }
+  if (!line)
+    return;
+  emit_log_line(stdout, line);
+  free(line);
 }
 
 void log_info_message(LogInfoFlag flag, const char* format, ...) {
@@ -129,14 +177,12 @@ void log_info_message(LogInfoFlag flag, const char* format, ...) {
 
   va_list args;
   va_start(args, format);
-  write_message(stdout, LOG_LEVEL_INFO, t, format, args);
+  char* line = format_log_line(LOG_LEVEL_INFO, &t, format, args);
   va_end(args);
-
-  if (log_fp) {
-    va_start(args, format);
-    write_message(log_fp, LOG_LEVEL_INFO, t, format, args);
-    va_end(args);
-  }
+  if (!line)
+    return;
+  emit_log_line(stdout, line);
+  free(line);
 }
 
 void log_perror(const char* context) {

src/shared/transport_tcp.c

@@ -152,9 +152,18 @@ static void accept_loop(Server* server, void (*child_fn)(int, void*), void* chil
     log_message(LOG_LEVEL_INFO, "%s", log_fmt);
     pid_t pid = fork();
     if (pid == 0) {
+      /* Connection children must not run the parent's global cleanup(): it
+       * frees state (credentials / daemon conf) that the child's worker
+       * threads may still be reading and closes fd numbers the child could
+       * already have reused.  Reset the inherited handlers so a signal
+       * terminates the child directly; SIGCHLD is reset too since a child
+       * must never reap the parent's children.  This runs before the child
+       * spawns any thread, so it cannot race one. */
+      signal(SIGINT, SIG_DFL);
+      signal(SIGTERM, SIG_DFL);
+      signal(SIGCHLD, SIG_DFL);
       close(server->file_descriptor);
       child_fn(fd, child_ctx);
-      close(fd);
       _exit(0);
     } else if (pid > 0) {
       g_active_connections++;
@@ -169,6 +178,9 @@ struct plain_ctx {
 
 static void plain_child_fn(int fd, void* ctx) {
   ((struct plain_ctx*)ctx)->handler(fd);
+  /* handler() never closes the connection fd; the child owns its single
+   * close here after the handler has fully torn down. */
+  close(fd);
 }
 
 bool server_listen(Server* server, void (*handler)(int file_descriptor)) {

src/shared/transport_tls.c

@@ -192,13 +192,18 @@ static void tls_child_fn(int fd, void* arg) {
   SSL* ssl = wrap_fd_with_ssl(fd, ctx->ssl_ctx, true, NULL);
   if (!ssl) {
     io_set_ssl(NULL);
+    close(fd);
     return;
   }
   io_set_ssl(ssl);
   ctx->handler(fd);
+  /* Shut the TLS layer down before releasing the fd: handler() no longer
+   * closes it, so SSL_shutdown still has a valid socket.  The child owns the
+   * single fd close, performed last. */
   SSL_shutdown(ssl);
   SSL_free(ssl);
   io_set_ssl(NULL);
+  close(fd);
 }
 
 bool server_listen_tls(Server* server, void (*handler)(int file_descriptor)) {

tests/test_log.c

@@ -1,7 +1,9 @@
 #include "test_log.h"
 #include "log.h"
 #include "test_utils.h"
+#include <fcntl.h>
 #include <string.h>
+#include <threads.h>
 #include <unistd.h>
 
 /* Test default log level: WARNING and ERROR should print, DEBUG and INFO should not.
@@ -147,6 +149,118 @@ static void test_log_debug_enabled_matches_gate() {
   set_log_debug_flags(LOG_DEBUG_ALL);
 }
 
+#define LOG_CONCURRENCY_THREADS 8
+#define LOG_CONCURRENCY_LINES 250
+
+typedef struct {
+  int id;
+} LogConcurrencyArg;
+
+static int log_concurrency_worker(void* context) {
+  LogConcurrencyArg* arg = context;
+  for (int i = 0; i < LOG_CONCURRENCY_LINES; i++) {
+    log_message(LOG_LEVEL_WARNING, "worker %d line %d", arg->id, i);
+  }
+  return 0;
+}
+
+static int count_substring(const char* haystack, const char* needle) {
+  int count = 0;
+  size_t needle_length = strlen(needle);
+  const char* cursor = haystack;
+  while ((cursor = strstr(cursor, needle)) != NULL) {
+    count++;
+    cursor += needle_length;
+  }
+  return count;
+}
+
+/* Concurrent log_message() calls from many threads must never interleave a
+ * single line: every emitted line has exactly one timestamp prefix and one
+ * body.  Before write_message() was serialized, the three separate fprintf
+ * calls (prefix, body, newline) let lines tear. */
+static void test_log_concurrent_no_torn_lines(void) {
+  FILE* fp = tmpfile();
+  EXPECT_NOT_NULL(fp);
+
+  /* Mute the console mirror so the workers don't flood the test output. */
+  fflush(stdout);
+  fflush(stderr);
+  int saved_stdout = dup(STDOUT_FILENO);
+  int saved_stderr = dup(STDERR_FILENO);
+  int null_fd = open("/dev/null", O_WRONLY);
+  EXPECT_TRUE(saved_stdout >= 0);
+  EXPECT_TRUE(saved_stderr >= 0);
+  EXPECT_TRUE(null_fd >= 0);
+  EXPECT_TRUE(dup2(null_fd, STDOUT_FILENO) >= 0);
+  EXPECT_TRUE(dup2(null_fd, STDERR_FILENO) >= 0);
+  close(null_fd);
+
+  set_log_level(LOG_LEVEL_WARNING);
+  log_set_stderr_mode(LOG_STDERR_ERRORS);
+  log_set_file(fp);
+
+  thrd_t threads[LOG_CONCURRENCY_THREADS];
+  LogConcurrencyArg args[LOG_CONCURRENCY_THREADS];
+  int created = 0;
+  for (int i = 0; i < LOG_CONCURRENCY_THREADS; i++) {
+    args[i].id = i;
+    if (thrd_create(&threads[i], log_concurrency_worker, &args[i]) != thrd_success)
+      break;
+    created++;
+  }
+  for (int i = 0; i < created; i++) {
+    thrd_join(threads[i], NULL);
+  }
+
+  log_set_file(NULL);
+  fflush(fp);
+
+  fflush(stdout);
+  fflush(stderr);
+  dup2(saved_stdout, STDOUT_FILENO);
+  dup2(saved_stderr, STDERR_FILENO);
+  close(saved_stdout);
+  close(saved_stderr);
+
+  rewind(fp);
+  char line[512];
+  int total_lines = 0;
+  int malformed_lines = 0;
+  bool saw_missing_newline = false;
+  while (fgets(line, sizeof(line), fp) != NULL) {
+    size_t length = strlen(line);
+    if (length == 0 || line[length - 1] != '\n')
+      saw_missing_newline = true;
+    if (strncmp(line, "20", 2) != 0 || count_substring(line, "[WARN]: worker ") != 1)
+      malformed_lines++;
+    total_lines++;
+  }
+  fclose(fp);
+
+  EXPECT_EQ_INT(created, LOG_CONCURRENCY_THREADS);
+  EXPECT_FALSE(saw_missing_newline);
+  EXPECT_EQ_INT(malformed_lines, 0);
+  EXPECT_EQ_INT(total_lines, LOG_CONCURRENCY_THREADS * LOG_CONCURRENCY_LINES);
+  log_set_stderr_mode(LOG_STDERR_ERRORS);
+}
+
+/* Detaching the logger from a FILE* before it is closed must leave the logging
+ * subsystem safe: later calls must not touch the freed handle. */
+static void test_log_set_file_null_before_fclose(void) {
+  FILE* fp = tmpfile();
+  EXPECT_NOT_NULL(fp);
+
+  set_log_level(LOG_LEVEL_ERROR);
+  log_set_file(fp);
+  log_message(LOG_LEVEL_ERROR, "line before detach");
+  log_set_file(NULL);
+  fclose(fp);
+
+  log_message(LOG_LEVEL_ERROR, "line after close");
+  EXPECT_TRUE(true);
+}
+
 void test_log() {
   test_log_message_debug();
   test_log_message_info();
@@ -159,4 +273,6 @@ void test_log() {
   test_log_stderr_mode_all();
   test_log_message_formats();
   test_log_debug_enabled_matches_gate();
+  test_log_concurrent_no_torn_lines();
+  test_log_set_file_null_before_fclose();
 }

tests/test_scanner.c

@@ -1317,6 +1317,58 @@ static void test_scanner_captures_directory_times() {
   rmdir(root);
 }
 
+/* Ownership guard for chunk_data_to_chunk(): a returned Chunk owns its File
+ * objects, so destroying the chunk must free them exactly once and the scanner
+ * must never free them again.  chunk_size = 1 forces the mid-directory
+ * conversion branch (chunk_data_size > chunk_size) for every file, and the
+ * chunk is destroyed immediately, catching a double free / use-after-free under
+ * ASan if ownership transfer regressed.
+ *
+ * The failure path (array_list_to_array() or chunk_create() returning NULL) is
+ * not reachable from a unit test: both allocate through protocol_alloc(), and
+ * each allocation they perform is no larger than the array_list allocations
+ * that already succeeded while building the list (array_list_to_array() copies
+ * exactly `size` pointers, which never exceeds the capacity just grown, and
+ * sizeof(Chunk) is far below the initial 100-entry item array).  Binding a
+ * small --max-alloc session therefore always fails *before* this function, not
+ * inside it, so fault injection cannot isolate these paths. */
+static void test_scanner_chunk_ownership() {
+  const char* dir = "test_scan_ownership";
+  const char* file1 = "test_scan_ownership/a.txt";
+  const char* file2 = "test_scan_ownership/b.txt";
+  const char* file3 = "test_scan_ownership/c.txt";
+
+  EXPECT_EQ_INT(mkdir(dir, 0755), 0);
+  create_test_file(file1, "aaaa");
+  create_test_file(file2, "bbbb");
+  create_test_file(file3, "cccc");
+
+  ScannerOptions options = {0};
+  options.chunk_size = 1;
+  DirectoryScanner* scanner = directory_scanner_create_with_options(dir, &options);
+  EXPECT_NOT_NULL(scanner);
+
+  int chunks = 0;
+  int files = 0;
+  Chunk* chunk;
+  while ((chunk = directory_scanner_next(scanner)) != NULL) {
+    chunks++;
+    files += chunk->element_count;
+    EXPECT_EQ_INT(chunk->element_count, 1);
+    chunk_destroy(chunk);
+    EXPECT_FALSE(directory_scanner_failed(scanner));
+  }
+  EXPECT_EQ_INT(files, 3);
+  EXPECT_EQ_INT(chunks, 3);
+  EXPECT_FALSE(directory_scanner_failed(scanner));
+
+  directory_scanner_destroy(scanner);
+  unlink(file1);
+  unlink(file2);
+  unlink(file3);
+  rmdir(dir);
+}
+
 void test_scanner() {
   test_scanner_single_file();
   test_scanner_multiple_files();
@@ -1353,4 +1405,5 @@ void test_scanner() {
   test_dirs_files_from();
   test_files_from_relative_send_path();
   test_scanner_captures_directory_times();
+  test_scanner_chunk_ownership();
 }