diff --git a/modules/affile/logproto-file-writer.c b/modules/affile/logproto-file-writer.c
index 8af0ed490..5662015f7 100644
--- a/modules/affile/logproto-file-writer.c
+++ b/modules/affile/logproto-file-writer.c
@@ -1,6 +1,8 @@
 /*
  * Copyright (c) 2002-2012 Balabit
+ * Copyright (c) 2024 Axoflow
  * Copyright (c) 1998-2012 Balázs Scheidler
+ * Copyright (c) 2024 László Várady
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 as published
@@ -38,11 +40,92 @@ typedef struct _LogProtoFileWriter
   gint buf_size;
   gint buf_count;
   gint fd;
-  gint sum_len;
+  gsize sum_len;
   gboolean fsync;
   struct iovec buffer[0];
 } LogProtoFileWriter;
 
+static inline gboolean
+_flush_partial(LogProtoFileWriter *self, LogProtoStatus *status)
+{
+  /* there is still some data from the previous file writing process */
+
+  gint len = self->partial_len - self->partial_pos;
+  gssize rc = log_transport_write(self->super.transport, self->partial + self->partial_pos, len);
+
+  if (rc > 0 && self->fsync)
+    fsync(self->fd);
+
+  if (rc < 0)
+    {
+      if (errno == EINTR || errno == EAGAIN)
+        {
+          *status = LPS_SUCCESS;
+          return FALSE;
+        }
+
+      log_proto_client_msg_rewind(&self->super);
+      msg_error("I/O error occurred while writing",
+                evt_tag_int("fd", self->super.transport->fd),
+                evt_tag_error(EVT_TAG_OSERROR));
+
+      *status = LPS_ERROR;
+      return FALSE;
+    }
+
+  if (rc != len)
+    {
+      self->partial_pos += rc;
+      *status = LPS_PARTIAL;
+      return FALSE;
+    }
+
+  log_proto_client_msg_ack(&self->super, self->partial_messages);
+  g_free(self->partial);
+  self->partial = NULL;
+  self->partial_messages = 0;
+  return TRUE;
+}
+
+static inline void
+_process_partial_write(LogProtoFileWriter *self, gsize written)
+{
+  /* partial success: not everything has been written out */
+
+  /* look for the first chunk that has been cut */
+  gsize sum = self->buffer[0].iov_len; /* sum is the cumulated length of the already processed items */
+  gint i = 0;
+  while (written > sum)
+    sum += self->buffer[++i].iov_len;
+
+  gsize first_non_written_msg_chunk_len = sum - written;
+  self->partial_len = first_non_written_msg_chunk_len;
+  gint first_non_written_chunk_index = i;
+  ++i;
+
+  /* add the lengths of the following messages */
+  while (i < self->buf_count)
+    self->partial_len += self->buffer[i++].iov_len;
+
+  /* allocate and copy the remaining data */
+  self->partial = (guchar *)g_malloc(self->partial_len);
+  gsize ofs = first_non_written_msg_chunk_len;
+  gsize pos = self->buffer[first_non_written_chunk_index].iov_len - ofs;
+  memcpy(self->partial, (guchar *) self->buffer[first_non_written_chunk_index].iov_base + pos, ofs);
+  i = first_non_written_chunk_index + 1;
+  while (i < self->buf_count)
+    {
+      memcpy(self->partial + ofs, self->buffer[i].iov_base, self->buffer[i].iov_len);
+      ofs += self->buffer[i].iov_len;
+      ++i;
+    }
+
+  self->partial_pos = 0;
+  self->partial_messages = self->buf_count - first_non_written_chunk_index;
+
+  log_proto_client_msg_ack(&self->super, self->buf_count - self->partial_messages);
+}
+
 /*
  * log_proto_file_writer_flush:
  *
@@ -55,90 +138,28 @@ static LogProtoStatus
 log_proto_file_writer_flush(LogProtoClient *s)
 {
   LogProtoFileWriter *self = (LogProtoFileWriter *)s;
-  gint rc, i, i0, sum, ofs, pos;
 
   if (self->partial)
     {
-      /* there is still some data from the previous file writing process */
-      gint len = self->partial_len - self->partial_pos;
-
-      rc = log_transport_write(self->super.transport, self->partial + self->partial_pos, len);
-      if (rc > 0 && self->fsync)
-        fsync(self->fd);
-      if (rc < 0)
-        {
-          goto write_error;
-        }
-      else if (rc != len)
-        {
-          self->partial_pos += rc;
-          return LPS_PARTIAL;
-        }
-      else
-        {
-          log_proto_client_msg_ack(&self->super, self->partial_messages);
-          g_free(self->partial);
-          self->partial = NULL;
-        }
+      LogProtoStatus partial_flush_status;
+      if (!_flush_partial(self, &partial_flush_status))
+        return partial_flush_status;
     }
 
   /* we might be called from log_writer_deinit() without having a buffer at all */
   if (self->buf_count == 0)
     return LPS_SUCCESS;
 
-  rc = log_transport_writev(self->super.transport, self->buffer, self->buf_count);
+  gssize rc = log_transport_writev(self->super.transport, self->buffer, self->buf_count);
+
   if (rc > 0 && self->fsync)
     fsync(self->fd);
 
   if (rc < 0)
     {
-      goto write_error;
-    }
-  else if (rc != self->sum_len)
-    {
-      /* partial success: not everything has been written out */
-      /* look for the first chunk that has been cut */
-      sum = self->buffer[0].iov_len; /* sum is the cumulated length of the already processed items */
-      i = 0;
-      while (rc > sum)
-        sum += self->buffer[++i].iov_len;
-      self->partial_len = sum - rc; /* this is the length of the first non-written chunk */
-      i0 = i;
-      ++i;
-      /* add the lengths of the following messages */
-      while (i < self->buf_count)
-        self->partial_len += self->buffer[i++].iov_len;
-      /* allocate and copy the remaining data */
-      self->partial = (guchar *)g_malloc(self->partial_len);
-      ofs = sum - rc; /* the length of the remaining (not processed) chunk in the first message */
-      pos = self->buffer[i0].iov_len - ofs;
-      memcpy(self->partial, (guchar *) self->buffer[i0].iov_base + pos, ofs);
-      i = i0 + 1;
-      while (i < self->buf_count)
-        {
-          memcpy(self->partial + ofs, self->buffer[i].iov_base, self->buffer[i].iov_len);
-          ofs += self->buffer[i].iov_len;
-          ++i;
-        }
-      self->partial_pos = 0;
-      self->partial_messages = self->buf_count - i0;
-    }
-  else
-    {
-      log_proto_client_msg_ack(&self->super, self->buf_count);
-    }
+      if (errno == EINTR || errno == EAGAIN)
+        return LPS_SUCCESS;
 
-  /* free the previous message strings (the remaining part has been copied to the partial buffer) */
-  for (i = 0; i < self->buf_count; ++i)
-    g_free(self->buffer[i].iov_base);
-  self->buf_count = 0;
-  self->sum_len = 0;
-
-  return LPS_SUCCESS;
-
-write_error:
-  if (errno != EINTR && errno != EAGAIN)
-    {
       log_proto_client_msg_rewind(&self->super);
       msg_error("I/O error occurred while writing",
                 evt_tag_int("fd", self->super.transport->fd),
@@ -146,8 +167,18 @@ log_proto_file_writer_flush(LogProtoClient *s)
       return LPS_ERROR;
     }
 
-  return LPS_SUCCESS;
+  if (rc != self->sum_len)
+    _process_partial_write(self, rc);
+  else
+    log_proto_client_msg_ack(&self->super, self->buf_count);
+
+  /* free the previous message strings (the remaining part has been copied to the partial buffer) */
+  for (gint i = 0; i < self->buf_count; ++i)
+    g_free(self->buffer[i].iov_base);
+  self->buf_count = 0;
+  self->sum_len = 0;
 
+  return LPS_SUCCESS;
 }
 
 /*
diff --git a/news/bugfix-303.md b/news/bugfix-303.md
new file mode 100644
index 000000000..53ea8f9f6
--- /dev/null
+++ b/news/bugfix-303.md
@@ -0,0 +1,8 @@
+`file()`, `stdout()`: fix log sources getting stuck
+
+Due to an acknowledgment bug in the `file()` and `stdout()` destinations,
+sources routed to those destinations may have gotten stuck as they were
+flow-controlled incorrectly.
+
+This issue occured only in extremely rare cases with regular files, but it
+occured frequently with `/dev/stderr` and other slow pseudo-devices.