Event pipe: reduced number of file buffers used.

If possible we now just extend already present file buffer in p->out chain
instead of keeping ngx_buf_t for each buffer we've flushed to disk.  This
saves about 120 bytes of memory per buffer flushed to disk, and resolves
high CPU usage observed in edge cases (due to coalescing these buffers on
send).

src/event/ngx_event_pipe.c

@@ -680,10 +680,10 @@ ngx_event_pipe_write_to_downstream(ngx_event_pipe_t *p)
 static ngx_int_t
 ngx_event_pipe_write_chain_to_temp_file(ngx_event_pipe_t *p)
 {
-    ssize_t       size, bsize;
+    ssize_t       size, bsize, n;
     ngx_buf_t    *b;
     ngx_uint_t    prev_last_shadow;
-    ngx_chain_t  *cl, *tl, *next, *out, **ll, **last_free, fl;
+    ngx_chain_t  *cl, *tl, *next, *out, **ll, **last_out, **last_free, fl;
 
     if (p->buf_to_file) {
         fl.buf = p->buf_to_file;
@@ -748,10 +748,63 @@ ngx_event_pipe_write_chain_to_temp_file(ngx_event_pipe_t *p)
         p->last_in = &p->in;
     }
 
-    if (ngx_write_chain_to_temp_file(p->temp_file, out) == NGX_ERROR) {
+    n = ngx_write_chain_to_temp_file(p->temp_file, out);
+
+    if (n == NGX_ERROR) {
         return NGX_ABORT;
     }
 
+    if (p->buf_to_file) {
+        p->temp_file->offset = p->buf_to_file->last - p->buf_to_file->pos;
+        n -= p->buf_to_file->last - p->buf_to_file->pos;
+        p->buf_to_file = NULL;
+        out = out->next;
+    }
+
+    if (n > 0) {
+        /* update previous buffer or add new buffer */
+
+        if (p->out) {
+            for (cl = p->out; cl->next; cl = cl->next) { /* void */ }
+
+            b = cl->buf;
+
+            if (b->file_last == p->temp_file->offset) {
+                p->temp_file->offset += n;
+                b->file_last = p->temp_file->offset;
+                goto free;
+            }
+
+            last_out = &cl->next;
+
+        } else {
+            last_out = &p->out;
+        }
+
+        cl = ngx_chain_get_free_buf(p->pool, &p->free);
+        if (cl == NULL) {
+            return NGX_ABORT;
+        }
+
+        b = cl->buf;
+
+        ngx_memzero(b, sizeof(ngx_buf_t));
+
+        b->tag = p->tag;
+
+        b->file = &p->temp_file->file;
+        b->file_pos = p->temp_file->offset;
+        p->temp_file->offset += n;
+        b->file_last = p->temp_file->offset;
+
+        b->in_file = 1;
+        b->temp_file = 1;
+
+        *last_out = cl;
+    }
+
+free:
+
     for (last_free = &p->free_raw_bufs;
          *last_free != NULL;
          last_free = &(*last_free)->next)
@@ -759,31 +812,13 @@ ngx_event_pipe_write_chain_to_temp_file(ngx_event_pipe_t *p)
         /* void */
     }
 
-    if (p->buf_to_file) {
-        p->temp_file->offset = p->buf_to_file->last - p->buf_to_file->pos;
-        p->buf_to_file = NULL;
-        out = out->next;
-    }
-
     for (cl = out; cl; cl = next) {
         next = cl->next;
-        cl->next = NULL;
+
+        cl->next = p->free;
+        p->free = cl;
 
         b = cl->buf;
-        b->file = &p->temp_file->file;
-        b->file_pos = p->temp_file->offset;
-        p->temp_file->offset += b->last - b->pos;
-        b->file_last = p->temp_file->offset;
-
-        b->in_file = 1;
-        b->temp_file = 1;
-
-        if (p->out) {
-            *p->last_out = cl;
-        } else {
-            p->out = cl;
-        }
-        p->last_out = &cl->next;
 
         if (b->last_shadow) {
 

src/event/ngx_event_pipe.h

@@ -30,8 +30,6 @@ struct ngx_event_pipe_s {
     ngx_chain_t      **last_in;
 
     ngx_chain_t       *out;
-    ngx_chain_t      **last_out;
-
     ngx_chain_t       *free;
     ngx_chain_t       *busy;