Introducing port messages to notify about out of shared memory.

- OOSM (out of shared memory).  Sent by application process to router
  when application reaches the limit of allocated shared memory and
  needs more.
- SHM_ACK.  Sent by router to application when the application's shared
  memory is released and the OOSM flag is enabled for the segment.

This implements blocking mode (the library waits for SHM_ACK in case of
out of shared memory condition and retries allocating the required memory
amount) and non-blocking mode (the library notifies the application that
it's out of shared memory and returns control to the application module
that sets up the output queue and puts SHM_ACK in the main message loop).

6 files changed, 567 insertions, 68 deletions

diff --git a/src/nxt_port.h b/src/nxt_port.h
index 3f9302e8..c6f15238 100644
--- a/src/nxt_port.h
+++ b/src/nxt_port.h
@@ -44,6 +44,9 @@ struct nxt_port_handlers_s {
 
     /* Various data. */
     nxt_port_handler_t  data;
+
+    nxt_port_handler_t  oosm;
+    nxt_port_handler_t  shm_ack;
 };
 
 
@@ -82,6 +85,9 @@ typedef enum {
 
     _NXT_PORT_MSG_DATA          = nxt_port_handler_idx(data),
 
+    _NXT_PORT_MSG_OOSM          = nxt_port_handler_idx(oosm),
+    _NXT_PORT_MSG_SHM_ACK       = nxt_port_handler_idx(shm_ack),
+
     NXT_PORT_MSG_MAX            = sizeof(nxt_port_handlers_t)
                                   / sizeof(nxt_port_handler_t),
 
@@ -114,6 +120,9 @@ typedef enum {
 
     NXT_PORT_MSG_DATA           = _NXT_PORT_MSG_DATA,
     NXT_PORT_MSG_DATA_LAST      = _NXT_PORT_MSG_DATA | NXT_PORT_MSG_LAST,
+
+    NXT_PORT_MSG_OOSM           = _NXT_PORT_MSG_OOSM | NXT_PORT_MSG_LAST,
+    NXT_PORT_MSG_SHM_ACK        = _NXT_PORT_MSG_SHM_ACK | NXT_PORT_MSG_LAST,
 } nxt_port_msg_type_t;
 
 
diff --git a/src/nxt_port_memory.c b/src/nxt_port_memory.c
index b7068c88..24a40406 100644
--- a/src/nxt_port_memory.c
+++ b/src/nxt_port_memory.c
@@ -112,6 +112,8 @@ nxt_port_mmap_buf_completion(nxt_task_t *task, void *obj, void *data)
     u_char                   *p;
     nxt_mp_t                 *mp;
     nxt_buf_t                *b;
+    nxt_port_t               *port;
+    nxt_process_t            *process;
     nxt_chunk_id_t           c;
     nxt_port_mmap_header_t   *hdr;
     nxt_port_mmap_handler_t  *mmap_handler;
@@ -163,6 +165,21 @@ nxt_port_mmap_buf_completion(nxt_task_t *task, void *obj, void *data)
         c++;
     }
 
+    if (hdr->dst_pid == nxt_pid
+        && nxt_atomic_cmp_set(&hdr->oosm, 1, 0))
+    {
+        process = nxt_runtime_process_find(task->thread->runtime, hdr->src_pid);
+
+        if (process != NULL && !nxt_queue_is_empty(&process->ports)) {
+            port = nxt_process_port_first(process);
+
+            if (port->type == NXT_PROCESS_WORKER) {
+                (void) nxt_port_socket_write(task, port, NXT_PORT_MSG_SHM_ACK,
+                                             -1, 0, 0, NULL);
+            }
+        }
+    }
+
 release_buf:
 
     nxt_port_mmap_handler_use(mmap_handler, -1);
@@ -454,6 +471,8 @@ nxt_port_mmap_get(nxt_task_t *task, nxt_port_t *port, nxt_chunk_id_t *c,
                 goto unlock_return;
             }
         }
+
+        hdr->oosm = 1;
     }
 
     /* TODO introduce port_mmap limit and release wait. */
diff --git a/src/nxt_port_memory_int.h b/src/nxt_port_memory_int.h
index 53dfaebf..87c3d833 100644
--- a/src/nxt_port_memory_int.h
+++ b/src/nxt_port_memory_int.h
@@ -51,6 +51,7 @@ struct nxt_port_mmap_header_s {
     nxt_pid_t       src_pid; /* For sanity check. */
     nxt_pid_t       dst_pid; /* For sanity check. */
     nxt_port_id_t   sent_over;
+    nxt_atomic_t    oosm;
     nxt_free_map_t  free_map[MAX_FREE_IDX];
     nxt_free_map_t  free_map_padding;
     nxt_free_map_t  free_tracking_map[MAX_FREE_IDX];
diff --git a/src/nxt_router.c b/src/nxt_router.c
index 38396e86..6a1f3792 100644
--- a/src/nxt_router.c
+++ b/src/nxt_router.c
@@ -248,6 +248,7 @@ static nxt_int_t nxt_router_http_request_done(nxt_task_t *task,
     nxt_http_request_t *r);
 static void nxt_router_http_request_release(nxt_task_t *task, void *obj,
     void *data);
+static void nxt_router_oosm_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg);
 
 extern const nxt_http_request_state_t  nxt_http_websocket;
 
@@ -276,6 +277,7 @@ nxt_port_handlers_t  nxt_router_process_port_handlers = {
     .access_log   = nxt_router_access_log_reopen_handler,
     .rpc_ready    = nxt_port_rpc_handler,
     .rpc_error    = nxt_port_rpc_handler,
+    .oosm         = nxt_router_oosm_handler,
 };
 
 
@@ -2748,6 +2750,7 @@ static nxt_port_handlers_t  nxt_router_app_port_handlers = {
     .rpc_error = nxt_port_rpc_handler,
     .mmap      = nxt_port_mmap_handler,
     .data      = nxt_port_rpc_handler,
+    .oosm      = nxt_router_oosm_handler,
 };
 
 
@@ -5241,3 +5244,56 @@ nxt_router_http_request_release(nxt_task_t *task, void *obj, void *data)
 
     nxt_mp_release(r->mem_pool);
 }
+
+
+static void
+nxt_router_oosm_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
+{
+    size_t                   mi;
+    uint32_t                 i;
+    nxt_bool_t               ack;
+    nxt_process_t            *process;
+    nxt_free_map_t           *m;
+    nxt_port_mmap_header_t   *hdr;
+
+    nxt_debug(task, "oosm in %PI", msg->port_msg.pid);
+
+    process = nxt_runtime_process_find(task->thread->runtime,
+                                       msg->port_msg.pid);
+    if (nxt_slow_path(process == NULL)) {
+        return;
+    }
+
+    ack = 0;
+
+    /*
+     * To mitigate possible racing condition (when OOSM message received
+     * after some of the memory was already freed), need to try to find
+     * first free segment in shared memory and send ACK if found.
+     */
+
+    nxt_thread_mutex_lock(&process->incoming.mutex);
+
+    for (i = 0; i < process->incoming.size; i++) {
+        hdr = process->incoming.elts[i].mmap_handler->hdr;
+        m = hdr->free_map;
+
+        for (mi = 0; mi < MAX_FREE_IDX; mi++) {
+            if (m[mi] != 0) {
+                ack = 1;
+
+                nxt_debug(task, "oosm: already free #%uD %uz = 0x%08xA",
+                          i, mi, m[mi]);
+
+                break;
+            }
+        }
+    }
+
+    nxt_thread_mutex_unlock(&process->incoming.mutex);
+
+    if (ack) {
+        (void) nxt_port_socket_write(task, msg->port, NXT_PORT_MSG_SHM_ACK,
+                                     -1, 0, 0, NULL);
+    }
+}
diff --git a/src/nxt_unit.c b/src/nxt_unit.c
index 8bc7b679..95874db3 100644
--- a/src/nxt_unit.c
+++ b/src/nxt_unit.c
@@ -23,12 +23,17 @@
 #define NXT_UNIT_LOCAL_BUF_SIZE  \
     (NXT_UNIT_MAX_PLAIN_SIZE + sizeof(nxt_port_msg_t))
 
+#define NXT_UNIT_MAX_PLAIN_SIZE  1024
+#define NXT_UNIT_LOCAL_BUF_SIZE  \
+    (NXT_UNIT_MAX_PLAIN_SIZE + sizeof(nxt_port_msg_t))
+
 typedef struct nxt_unit_impl_s                  nxt_unit_impl_t;
 typedef struct nxt_unit_mmap_s                  nxt_unit_mmap_t;
 typedef struct nxt_unit_mmaps_s                 nxt_unit_mmaps_t;
 typedef struct nxt_unit_process_s               nxt_unit_process_t;
 typedef struct nxt_unit_mmap_buf_s              nxt_unit_mmap_buf_t;
 typedef struct nxt_unit_recv_msg_s              nxt_unit_recv_msg_t;
+typedef struct nxt_unit_read_buf_s              nxt_unit_read_buf_t;
 typedef struct nxt_unit_ctx_impl_s              nxt_unit_ctx_impl_t;
 typedef struct nxt_unit_port_impl_s             nxt_unit_port_impl_t;
 typedef struct nxt_unit_request_info_impl_s     nxt_unit_request_info_impl_t;
@@ -53,6 +58,7 @@ static int nxt_unit_process_req_headers(nxt_unit_ctx_t *ctx,
     nxt_unit_recv_msg_t *recv_msg);
 static int nxt_unit_process_websocket(nxt_unit_ctx_t *ctx,
     nxt_unit_recv_msg_t *recv_msg);
+static int nxt_unit_process_shm_ack(nxt_unit_ctx_t *ctx);
 static nxt_unit_request_info_impl_t *nxt_unit_request_info_get(
     nxt_unit_ctx_t *ctx);
 static void nxt_unit_request_info_release(nxt_unit_request_info_t *req);
@@ -69,11 +75,18 @@ static int nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
     nxt_unit_mmap_buf_t *mmap_buf, int last);
 static void nxt_unit_mmap_buf_free(nxt_unit_mmap_buf_t *mmap_buf);
 static void nxt_unit_free_outgoing_buf(nxt_unit_mmap_buf_t *mmap_buf);
+static nxt_unit_read_buf_t *nxt_unit_read_buf_get(nxt_unit_ctx_t *ctx);
+static nxt_unit_read_buf_t *nxt_unit_read_buf_get_impl(
+    nxt_unit_ctx_impl_t *ctx_impl);
+static void nxt_unit_read_buf_release(nxt_unit_ctx_t *ctx,
+    nxt_unit_read_buf_t *rbuf);
 static ssize_t nxt_unit_buf_read(nxt_unit_buf_t **b, uint64_t *len, void *dst,
     size_t size);
 static nxt_port_mmap_header_t *nxt_unit_mmap_get(nxt_unit_ctx_t *ctx,
     nxt_unit_process_t *process, nxt_unit_port_id_t *port_id,
-    nxt_chunk_id_t *c, int n);
+    nxt_chunk_id_t *c, int *n, int min_n);
+static int nxt_unit_send_oosm(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id);
+static int nxt_unit_wait_shm_ack(nxt_unit_ctx_t *ctx);
 static nxt_unit_mmap_t *nxt_unit_mmap_at(nxt_unit_mmaps_t *mmaps, uint32_t i);
 static nxt_port_mmap_header_t *nxt_unit_new_mmap(nxt_unit_ctx_t *ctx,
     nxt_unit_process_t *process, nxt_unit_port_id_t *port_id, int n);
@@ -81,7 +94,7 @@ static int nxt_unit_send_mmap(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id,
     int fd);
 static int nxt_unit_get_outgoing_buf(nxt_unit_ctx_t *ctx,
     nxt_unit_process_t *process, nxt_unit_port_id_t *port_id, uint32_t size,
-    nxt_unit_mmap_buf_t *mmap_buf, char *local_buf);
+    uint32_t min_size, nxt_unit_mmap_buf_t *mmap_buf, char *local_buf);
 static int nxt_unit_incoming_mmap(nxt_unit_ctx_t *ctx, pid_t pid, int fd);
 
 static void nxt_unit_mmaps_init(nxt_unit_mmaps_t *mmaps);
@@ -94,14 +107,18 @@ static int nxt_unit_tracking_read(nxt_unit_ctx_t *ctx,
     nxt_unit_recv_msg_t *recv_msg);
 static int nxt_unit_mmap_read(nxt_unit_ctx_t *ctx,
     nxt_unit_recv_msg_t *recv_msg);
-static int nxt_unit_mmap_release(nxt_port_mmap_header_t *hdr, void *start,
-    uint32_t size);
+static void nxt_unit_mmap_release(nxt_unit_ctx_t *ctx,
+    nxt_unit_process_t *process,
+    nxt_port_mmap_header_t *hdr, void *start, uint32_t size);
+static int nxt_unit_send_shm_ack(nxt_unit_ctx_t *ctx, pid_t pid);
 
 static nxt_unit_process_t *nxt_unit_process_get(nxt_unit_ctx_t *ctx,
     pid_t pid);
 static nxt_unit_process_t *nxt_unit_process_find(nxt_unit_ctx_t *ctx,
     pid_t pid, int remove);
 static nxt_unit_process_t *nxt_unit_process_pop_first(nxt_unit_impl_t *lib);
+static void nxt_unit_read_buf(nxt_unit_ctx_t *ctx,
+    nxt_unit_read_buf_t *rbuf);
 static int nxt_unit_create_port(nxt_unit_ctx_t *ctx,
     nxt_unit_port_id_t *port_id, int *fd);
 
@@ -144,6 +161,7 @@ struct nxt_unit_mmap_buf_s {
     nxt_unit_port_id_t       port_id;
     nxt_unit_request_info_t  *req;
     nxt_unit_ctx_impl_t      *ctx_impl;
+    nxt_unit_process_t       *process;
     char                     *free_ptr;
     char                     *plain_ptr;
 };
@@ -206,6 +224,14 @@ struct nxt_unit_websocket_frame_impl_s {
 };
 
 
+struct nxt_unit_read_buf_s {
+    nxt_unit_read_buf_t           *next;
+    ssize_t                       size;
+    char                          buf[16384];
+    char                          oob[256];
+};
+
+
 struct nxt_unit_ctx_impl_s {
     nxt_unit_ctx_t                ctx;
 
@@ -230,7 +256,12 @@ struct nxt_unit_ctx_impl_s {
     /*  of nxt_unit_request_info_impl_t */
     nxt_lvlhsh_t                  requests;
 
+    nxt_unit_read_buf_t           *pending_read_head;
+    nxt_unit_read_buf_t           **pending_read_tail;
+    nxt_unit_read_buf_t           *free_read_buf;
+
     nxt_unit_mmap_buf_t           ctx_buf[2];
+    nxt_unit_read_buf_t           ctx_read_buf;
 
     nxt_unit_request_info_impl_t  req;
 };
@@ -277,6 +308,7 @@ struct nxt_unit_mmaps_s {
     pthread_mutex_t          mutex;
     uint32_t                 size;
     uint32_t                 cap;
+    nxt_atomic_t             allocated_chunks;
     nxt_unit_mmap_t          *elts;
 };
 
@@ -495,6 +527,11 @@ nxt_unit_ctx_init(nxt_unit_impl_t *lib, nxt_unit_ctx_impl_t *ctx_impl,
 
     nxt_queue_insert_tail(&ctx_impl->free_req, &ctx_impl->req.link);
 
+    ctx_impl->pending_read_head = NULL;
+    ctx_impl->pending_read_tail = &ctx_impl->pending_read_head;
+    ctx_impl->free_read_buf = &ctx_impl->ctx_read_buf;
+    ctx_impl->ctx_read_buf.next = NULL;
+
     ctx_impl->req.req.ctx = &ctx_impl->ctx;
     ctx_impl->req.req.unit = &lib->unit;
 
@@ -772,6 +809,10 @@ nxt_unit_process_msg(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id,
         rc = NXT_UNIT_OK;
         break;
 
+    case _NXT_PORT_MSG_SHM_ACK:
+        rc = nxt_unit_process_shm_ack(ctx);
+        break;
+
     default:
         nxt_unit_debug(ctx, "#%"PRIu32": ignore message type: %d",
                        port_msg->stream, (int) port_msg->type);
@@ -1052,6 +1093,23 @@ nxt_unit_process_websocket(nxt_unit_ctx_t *ctx, nxt_unit_recv_msg_t *recv_msg)
 }
 
 
+static int
+nxt_unit_process_shm_ack(nxt_unit_ctx_t *ctx)
+{
+    nxt_unit_impl_t       *lib;
+    nxt_unit_callbacks_t  *cb;
+
+    lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
+    cb = &lib->callbacks;
+
+    if (cb->shm_ack_handler != NULL) {
+        cb->shm_ack_handler(ctx);
+    }
+
+    return NXT_UNIT_OK;
+}
+
+
 static nxt_unit_request_info_impl_t *
 nxt_unit_request_info_get(nxt_unit_ctx_t *ctx)
 {
@@ -1705,7 +1763,8 @@ nxt_unit_response_buf_alloc(nxt_unit_request_info_t *req, uint32_t size)
     nxt_unit_mmap_buf_insert_tail(&req_impl->outgoing_buf, mmap_buf);
 
     rc = nxt_unit_get_outgoing_buf(req->ctx, req_impl->process,
-                                   &req->response_port, size, mmap_buf, NULL);
+                                   &req->response_port, size, size, mmap_buf,
+                                   NULL);
     if (nxt_slow_path(rc != NXT_UNIT_OK)) {
         nxt_unit_mmap_buf_release(mmap_buf);
 
@@ -1947,6 +2006,7 @@ nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
         nxt_port_mmap_msg_t  mmap_msg;
     } m;
 
+    int                      rc;
     u_char                   *last_used, *first_free;
     ssize_t                  res;
     nxt_chunk_id_t           first_free_chunk;
@@ -1971,6 +2031,8 @@ nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
     m.msg.mf = 0;
     m.msg.tracking = 0;
 
+    rc = NXT_UNIT_ERROR;
+
     if (m.msg.mmap) {
         m.mmap_msg.mmap_id = hdr->id;
         m.mmap_msg.chunk_id = nxt_port_mmap_chunk_id(hdr,
@@ -1985,13 +2047,13 @@ nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
         res = lib->callbacks.port_send(ctx, &mmap_buf->port_id, &m, sizeof(m),
                                        NULL, 0);
         if (nxt_slow_path(res != sizeof(m))) {
-            return NXT_UNIT_ERROR;
+            goto free_buf;
         }
 
-        if (buf->end - buf->free >= PORT_MMAP_CHUNK_SIZE) {
-            last_used = (u_char *) buf->free - 1;
+        last_used = (u_char *) buf->free - 1;
+        first_free_chunk = nxt_port_mmap_chunk_id(hdr, last_used) + 1;
 
-            first_free_chunk = nxt_port_mmap_chunk_id(hdr, last_used) + 1;
+        if (buf->end - buf->free >= PORT_MMAP_CHUNK_SIZE) {
             first_free = nxt_port_mmap_chunk_start(hdr, first_free_chunk);
 
             buf->start = (char *) first_free;
@@ -2009,6 +2071,13 @@ nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
             mmap_buf->hdr = NULL;
         }
 
+        nxt_atomic_fetch_add(&mmap_buf->process->outgoing.allocated_chunks,
+                            (int) m.mmap_msg.chunk_id - (int) first_free_chunk);
+
+        nxt_unit_debug(ctx, "process %d allocated_chunks %d",
+                       mmap_buf->process->pid,
+                       mmap_buf->process->outgoing.allocated_chunks);
+
     } else {
         if (nxt_slow_path(mmap_buf->plain_ptr == NULL
                           || mmap_buf->plain_ptr > buf->start - sizeof(m.msg)))
@@ -2016,7 +2085,7 @@ nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
             nxt_unit_warn(ctx, "#%"PRIu32": failed to send plain memory buffer"
                           ": no space reserved for message header", stream);
 
-            return NXT_UNIT_ERROR;
+            goto free_buf;
         }
 
         memcpy(buf->start - sizeof(m.msg), &m.msg, sizeof(m.msg));
@@ -2030,11 +2099,17 @@ nxt_unit_mmap_buf_send(nxt_unit_ctx_t *ctx, uint32_t stream,
                                        m.mmap_msg.size + sizeof(m.msg),
                                        NULL, 0);
         if (nxt_slow_path(res != (ssize_t) (m.mmap_msg.size + sizeof(m.msg)))) {
-            return NXT_UNIT_ERROR;
+            goto free_buf;
         }
     }
 
-    return NXT_UNIT_OK;
+    rc = NXT_UNIT_OK;
+
+free_buf:
+
+    nxt_unit_free_outgoing_buf(mmap_buf);
+
+    return rc;
 }
 
 
@@ -2058,15 +2133,76 @@ static void
 nxt_unit_free_outgoing_buf(nxt_unit_mmap_buf_t *mmap_buf)
 {
     if (mmap_buf->hdr != NULL) {
-        nxt_unit_mmap_release(mmap_buf->hdr, mmap_buf->buf.start,
+        nxt_unit_mmap_release(&mmap_buf->ctx_impl->ctx,
+                              mmap_buf->process,
+                              mmap_buf->hdr, mmap_buf->buf.start,
                               mmap_buf->buf.end - mmap_buf->buf.start);
 
-    } else if (mmap_buf->free_ptr != NULL) {
+        mmap_buf->hdr = NULL;
+
+        return;
+    }
+
+    if (mmap_buf->free_ptr != NULL) {
         free(mmap_buf->free_ptr);
+
+        mmap_buf->free_ptr = NULL;
     }
 }
 
 
+static nxt_unit_read_buf_t *
+nxt_unit_read_buf_get(nxt_unit_ctx_t *ctx)
+{
+    nxt_unit_ctx_impl_t  *ctx_impl;
+
+    ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
+
+    pthread_mutex_lock(&ctx_impl->mutex);
+
+    return nxt_unit_read_buf_get_impl(ctx_impl);
+}
+
+
+static nxt_unit_read_buf_t *
+nxt_unit_read_buf_get_impl(nxt_unit_ctx_impl_t *ctx_impl)
+{
+    nxt_unit_read_buf_t  *rbuf;
+
+    if (ctx_impl->free_read_buf != NULL) {
+        rbuf = ctx_impl->free_read_buf;
+        ctx_impl->free_read_buf = rbuf->next;
+
+        pthread_mutex_unlock(&ctx_impl->mutex);
+
+        return rbuf;
+    }
+
+    pthread_mutex_unlock(&ctx_impl->mutex);
+
+    rbuf = malloc(sizeof(nxt_unit_read_buf_t));
+
+    return rbuf;
+}
+
+
+static void
+nxt_unit_read_buf_release(nxt_unit_ctx_t *ctx,
+    nxt_unit_read_buf_t *rbuf)
+{
+    nxt_unit_ctx_impl_t  *ctx_impl;
+
+    ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
+
+    pthread_mutex_lock(&ctx_impl->mutex);
+
+    rbuf->next = ctx_impl->free_read_buf;
+    ctx_impl->free_read_buf = rbuf;
+
+    pthread_mutex_unlock(&ctx_impl->mutex);
+}
+
+
 nxt_unit_buf_t *
 nxt_unit_buf_next(nxt_unit_buf_t *buf)
 {
@@ -2100,8 +2236,21 @@ int
 nxt_unit_response_write(nxt_unit_request_info_t *req, const void *start,
     size_t size)
 {
+    ssize_t  res;
+
+    res = nxt_unit_response_write_nb(req, start, size, size);
+
+    return res < 0 ? -res : NXT_UNIT_OK;
+}
+
+
+ssize_t
+nxt_unit_response_write_nb(nxt_unit_request_info_t *req, const void *start,
+    size_t size, size_t min_size)
+{
     int                           rc;
-    uint32_t                      part_size;
+    ssize_t                       sent;
+    uint32_t                      part_size, min_part_size, buf_size;
     const char                    *part_start;
     nxt_unit_mmap_buf_t           mmap_buf;
     nxt_unit_request_info_impl_t  *req_impl;
@@ -2110,58 +2259,70 @@ nxt_unit_response_write(nxt_unit_request_info_t *req, const void *start,
     req_impl = nxt_container_of(req, nxt_unit_request_info_impl_t, req);
 
     part_start = start;
+    sent = 0;
 
     if (nxt_slow_path(req_impl->state < NXT_UNIT_RS_RESPONSE_INIT)) {
         nxt_unit_req_warn(req, "write: response not initialized yet");
 
-        return NXT_UNIT_ERROR;
+        return -NXT_UNIT_ERROR;
     }
 
     /* Check if response is not send yet. */
-    if (nxt_slow_path(req->response_buf)) {
+    if (nxt_slow_path(req->response_buf != NULL)) {
         part_size = req->response_buf->end - req->response_buf->free;
         part_size = nxt_min(size, part_size);
 
         rc = nxt_unit_response_add_content(req, part_start, part_size);
         if (nxt_slow_path(rc != NXT_UNIT_OK)) {
-            return rc;
+            return -rc;
         }
 
         rc = nxt_unit_response_send(req);
         if (nxt_slow_path(rc != NXT_UNIT_OK)) {
-            return rc;
+            return -rc;
         }
 
         size -= part_size;
         part_start += part_size;
+        sent += part_size;
+
+        min_size -= nxt_min(min_size, part_size);
     }
 
     while (size > 0) {
         part_size = nxt_min(size, PORT_MMAP_DATA_SIZE);
+        min_part_size = nxt_min(min_size, part_size);
+        min_part_size = nxt_min(min_part_size, PORT_MMAP_CHUNK_SIZE);
 
         rc = nxt_unit_get_outgoing_buf(req->ctx, req_impl->process,
                                        &req->response_port, part_size,
-                                       &mmap_buf, local_buf);
+                                       min_part_size, &mmap_buf, local_buf);
         if (nxt_slow_path(rc != NXT_UNIT_OK)) {
-            return rc;
+            return -rc;
+        }
+
+        buf_size = mmap_buf.buf.end - mmap_buf.buf.free;
+        if (nxt_slow_path(buf_size == 0)) {
+            return sent;
         }
+        part_size = nxt_min(buf_size, part_size);
 
         mmap_buf.buf.free = nxt_cpymem(mmap_buf.buf.free,
                                        part_start, part_size);
 
         rc = nxt_unit_mmap_buf_send(req->ctx, req_impl->stream, &mmap_buf, 0);
-
-        nxt_unit_free_outgoing_buf(&mmap_buf);
-
         if (nxt_slow_path(rc != NXT_UNIT_OK)) {
-            return rc;
+            return -rc;
         }
 
         size -= part_size;
         part_start += part_size;
+        sent += part_size;
+
+        min_size -= nxt_min(min_size, part_size);
     }
 
-    return NXT_UNIT_OK;
+    return sent;
 }
 
 
@@ -2171,6 +2332,7 @@ nxt_unit_response_write_cb(nxt_unit_request_info_t *req,
 {
     int                           rc;
     ssize_t                       n;
+    uint32_t                      buf_size;
     nxt_unit_buf_t                *buf;
     nxt_unit_mmap_buf_t           mmap_buf;
     nxt_unit_request_info_impl_t  *req_impl;
@@ -2224,10 +2386,11 @@ nxt_unit_response_write_cb(nxt_unit_request_info_t *req,
         nxt_unit_req_debug(req, "write_cb, alloc %"PRIu32"",
                            read_info->buf_size);
 
+        buf_size = nxt_min(read_info->buf_size, PORT_MMAP_DATA_SIZE);
+
         rc = nxt_unit_get_outgoing_buf(req->ctx, req_impl->process,
                                        &req->response_port,
-                                       nxt_min(read_info->buf_size,
-                                               PORT_MMAP_DATA_SIZE),
+                                       buf_size, buf_size,
                                        &mmap_buf, local_buf);
         if (nxt_slow_path(rc != NXT_UNIT_OK)) {
             return rc;
@@ -2249,9 +2412,6 @@ nxt_unit_response_write_cb(nxt_unit_request_info_t *req,
         }
 
         rc = nxt_unit_mmap_buf_send(req->ctx, req_impl->stream, &mmap_buf, 0);
-
-        nxt_unit_free_outgoing_buf(&mmap_buf);
-
         if (nxt_slow_path(rc != NXT_UNIT_OK)) {
             nxt_unit_req_error(req, "Failed to send content");
 
@@ -2398,7 +2558,7 @@ nxt_unit_websocket_sendv(nxt_unit_request_info_t *req, uint8_t opcode,
 {
     int                           i, rc;
     size_t                        l, copy;
-    uint32_t                      payload_len, buf_size;
+    uint32_t                      payload_len, buf_size, alloc_size;
     const uint8_t                 *b;
     nxt_unit_buf_t                *buf;
     nxt_unit_mmap_buf_t           mmap_buf;
@@ -2415,10 +2575,11 @@ nxt_unit_websocket_sendv(nxt_unit_request_info_t *req, uint8_t opcode,
     }
 
     buf_size = 10 + payload_len;
+    alloc_size = nxt_min(buf_size, PORT_MMAP_DATA_SIZE);
 
     rc = nxt_unit_get_outgoing_buf(req->ctx, req_impl->process,
                                    &req->response_port,
-                                   nxt_min(buf_size, PORT_MMAP_DATA_SIZE),
+                                   alloc_size, alloc_size,
                                    &mmap_buf, local_buf);
     if (nxt_slow_path(rc != NXT_UNIT_OK)) {
         return rc;
@@ -2454,17 +2615,16 @@ nxt_unit_websocket_sendv(nxt_unit_request_info_t *req, uint8_t opcode,
                     rc = nxt_unit_mmap_buf_send(req->ctx, req_impl->stream,
                                                 &mmap_buf, 0);
 
-                    nxt_unit_free_outgoing_buf(&mmap_buf);
-
                     if (nxt_slow_path(rc != NXT_UNIT_OK)) {
                         return rc;
                     }
                 }
 
+                alloc_size = nxt_min(buf_size, PORT_MMAP_DATA_SIZE);
+
                 rc = nxt_unit_get_outgoing_buf(req->ctx, req_impl->process,
                                                &req->response_port,
-                                               nxt_min(buf_size,
-                                                       PORT_MMAP_DATA_SIZE),
+                                               alloc_size, alloc_size,
                                                &mmap_buf, local_buf);
                 if (nxt_slow_path(rc != NXT_UNIT_OK)) {
                     return rc;
@@ -2478,8 +2638,6 @@ nxt_unit_websocket_sendv(nxt_unit_request_info_t *req, uint8_t opcode,
     if (buf->free > buf->start) {
         rc = nxt_unit_mmap_buf_send(req->ctx, req_impl->stream,
                                     &mmap_buf, 0);
-
-        nxt_unit_free_outgoing_buf(&mmap_buf);
     }
 
     return rc;
@@ -2553,15 +2711,23 @@ nxt_unit_websocket_done(nxt_unit_websocket_frame_t *ws)
 
 static nxt_port_mmap_header_t *
 nxt_unit_mmap_get(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
-    nxt_unit_port_id_t *port_id, nxt_chunk_id_t *c, int n)
+    nxt_unit_port_id_t *port_id, nxt_chunk_id_t *c, int *n, int min_n)
 {
     int                     res, nchunks, i;
+    uint32_t                outgoing_size;
     nxt_unit_mmap_t         *mm, *mm_end;
+    nxt_unit_impl_t         *lib;
     nxt_port_mmap_header_t  *hdr;
 
+    lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
+
     pthread_mutex_lock(&process->outgoing.mutex);
 
-    mm_end = process->outgoing.elts + process->outgoing.size;
+retry:
+
+    outgoing_size = process->outgoing.size;
+
+    mm_end = process->outgoing.elts + outgoing_size;
 
     for (mm = process->outgoing.elts; mm < mm_end; mm++) {
         hdr = mm->hdr;
@@ -2575,11 +2741,17 @@ nxt_unit_mmap_get(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
         while (nxt_port_mmap_get_free_chunk(hdr->free_map, c)) {
             nchunks = 1;
 
-            while (nchunks < n) {
+            while (nchunks < *n) {
                 res = nxt_port_mmap_chk_set_chunk_busy(hdr->free_map,
                                                        *c + nchunks);
 
                 if (res == 0) {
+                    if (nchunks >= min_n) {
+                        *n = nchunks;
+
+                        goto unlock;
+                    }
+
                     for (i = 0; i < nchunks; i++) {
                         nxt_port_mmap_set_chunk_free(hdr->free_map, *c + i);
                     }
@@ -2592,23 +2764,155 @@ nxt_unit_mmap_get(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
                 nchunks++;
             }
 
-            if (nchunks == n) {
+            if (nchunks >= min_n) {
+                *n = nchunks;
+
                 goto unlock;
             }
         }
+
+        hdr->oosm = 1;
+    }
+
+    if (outgoing_size >= lib->shm_mmap_limit) {
+        /* Cannot allocate more shared memory. */
+        pthread_mutex_unlock(&process->outgoing.mutex);
+
+        if (min_n == 0) {
+            *n = 0;
+        }
+
+        if (nxt_slow_path(process->outgoing.allocated_chunks + min_n
+                          >= lib->shm_mmap_limit * PORT_MMAP_CHUNK_COUNT))
+        {
+            /* Memory allocated by application, but not send to router. */
+            return NULL;
+        }
+
+        /* Notify router about OOSM condition. */
+
+        res = nxt_unit_send_oosm(ctx, port_id);
+        if (nxt_slow_path(res != NXT_UNIT_OK)) {
+            return NULL;
+        }
+
+        /* Return if caller can handle OOSM condition. Non-blocking mode. */
+
+        if (min_n == 0) {
+            return NULL;
+        }
+
+        nxt_unit_debug(ctx, "oosm: waiting for ACK");
+
+        res = nxt_unit_wait_shm_ack(ctx);
+        if (nxt_slow_path(res != NXT_UNIT_OK)) {
+            return NULL;
+        }
+
+        nxt_unit_debug(ctx, "oosm: retry");
+
+        pthread_mutex_lock(&process->outgoing.mutex);
+
+        goto retry;
     }
 
     *c = 0;
-    hdr = nxt_unit_new_mmap(ctx, process, port_id, n);
+    hdr = nxt_unit_new_mmap(ctx, process, port_id, *n);
 
 unlock:
 
+    nxt_atomic_fetch_add(&process->outgoing.allocated_chunks, *n);
+
+    nxt_unit_debug(ctx, "process %d allocated_chunks %d",
+                   process->pid,
+                   process->outgoing.allocated_chunks);
+
     pthread_mutex_unlock(&process->outgoing.mutex);
 
     return hdr;
 }
 
 
+static int
+nxt_unit_send_oosm(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id)
+{
+    ssize_t          res;
+    nxt_port_msg_t   msg;
+    nxt_unit_impl_t  *lib;
+
+    lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
+
+    msg.stream = 0;
+    msg.pid = lib->pid;
+    msg.reply_port = 0;
+    msg.type = _NXT_PORT_MSG_OOSM;
+    msg.last = 0;
+    msg.mmap = 0;
+    msg.nf = 0;
+    msg.mf = 0;
+    msg.tracking = 0;
+
+    res = lib->callbacks.port_send(ctx, port_id, &msg, sizeof(msg), NULL, 0);
+    if (nxt_slow_path(res != sizeof(msg))) {
+        nxt_unit_warn(ctx, "failed to send oosm to %d: %s (%d)",
+                      (int) port_id->pid, strerror(errno), errno);
+
+        return NXT_UNIT_ERROR;
+    }
+
+    return NXT_UNIT_OK;
+}
+
+
+static int
+nxt_unit_wait_shm_ack(nxt_unit_ctx_t *ctx)
+{
+    nxt_port_msg_t       *port_msg;
+    nxt_unit_ctx_impl_t  *ctx_impl;
+    nxt_unit_read_buf_t  *rbuf;
+
+    ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
+
+    while (1) {
+        rbuf = nxt_unit_read_buf_get(ctx);
+        if (nxt_slow_path(rbuf == NULL)) {
+            return NXT_UNIT_ERROR;
+        }
+
+        nxt_unit_read_buf(ctx, rbuf);
+        if (nxt_slow_path(rbuf->size < (ssize_t) sizeof(nxt_port_msg_t))) {
+            nxt_unit_read_buf_release(ctx, rbuf);
+
+            return NXT_UNIT_ERROR;
+        }
+
+        port_msg = (nxt_port_msg_t *) rbuf->buf;
+
+        if (port_msg->type == _NXT_PORT_MSG_SHM_ACK) {
+            nxt_unit_read_buf_release(ctx, rbuf);
+
+            break;
+        }
+
+        pthread_mutex_lock(&ctx_impl->mutex);
+
+        *ctx_impl->pending_read_tail = rbuf;
+        ctx_impl->pending_read_tail = &rbuf->next;
+        rbuf->next = NULL;
+
+        pthread_mutex_unlock(&ctx_impl->mutex);
+
+        if (port_msg->type == _NXT_PORT_MSG_QUIT) {
+            nxt_unit_debug(ctx, "oosm: quit received");
+
+            return NXT_UNIT_ERROR;
+        }
+    }
+
+    return NXT_UNIT_OK;
+}
+
+
 static nxt_unit_mmap_t *
 nxt_unit_mmap_at(nxt_unit_mmaps_t *mmaps, uint32_t i)
 {
@@ -2843,10 +3147,10 @@ nxt_unit_send_mmap(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id, int fd)
 
 static int
 nxt_unit_get_outgoing_buf(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
-    nxt_unit_port_id_t *port_id, uint32_t size,
+    nxt_unit_port_id_t *port_id, uint32_t size, uint32_t min_size,
     nxt_unit_mmap_buf_t *mmap_buf, char *local_buf)
 {
-    uint32_t                nchunks;
+    int                     nchunks, min_nchunks;
     nxt_chunk_id_t          c;
     nxt_port_mmap_header_t  *hdr;
 
@@ -2869,6 +3173,7 @@ nxt_unit_get_outgoing_buf(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
         mmap_buf->buf.free = mmap_buf->buf.start;
         mmap_buf->buf.end = mmap_buf->buf.start + size;
         mmap_buf->port_id = *port_id;
+        mmap_buf->process = process;
 
         nxt_unit_debug(ctx, "outgoing plain buffer allocation: (%p, %d)",
                        mmap_buf->buf.start, (int) size);
@@ -2877,9 +3182,20 @@ nxt_unit_get_outgoing_buf(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
     }
 
     nchunks = (size + PORT_MMAP_CHUNK_SIZE - 1) / PORT_MMAP_CHUNK_SIZE;
+    min_nchunks = (min_size + PORT_MMAP_CHUNK_SIZE - 1) / PORT_MMAP_CHUNK_SIZE;
 
-    hdr = nxt_unit_mmap_get(ctx, process, port_id, &c, nchunks);
+    hdr = nxt_unit_mmap_get(ctx, process, port_id, &c, &nchunks, min_nchunks);
     if (nxt_slow_path(hdr == NULL)) {
+        if (nxt_fast_path(min_nchunks == 0 && nchunks == 0)) {
+            mmap_buf->hdr = NULL;
+            mmap_buf->buf.start = NULL;
+            mmap_buf->buf.free = NULL;
+            mmap_buf->buf.end = NULL;
+            mmap_buf->free_ptr = NULL;
+
+            return NXT_UNIT_OK;
+        }
+
         return NXT_UNIT_ERROR;
     }
 
@@ -2888,6 +3204,7 @@ nxt_unit_get_outgoing_buf(nxt_unit_ctx_t *ctx, nxt_unit_process_t *process,
     mmap_buf->buf.free = mmap_buf->buf.start;
     mmap_buf->buf.end = mmap_buf->buf.start + nchunks * PORT_MMAP_CHUNK_SIZE;
     mmap_buf->port_id = *port_id;
+    mmap_buf->process = process;
     mmap_buf->free_ptr = NULL;
 
     nxt_unit_debug(ctx, "outgoing mmap allocation: (%d,%d,%d)",
@@ -2991,6 +3308,7 @@ nxt_unit_mmaps_init(nxt_unit_mmaps_t *mmaps)
     mmaps->size = 0;
     mmaps->cap = 0;
     mmaps->elts = NULL;
+    mmaps->allocated_chunks = 0;
 }
 
 
@@ -3174,6 +3492,7 @@ nxt_unit_mmap_read(nxt_unit_ctx_t *ctx, nxt_unit_recv_msg_t *recv_msg)
         b->buf.free = start;
         b->buf.end = b->buf.start + size;
         b->hdr = hdr;
+        b->process = process;
 
         b = b->next;
 
@@ -3191,23 +3510,79 @@ nxt_unit_mmap_read(nxt_unit_ctx_t *ctx, nxt_unit_recv_msg_t *recv_msg)
 }
 
 
-static int
-nxt_unit_mmap_release(nxt_port_mmap_header_t *hdr, void *start, uint32_t size)
+static void
+nxt_unit_mmap_release(nxt_unit_ctx_t *ctx,
+    nxt_unit_process_t *process, nxt_port_mmap_header_t *hdr,
+    void *start, uint32_t size)
 {
-    u_char          *p, *end;
-    nxt_chunk_id_t  c;
+    int              freed_chunks;
+    u_char           *p, *end;
+    nxt_chunk_id_t   c;
+    nxt_unit_impl_t  *lib;
 
     memset(start, 0xA5, size);
 
     p = start;
     end = p + size;
     c = nxt_port_mmap_chunk_id(hdr, p);
+    freed_chunks = 0;
 
     while (p < end) {
         nxt_port_mmap_set_chunk_free(hdr->free_map, c);
 
         p += PORT_MMAP_CHUNK_SIZE;
         c++;
+        freed_chunks++;
+    }
+
+    lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
+
+    if (hdr->src_pid == lib->pid && freed_chunks != 0) {
+        nxt_atomic_fetch_add(&process->outgoing.allocated_chunks,
+                             -freed_chunks);
+
+        nxt_unit_debug(ctx, "process %d allocated_chunks %d",
+                       process->pid,
+                       process->outgoing.allocated_chunks);
+    }
+
+    if (hdr->dst_pid == lib->pid
+        && freed_chunks != 0
+        && nxt_atomic_cmp_set(&hdr->oosm, 1, 0))
+    {
+        nxt_unit_send_shm_ack(ctx, hdr->src_pid);
+    }
+}
+
+
+static int
+nxt_unit_send_shm_ack(nxt_unit_ctx_t *ctx, pid_t pid)
+{
+    ssize_t             res;
+    nxt_port_msg_t      msg;
+    nxt_unit_impl_t     *lib;
+    nxt_unit_port_id_t  port_id;
+
+    lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
+
+    nxt_unit_port_id_init(&port_id, pid, 0);
+
+    msg.stream = 0;
+    msg.pid = lib->pid;
+    msg.reply_port = 0;
+    msg.type = _NXT_PORT_MSG_SHM_ACK;
+    msg.last = 0;
+    msg.mmap = 0;
+    msg.nf = 0;
+    msg.mf = 0;
+    msg.tracking = 0;
+
+    res = lib->callbacks.port_send(ctx, &port_id, &msg, sizeof(msg), NULL, 0);
+    if (nxt_slow_path(res != sizeof(msg))) {
+        nxt_unit_warn(ctx, "failed to send ack to %d: %s (%d)",
+                      (int) port_id.pid, strerror(errno), errno);
+
+        return NXT_UNIT_ERROR;
     }
 
     return NXT_UNIT_OK;
@@ -3369,43 +3744,76 @@ int
 nxt_unit_run_once(nxt_unit_ctx_t *ctx)
 {
     int                  rc;
-    char                 buf[4096];
-    char                 oob[256];
-    ssize_t              rsize;
-    nxt_unit_impl_t      *lib;
     nxt_unit_ctx_impl_t  *ctx_impl;
+    nxt_unit_read_buf_t  *rbuf;
 
-    lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
     ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
 
-    memset(oob, 0, sizeof(struct cmsghdr));
+    pthread_mutex_lock(&ctx_impl->mutex);
+
+    if (ctx_impl->pending_read_head != NULL) {
+        rbuf = ctx_impl->pending_read_head;
+        ctx_impl->pending_read_head = rbuf->next;
+
+        if (ctx_impl->pending_read_tail == &rbuf->next) {
+            ctx_impl->pending_read_tail = &ctx_impl->pending_read_head;
+        }
+
+        pthread_mutex_unlock(&ctx_impl->mutex);
 
-    if (ctx_impl->read_port_fd != -1) {
-        rsize = nxt_unit_port_recv(ctx, ctx_impl->read_port_fd,
-                                         buf, sizeof(buf),
-                                         oob, sizeof(oob));
     } else {
-        rsize = lib->callbacks.port_recv(ctx, &ctx_impl->read_port_id,
-                                         buf, sizeof(buf),
-                                         oob, sizeof(oob));
+        rbuf = nxt_unit_read_buf_get_impl(ctx_impl);
+        if (nxt_slow_path(rbuf == NULL)) {
+            return NXT_UNIT_ERROR;
+        }
+
+        nxt_unit_read_buf(ctx, rbuf);
     }
 
-    if (nxt_fast_path(rsize > 0)) {
-        rc = nxt_unit_process_msg(ctx, &ctx_impl->read_port_id, buf, rsize,
-                                  oob, sizeof(oob));
+    if (nxt_fast_path(rbuf->size > 0)) {
+        rc = nxt_unit_process_msg(ctx, &ctx_impl->read_port_id,
+                                  rbuf->buf, rbuf->size,
+                                  rbuf->oob, sizeof(rbuf->oob));
 
 #if (NXT_DEBUG)
-        memset(buf, 0xAC, rsize);
+        memset(rbuf->buf, 0xAC, rbuf->size);
 #endif
 
     } else {
         rc = NXT_UNIT_ERROR;
     }
 
+    nxt_unit_read_buf_release(ctx, rbuf);
+
     return rc;
 }
 
 
+static void
+nxt_unit_read_buf(nxt_unit_ctx_t *ctx, nxt_unit_read_buf_t *rbuf)
+{
+    nxt_unit_impl_t      *lib;
+    nxt_unit_ctx_impl_t  *ctx_impl;
+
+    ctx_impl = nxt_container_of(ctx, nxt_unit_ctx_impl_t, ctx);
+
+    memset(rbuf->oob, 0, sizeof(struct cmsghdr));
+
+    if (ctx_impl->read_port_fd != -1) {
+        rbuf->size = nxt_unit_port_recv(ctx, ctx_impl->read_port_fd,
+                                        rbuf->buf, sizeof(rbuf->buf),
+                                        rbuf->oob, sizeof(rbuf->oob));
+
+    } else {
+        lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
+
+        rbuf->size = lib->callbacks.port_recv(ctx, &ctx_impl->read_port_id,
+                                              rbuf->buf, sizeof(rbuf->buf),
+                                              rbuf->oob, sizeof(rbuf->oob));
+    }
+}
+
+
 void
 nxt_unit_done(nxt_unit_ctx_t *ctx)
 {
diff --git a/src/nxt_unit.h b/src/nxt_unit.h
index 6948b253..c8aaa124 100644
--- a/src/nxt_unit.h
+++ b/src/nxt_unit.h
@@ -137,6 +137,9 @@ struct nxt_unit_callbacks_s {
     /* Gracefully quit the application. Optional. */
     void     (*quit)(nxt_unit_ctx_t *);
 
+    /* Shared memory release acknowledgement. */
+    void     (*shm_ack_handler)(nxt_unit_ctx_t *);
+
     /* Send data and control to process pid using port id. Optional. */
     ssize_t  (*port_send)(nxt_unit_ctx_t *, nxt_unit_port_id_t *port_id,
                  const void *buf, size_t buf_size,
@@ -323,6 +326,9 @@ uint32_t nxt_unit_buf_min(void);
 int nxt_unit_response_write(nxt_unit_request_info_t *req, const void *start,
     size_t size);
 
+ssize_t nxt_unit_response_write_nb(nxt_unit_request_info_t *req,
+    const void *start, size_t size, size_t min_size);
+
 int nxt_unit_response_write_cb(nxt_unit_request_info_t *req,
     nxt_unit_read_info_t *read_info);