cord_buf: introduce ownership management

The global ibuf used for hot Lua and Lua C code didn't have
ownership management. As a result, it could be reused in some
unexpected ways during Lua GC via __gc handlers, even if it was
currently in use in some code below the stack.

The patch makes cord_ibuf_take() steal the global buffer from its
global stash, and assign to the current fiber. cord_ibuf_put()
puts it back to the stash, and detaches from the fiber. If yield
happens before cord_ibuf_put(), the buffer is detached
automatically.

Fiber attach/detach is done via on_yield/on_stop triggers. The
buffer is not supposed to survive a yield, so this allows to
free/put the buffer back to the stash even if the owner didn't do
that. For instance, if a Lua exception was raised before
cord_ibuf_put() was called.

This makes cord buffer being safe to use in any yield-free code,
even if Lua GC might be started. And in non-Lua code as well.

Part of #5632

src/lib/core/cord_buf.c

@@ -5,6 +5,7 @@
  */
 #include "cord_buf.h"
 #include "fiber.h"
+#include "trigger.h"
 
 #include "small/ibuf.h"
 
@@ -13,35 +14,154 @@ enum {
 	CORD_IBUF_START_CAPACITY = 16384,
 };
 
-static struct ibuf *cord_buf_global = NULL;
+/** Global buffer with automatic collection on fiber yield. */
+struct cord_buf {
+	/** Base buffer. */
+	struct ibuf base;
+	/**
+	 * Triggers on fiber stop/yield when the buffer is either destroyed or
+	 * cached to the global stash for later reuse.
+	 */
+	struct trigger on_stop;
+	struct trigger on_yield;
+#ifndef NDEBUG
+	/**
+	 * Fiber owning the buffer right now. Used for debug and sanity checks
+	 * only.
+	 */
+	struct fiber *owner;
+#endif
+};
 
-struct ibuf *
-cord_ibuf_take(void)
+/**
+ * The global buffer last saved to the cache. Having it here is supposed to
+ * help to reuse the buffer's already allocated data sometimes.
+ */
+static struct cord_buf *cord_buf_global = NULL;
+
+static inline void
+cord_buf_put(struct cord_buf *buf);
+
+static void
+cord_buf_delete(struct cord_buf *buf);
+
+static inline void
+cord_buf_set_owner(struct cord_buf *buf)
 {
-	assert(cord_is_main());
-	struct ibuf *buf = cord_buf_global;
-	if (buf != NULL) {
-		ibuf_reset(buf);
-		return buf;
-	}
-	buf = malloc(sizeof(*buf));
+	assert(buf->owner == NULL);
+	struct fiber *f = fiber();
+	trigger_add(&f->on_stop, &buf->on_stop);
+	trigger_add(&f->on_yield, &buf->on_yield);
+#ifndef NDEBUG
+	buf->owner = f;
+#endif
+	ibuf_reset(&buf->base);
+}
+
+static inline void
+cord_buf_clear_owner(struct cord_buf *buf)
+{
+	assert(buf->owner == fiber());
+	trigger_clear(&buf->on_stop);
+	trigger_clear(&buf->on_yield);
+#ifndef NDEBUG
+	buf->owner = NULL;
+#endif
+}
+
+static int
+cord_buf_on_stop(struct trigger *trigger, void *event)
+{
+	(void)event;
+	struct cord_buf *buf = trigger->data;
+	assert(trigger == &buf->on_stop);
+	cord_buf_put(buf);
+	return 0;
+}
+
+static int
+cord_buf_on_yield(struct trigger *trigger, void *event)
+{
+	(void)event;
+	struct cord_buf *buf = trigger->data;
+	assert(trigger == &buf->on_yield);
+	cord_buf_put(buf);
+	return 0;
+}
+
+static struct cord_buf *
+cord_buf_new(void)
+{
+	struct cord_buf *buf = malloc(sizeof(*buf));
 	if (buf == NULL)
 		panic("Couldn't allocate thread buffer");
-	ibuf_create(buf, &cord()->slabc, CORD_IBUF_START_CAPACITY);
-	cord_buf_global = buf;
+	ibuf_create(&buf->base, &cord()->slabc, CORD_IBUF_START_CAPACITY);
+	trigger_create(&buf->on_stop, cord_buf_on_stop, buf, NULL);
+	trigger_create(&buf->on_yield, cord_buf_on_yield, buf, NULL);
+#ifndef NDEBUG
+	buf->owner = NULL;
+#endif
+	return buf;
+}
+
+static inline void
+cord_buf_put(struct cord_buf *buf)
+{
+	assert(cord_is_main());
+	cord_buf_clear_owner(buf);
+	/*
+	 * Delete if the stash is busy. It could happen if there was >= 2
+	 * buffers at some point and one of them is already saved back to the
+	 * stash.
+	 *
+	 * XXX: in future it might be useful to consider saving the buffers into
+	 * a list. Maybe keep always at most 2 buffers, because usually there
+	 * are at most 2 contexts: normal Lua and Lua during GC. Recursive
+	 * GC is supposed to be rare, no need to optimize it.
+	 */
+	if (cord_buf_global == NULL)
+		cord_buf_global = buf;
+	else
+		cord_buf_delete(buf);
+}
+
+static inline struct cord_buf *
+cord_buf_take(void)
+{
+	assert(cord_is_main());
+	struct cord_buf *buf = cord_buf_global;
+	if (buf != NULL)
+		cord_buf_global = NULL;
+	else
+		buf = cord_buf_new();
+	cord_buf_set_owner(buf);
 	return buf;
 }
 
+static void
+cord_buf_delete(struct cord_buf *buf)
+{
+	assert(buf->owner == NULL);
+	ibuf_destroy(&buf->base);
+	TRASH(buf);
+	free(buf);
+}
+
+struct ibuf *
+cord_ibuf_take(void)
+{
+	return &cord_buf_take()->base;
+}
+
 void
 cord_ibuf_put(struct ibuf *ibuf)
 {
-	(void)ibuf;
-	assert(ibuf == cord_buf_global);
+	cord_buf_put((struct cord_buf *)ibuf);
 }
 
 void
 cord_ibuf_drop(struct ibuf *ibuf)
 {
 	ibuf_reinit(ibuf);
-	assert(ibuf == cord_buf_global);
+	cord_ibuf_put(ibuf);
 }

src/lib/core/cord_buf.h

@@ -18,7 +18,9 @@ struct ibuf *
 cord_ibuf_take(void);
 
 /**
- * Put the global ibuf back.
+ * Put the global ibuf back. It is not necessary - the buffer is put back on the
+ * next yield. But then it can't be reused/freed until the yield. Put it back
+ * manually when possible.
  */
 void
 cord_ibuf_put(struct ibuf *ibuf);
@@ -29,6 +31,8 @@ cord_ibuf_put(struct ibuf *ibuf);
  * because it is often needed from Lua, and allows not to call :recycle() there,
  * which would be an additional FFI call before cord_ibuf_put().
  *
+ * Drop is not necessary though, see the put() comment.
+ *
  * XXX: recycle of the global buffer is a workaround for the ibuf being used in
  * some places working with Lua API, where it wasn't wanted to "reuse" it
  * anyhow. Instead, the global buffer is used to protect from the buffer leak in

test/app-tap/buffer.test.lua

+#!/usr/bin/env tarantool
+
+local tap = require('tap')
+local fiber = require('fiber')
+local buffer = require('buffer')
+local cord_ibuf_take = buffer.internal.cord_ibuf_take
+local cord_ibuf_put = buffer.internal.cord_ibuf_put
+local cord_ibuf_drop = buffer.internal.cord_ibuf_drop
+
+local function test_cord_ibuf(test)
+    test:plan(10)
+
+    local ibuf1 = cord_ibuf_take()
+    test:is(ibuf1:size(), 0, 'is empty')
+    ibuf1:alloc(1)
+    test:is(ibuf1:size(), 1, 'alloc 1')
+    cord_ibuf_put(ibuf1)
+
+    ibuf1 = cord_ibuf_take()
+    test:is(ibuf1:size(), 0, 'is empty again')
+    ibuf1:alloc(1)
+    cord_ibuf_drop(ibuf1)
+
+    ibuf1 = cord_ibuf_take()
+    test:is(ibuf1:capacity(), 0, 'has no capacity')
+    local pos1 = ibuf1:alloc(1)
+    pos1[0] = 1
+
+    local ibuf2 = cord_ibuf_take()
+    test:isnt(ibuf1, ibuf2, 'can have 2 cord buffers')
+    test:is(ibuf2:size(), 0, 'second is empty')
+    local pos2 = ibuf2:alloc(1)
+    pos2[0] = 2
+    test:is(pos1[0], 1, 'change does not affect the first buffer')
+    cord_ibuf_put(ibuf2)
+    ibuf1 = ibuf2
+
+    fiber.yield()
+    ibuf2 = cord_ibuf_take()
+    test:is(ibuf1, ibuf2, 'yield drops the ownership')
+    cord_ibuf_put(ibuf2)
+
+    ibuf1 = nil
+    local f = fiber.new(function()
+        ibuf1 = cord_ibuf_take()
+    end)
+    f:set_joinable(true)
+    f:join()
+    test:isnt(ibuf1, nil, 'took a cord buf in a new fiber')
+    ibuf2 = cord_ibuf_take()
+    test:is(ibuf1, ibuf2, 'was freed on fiber stop and reused')
+    cord_ibuf_put(ibuf2)
+end
+
+local test = tap.test('buffer')
+test:plan(1)
+test:test("cord buffer", test_cord_ibuf)
+
+os.exit(test:check() and 0 or 1)