path: root/buffer_tree.c

/* SPDX-License-Identifier: GPL-2.0 */

/** \file buffer_tree.c Buffer tree and buffer pool implementations. */

#include "para.h"
#include "list.h"
#include "string.h"
#include "buffer_tree.h"
#include "error.h"
#include "sched.h"

/* whead = NULL means area full */
struct btr_pool {
	char *name;
	char *area_start;
	char *area_end;
	char *rhead;
	char *whead;
};

struct btr_buffer {
	char *buf;
	size_t size;
	/** The number of references to this buffer. */
	int refcount;
	/* NULL means no buffer pool but a heap buffer. */
	struct btr_pool *pool;
	/* Only relevant if pool is NULL. */
	bool dont_free;
};

struct btr_buffer_reference {
	struct btr_buffer *btrb;
	size_t consumed;
	/* Each buffer reference belongs to the buffer queue list of some buffer tree node. */
	struct list_head node;
	size_t wrap_count;
};

struct btr_node {
	char *name;
	struct btr_node *parent;
	/* The position of this btr node in the buffer tree. */
	struct list_head node;
	/* The children nodes of this btr node are linked together in a list. */
	struct list_head children;
	/* Time of first data transfer. */
	struct timeval start;
	/**
	 * The input queue is a list of references to btr buffers. Each item on
	 * the list represents an input buffer which has not been completely
	 * used by this btr node.
	 */
	struct list_head input_queue;
	btr_command_handler execute;
	void *context;
};

/**
 * Create a new buffer pool.
 *
 * \param name The name of the new buffer pool.
 * \param area_size The size in bytes of the pool area.
 *
 * \return An opaque pointer to the newly created buffer pool. It must be
 * passed to \ref btr_pool_free() after it is no longer used to deallocate
 * all resources.
 */
struct btr_pool *btr_pool_new(const char *name, size_t area_size)
{
	struct btr_pool *btrp;

	PARA_INFO_LOG("%s, %zu bytes\n", name, area_size);
	btrp = alloc(sizeof(*btrp));
	btrp->area_start = alloc(area_size);
	btrp->area_end = btrp->area_start + area_size;
	btrp->rhead = btrp->area_start;
	btrp->whead = btrp->area_start;
	btrp->name = para_strdup(name);
	return btrp;
}

/**
 * Deallocate resources used by a buffer pool.
 *
 * \param btrp A pointer obtained via \ref btr_pool_new().
 */
void btr_pool_free(struct btr_pool *btrp)
{
	if (!btrp)
		return;
	free(btrp->area_start);
	free(btrp->name);
	free(btrp);
}

/**
 * Return the size of the buffer pool area.
 *
 * \param btrp The buffer pool.
 *
 * \return The same value which was passed during creation time to
 * \ref btr_pool_new().
 */
size_t btr_pool_size(const struct btr_pool *btrp)
{
	return btrp->area_end - btrp->area_start;
}

static size_t btr_pool_filled(const struct btr_pool *btrp)
{
	if (!btrp->whead)
		return btr_pool_size(btrp);
	if (btrp->rhead <= btrp->whead)
		return  btrp->whead - btrp->rhead;
	return btr_pool_size(btrp) - (btrp->rhead - btrp->whead);
}

/**
 * Get the number of unused bytes in the buffer pool.
 *
 * \param btrp The pool.
 *
 * \return The number of bytes that can currently be allocated.
 *
 * Note that in general the returned number of bytes is not available as a
 * single contiguous buffer.
 */
size_t btr_pool_unused(const struct btr_pool *btrp)
{
	return btr_pool_size(btrp) - btr_pool_filled(btrp);
}

/*
 * Return maximal size available for one read. This is smaller than the
 * value returned by btr_pool_unused().
 */
static size_t btr_pool_available(const struct btr_pool *btrp)
{
	if (!btrp->whead)
		return 0;
	if (btrp->rhead <= btrp->whead)
		return btrp->area_end - btrp->whead;
	return btrp->rhead - btrp->whead;
}

/**
 * Obtain the current write head.
 *
 * \param btrp The buffer pool.
 * \param result The write head is returned here.
 *
 * \return The maximal amount of bytes that may be written to the returned
 * buffer.
 */
size_t btr_pool_get_buffer(const struct btr_pool *btrp, char **result)
{
	if (result)
		*result = btrp->whead;
	return btr_pool_available(btrp);
}

/**
 * Get references to buffers pointing to free space of the buffer pool area.
 *
 * \param btrp The buffer pool.
 * \param iov The I/O vector.
 *
 * \return Zero if the buffer pool is full, one if the free space of the
 * buffer pool area is available as a single contiguous buffer, two if the
 * free space consists of two buffers. If this function returns the value n,
 * then n elements of the I/O vector are initialized.
 */
int btr_pool_get_buffers(const struct btr_pool *btrp, struct iovec iov[2])
{
	size_t sz, unused;
	char *buf;

	sz = btr_pool_get_buffer(btrp, &buf);
	if (sz == 0)
		return 0;
	iov[0].iov_len = sz;
	iov[0].iov_base = buf;
	unused = btr_pool_unused(btrp);
	if (sz == unused)
		return 1;
	iov[1].iov_len = unused - sz;
	iov[1].iov_base = btrp->area_start;
	return 2;
}

/**
 * Mark a part of the buffer pool area as allocated.
 *
 * \param btrp The buffer pool.
 * \param size The amount of bytes to be allocated.
 *
 * This is usually called after the caller wrote to the buffer obtained by
 * \ref btr_pool_get_buffer().
 */
static void btr_pool_allocate(struct btr_pool *btrp, size_t size)
{
	char *end;

	if (size == 0)
		return;
	assert(size <= btr_pool_available(btrp));
	end = btrp->whead + size;
	assert(end <= btrp->area_end);

	if (end == btrp->area_end) {
		PARA_DEBUG_LOG("%s: end of pool area reached\n", btrp->name);
		end = btrp->area_start;
	}
	if (end == btrp->rhead) {
		PARA_DEBUG_LOG("%s btrp buffer full\n", btrp->name);
		end = NULL; /* buffer full */
	}
	btrp->whead = end;
}

static void btr_pool_deallocate(struct btr_pool *btrp, size_t size)
{
	char *end = btrp->rhead + size;

	if (size == 0)
		return;
	assert(end <= btrp->area_end);
	assert(size <= btr_pool_filled(btrp));
	if (end == btrp->area_end)
		end = btrp->area_start;
	if (!btrp->whead)
		btrp->whead = btrp->rhead;
	btrp->rhead = end;
	if (btrp->rhead == btrp->whead)
		btrp->rhead = btrp->whead = btrp->area_start;
}

#define FOR_EACH_CHILD(_tn, _btrn) list_for_each_entry((_tn), \
	&((_btrn)->children), node)
#define FOR_EACH_CHILD_SAFE(_tn, _tmp, _btrn) \
	list_for_each_entry_safe((_tn), (_tmp), &((_btrn)->children), node)

#define FOR_EACH_BUFFER_REF(_br, _btrn) \
	list_for_each_entry((_br), &(_btrn)->input_queue, node)
#define FOR_EACH_BUFFER_REF_SAFE(_br, _tmp, _btrn) \
	list_for_each_entry_safe((_br), (_tmp), &(_btrn)->input_queue, node)

/**
 * Create a new buffer tree node.
 *
 * \param bnd Specifies how to create the new node.
 *
 * \return A pointer to the newly allocated node.
 *
 * This function always succeeds (or calls exit()). The returned pointer
 * must be freed using \ref btr_remove_node().
 */
struct btr_node *btr_new_node(struct btr_node_description *bnd)
{
	struct btr_node *btrn = alloc(sizeof(*btrn));

	btrn->name = para_strdup(bnd->name);
	btrn->parent = bnd->parent;
	btrn->execute = bnd->handler;
	btrn->context = bnd->context;
	btrn->start.tv_sec = 0;
	btrn->start.tv_usec = 0;
	init_list_head(&btrn->children);
	init_list_head(&btrn->input_queue);
	if (!bnd->child) {
		if (bnd->parent) {
			list_add_tail(&btrn->node, &bnd->parent->children);
			PARA_INFO_LOG("new leaf node: %s (child of %s)\n",
				bnd->name, bnd->parent->name);
		} else
			PARA_INFO_LOG("added %s as btr root\n", bnd->name);
		goto out;
	}
	if (!bnd->parent) {
		assert(!bnd->child->parent);
		PARA_INFO_LOG("new root: %s (was %s)\n",
			bnd->name, bnd->child->name);
		btrn->parent = NULL;
		list_add_tail(&bnd->child->node, &btrn->children);
		/* link it in */
		bnd->child->parent = btrn;
		goto out;
	}
	list_add_tail(&btrn->node, &bnd->parent->children);
	list_move(&bnd->child->node, &btrn->children);
	bnd->child->parent = btrn;
	PARA_INFO_LOG("added %s as internal node\n", bnd->name);
out:
	return btrn;
}

/*
 * Allocate a new btr buffer.
 *
 * The freshly allocated buffer will have a zero reference count and will
 * not be associated with a buffer tree pool.
 */
static struct btr_buffer *new_btrb(char *buf, size_t size)
{
	struct btr_buffer *btrb = zalloc(sizeof(*btrb));

	btrb->buf = buf;
	btrb->size = size;
	return btrb;
}

static void dealloc_buffer(struct btr_buffer *btrb)
{
	if (btrb->pool)
		btr_pool_deallocate(btrb->pool, btrb->size);
	else if (!btrb->dont_free)
		free(btrb->buf);
}

static struct btr_buffer_reference *get_first_input_br(const struct btr_node *btrn)
{
	if (list_empty(&btrn->input_queue))
		return NULL;
	return list_first_entry(&btrn->input_queue,
		struct btr_buffer_reference, node);
}

static struct btr_buffer_reference *get_last_input_br(const struct btr_node *btrn)
{
	if (list_empty(&btrn->input_queue))
		return NULL;
	return list_last_entry(&btrn->input_queue,
		struct btr_buffer_reference, node);
}

/*
 * Deallocate the buffer reference and decrement the reference count of the
 * underlying buffer. If the refcount becomes zero, deallocate the buffer.
 */
static void btr_drop_buffer_reference(struct btr_buffer_reference *br)
{
	struct btr_buffer *btrb = br->btrb;

	list_del(&br->node);
	free(br);
	btrb->refcount--;
	if (btrb->refcount == 0) {
		if (!btrb->dont_free)
			dealloc_buffer(btrb);
		free(btrb);
	}
}

static bool try_merge_btrb(const struct btr_buffer *btrb,
		const struct btr_node *btrn, size_t consumed)
{
	struct btr_node *ch;

	if (consumed > 0)
		return false;
	FOR_EACH_CHILD(ch, btrn) {
		struct btr_buffer_reference *br = get_last_input_br(ch);
		if (!br)
			return false;
		if (br->consumed > 0)
			return false;
		if (br->btrb->buf + br->btrb->size != btrb->buf)
			return false;
		if (!br->btrb->pool)
			continue;
		if (br->btrb->size + btrb->size >= btr_pool_size(br->btrb->pool) / 3)
			return false;
	}
	/* merge it with last input buffer references */
	FOR_EACH_CHILD(ch, btrn) {
		struct btr_buffer_reference *br = get_last_input_br(ch);
		br->btrb->size += btrb->size;
	}
	return true;
}

/* Returns whether buffers were merged. */
static bool add_btrb_to_children(struct btr_buffer *btrb,
		struct btr_node *btrn, size_t consumed)
{
	struct btr_node *ch;

	if (btrn->start.tv_sec == 0)
		btrn->start = *now;
	if (try_merge_btrb(btrb, btrn, consumed))
		return true;
	FOR_EACH_CHILD(ch, btrn) {
		struct btr_buffer_reference *br = zalloc(sizeof(*br));
		br->btrb = btrb;
		br->consumed = consumed;
		list_add_tail(&br->node, &ch->input_queue);
		btrb->refcount++;
		if (ch->start.tv_sec == 0)
			ch->start = *now;
	}
	return false;
}

/**
 * Insert a heap buffer into the buffer tree.
 *
 * \param buf The buffer to insert.
 * \param size The size of buf in bytes.
 * \param btrn Position in the buffer tree to create the output.
 *
 * This creates buffer references and adds these references to each child of
 * the given buffer tree node. The buffer will be freed using standard free()
 * once no buffer tree node is referencing it any more.
 *
 * Note that this function must not be used if the buffer was obtained from
 * a buffer pool. Use \ref btr_add_output_pool() in this case.
 */
void btr_add_output(char *buf, size_t size, struct btr_node *btrn)
{
	struct btr_buffer *btrb;

	if (size == 0)
		return;
	if (list_empty(&btrn->children)) {
		free(buf);
		return;
	}
	btrb = new_btrb(buf, size);
	add_btrb_to_children(btrb, btrn, 0);
}

/**
 * Insert a buffer into the buffer tree, non-freeing variant.
 *
 * \param buf See \ref btr_add_output().
 * \param size See \ref btr_add_output().
 * \param btrn See \ref btr_add_output().
 *
 * This is similar to \ref btr_add_output() but additionally sets the dont_free
 * flag on the buffer. If the reference count of the buffer drops to zero,
 * the buffer will *not* be deallocated, since this flag is set.
 *
 * The dont_free bit also prevents the children of buffer tree node from
 * modifying the buffer contents in-place. Specifically, \ref btr_inplace_ok()
 * returns false if there is any buffer in the input queue with the dont_free
 * bit set.
 */
void btr_add_output_dont_free(const char *buf, size_t size, struct btr_node *btrn)
{
	struct btr_buffer *btrb;

	if (size == 0)
		return;
	if (list_empty(&btrn->children))
		return;
	btrb = new_btrb((char *)buf, size);
	btrb->dont_free = true;
	if (add_btrb_to_children(btrb, btrn, 0))
		free(btrb);
}

/**
 * Feed data to child nodes of a buffer tree node.
 *
 * \param btrp The buffer pool.
 * \param size The number of bytes to be allocated and fed to each child.
 * \param btrn The node whose children are to be fed.
 *
 * This function allocates the amount of bytes from the buffer pool area,
 * starting at the current value of the write head, and creates buffer
 * references to the resulting part of the buffer pool area, one for each
 * child of the given buffer tree node. The references are then fed into
 * the input queue of each child.
 */
void btr_add_output_pool(struct btr_pool *btrp, size_t size,
		struct btr_node *btrn)
{
	struct btr_buffer *btrb;
	char *buf;
	size_t avail;

	if (size == 0)
		return;
	if (list_empty(&btrn->children))
		return;
	avail = btr_pool_get_buffer(btrp, &buf);
	assert(avail >= size);
	btr_pool_allocate(btrp, size);
	btrb = new_btrb(buf, size);
	btrb->pool = btrp;
	if (add_btrb_to_children(btrb, btrn, 0))
		free(btrb);
}

/**
 * Copy data to write head of a buffer pool and feed it to all children nodes.
 *
 * \param src The source buffer.
 * \param n The size of the source buffer in bytes.
 * \param btrp The destination buffer pool.
 * \param btrn Add the data as output of this node.
 *
 * This is expensive. The caller must make sure the data fits into the buffer
 * pool area.
 */
void btr_copy(const void *src, size_t n, struct btr_pool *btrp,
	struct btr_node *btrn)
{
	char *buf;
	size_t sz, copy;

	if (n == 0)
		return;
	assert(n <= btr_pool_unused(btrp));
	sz = btr_pool_get_buffer(btrp, &buf);
	copy = PARA_MIN(sz, n);
	memcpy(buf, src, copy);
	btr_add_output_pool(btrp, copy, btrn);
	if (copy == n)
		return;
	sz = btr_pool_get_buffer(btrp, &buf);
	assert(sz >= n - copy);
	assert(buf);
	memcpy(buf, src + copy, n - copy);
	btr_add_output_pool(btrp, n - copy, btrn);
}

static void btr_pushdown_br(struct btr_buffer_reference *br, struct btr_node *btrn)
{
	add_btrb_to_children(br->btrb, btrn, br->consumed);
	btr_drop_buffer_reference(br);
}

/**
 * Feed all buffer references of the input queue through the output channel.
 *
 * \param btrn The node whose buffer references should be pushed down.
 *
 * This function is useful for filters that do not change the contents of the
 * buffers at all, like the wav filter or the amp filter if no amplification
 * was specified. This function is rather cheap.
 *
 * \sa \ref btr_pushdown_one().
 */
void btr_pushdown(struct btr_node *btrn)
{
	struct btr_buffer_reference *br, *tmp;

	FOR_EACH_BUFFER_REF_SAFE(br, tmp, btrn)
		btr_pushdown_br(br, btrn);
}

/**
 * Feed the next buffer of the input queue through the output channel.
 *
 * \param btrn The node whose first input queue buffer should be pushed down.
 *
 * This works like \ref btr_pushdown() but pushes down only one buffer
 * reference.
 */
void btr_pushdown_one(struct btr_node *btrn)
{
	struct btr_buffer_reference *br;

	if (list_empty(&btrn->input_queue))
		return;
	br = list_first_entry(&btrn->input_queue, struct btr_buffer_reference, node);
	btr_pushdown_br(br, btrn);
}

/*
 * Find out whether a node is a leaf node.
 *
 * \param btrn The node to check.
 *
 * \return True if this node has no children. False otherwise.
 */
static bool btr_no_children(const struct btr_node *btrn)
{
	return list_empty(&btrn->children);
}

/**
 * Find out whether a node is an orphan.
 *
 * \param btrn The buffer tree node.
 *
 * \return True if the buffer tree node has no parent.
 *
 * This function returns true for the root node and false for any other node.
 *
 * After a (non-leaf) node was removed removed from the tree, the function
 * returns true for all child nodes.
 */
bool btr_no_parent(const struct btr_node *btrn)
{
	return !btrn->parent;
}

/**
 * Find out whether it is OK to change an input buffer.
 *
 * \param btrn The buffer tree node to check.
 *
 * This is used by filters that produce exactly the same amount of output
 * as there is input. The amp filter which multiplies each sample by some
 * number is an example of such a filter. If there are no other nodes in the
 * buffer tree that read the same input stream (i.e. if the buffer tree node
 * has no siblings), the node may modify its input buffer directly and push
 * down the modified buffer to its children, thereby avoiding to allocate
 * a possibly large additional buffer.
 *
 * Since the buffer tree may change at any time, this function should be called
 * during each post_monitor call.
 *
 * \return True if the buffer tree node has no siblings.
 */
bool btr_inplace_ok(const struct btr_node *btrn)
{
	struct btr_buffer_reference *br;
	FOR_EACH_BUFFER_REF(br, btrn) {
		struct btr_buffer *btrb = br->btrb;
		if (btrb->refcount > 1)
			return false;
		if (btrb->dont_free == true)
			return false;
	}
	return true;
}

static inline size_t br_available_bytes(const struct btr_buffer_reference *br)
{
	return br->btrb->size - br->consumed;
}

static size_t btr_get_buffer_by_reference(const struct btr_buffer_reference *br,
		char **buf)
{
	if (buf)
		*buf = br->btrb->buf + br->consumed;
	return br_available_bytes(br);
}

/**
 * Obtain the next buffer of the input queue, omitting data.
 *
 * \param btrn The node whose input queue is to be queried.
 * \param omit Number of bytes to be omitted.
 * \param bufp Result pointer. It is OK to pass NULL here.
 *
 * If a buffer tree node needs more input data but can not consume the data
 * it already has (because it might be needed again later) this function can
 * be used instead of \ref btr_next_buffer() to get a reference to the buffer
 * obtained by skipping the given number of bytes. Skipped input bytes are
 * not consumed.
 *
 * If omit is zero, this function acts identical to \ref btr_next_buffer().
 *
 * \return Number of bytes in the returned buffer. If there are less than or
 * equal to omit many bytes available in the input queue of the buffer tree
 * node, the function returns zero and the value of of the result buffer
 * is undefined.
 */
size_t btr_next_buffer_omit(const struct btr_node *btrn, size_t omit,
		char **bufp)
{
	struct btr_buffer_reference *br;
	size_t wrap_count, sz, rv = 0;
	char *buf, *result = NULL;

	br = get_first_input_br(btrn);
	if (!br)
		return 0;
	wrap_count = br->wrap_count;
	if (wrap_count > 0) { /* we have a wrap buffer */
		sz = btr_get_buffer_by_reference(br, &buf);
		if (sz > omit) { /* and it's big enough */
			result = buf + omit;
			rv = sz - omit;
			/*
			 * Wrap buffers are allocated with malloc(), so the next
			 * buffer ref will not align nicely, so we return the
			 * tail of the wrap buffer.
			 */
			goto out;
		}
		/*
		 * The next wrap_count bytes exist twice, in the wrap buffer
		 * and as a buffer reference in the buffer tree pool.
		 */
		omit += wrap_count;
	}
	/*
	 * For buffer tree pools, the buffers in the list align, i.e. the next
	 * buffer in the list starts directly at the end of its predecessor. In
	 * this case we merge adjacent buffers and return one larger buffer
	 * instead.
	 */
	FOR_EACH_BUFFER_REF(br, btrn) {
		sz = btr_get_buffer_by_reference(br, &buf);
		if (result) {
			if (result + rv != buf)
				goto out;
			rv += sz;
		} else if (sz > omit) {
			result = buf + omit;
			rv = sz - omit;
		} else
			omit -= sz;
	}
	if (!result)
		return 0;
out:
	if (bufp)
		*bufp = result;
	return rv;
}

/**
 * Obtain the next buffer of the input queue of a buffer tree node.
 *
 * \param btrn The node whose input queue is to be queried.
 * \param bufp Result pointer.
 *
 * \return The number of bytes that can be read from buf.
 *
 * The call of this function is is equivalent to calling \ref
 * btr_next_buffer_omit() with an omit value of zero.
 */
size_t btr_next_buffer(const struct btr_node *btrn, char **bufp)
{
	return btr_next_buffer_omit(btrn, 0, bufp);
}

/**
 * Deallocate the given number of bytes from the input queue.
 *
 * \param btrn The buffer tree node.
 * \param numbytes The number of bytes to be deallocated.
 *
 * This function must be used to get rid of existing buffer references in the
 * node's input queue. If no references to a buffer remain, the underlying
 * buffers are either freed (in the non-buffer pool case) or the read head of
 * the buffer pool is being advanced.
 *
 * If the number of bytes to consume is smaller than the buffer size, the
 * buffer is not deallocated and subsequent calls to \ref btr_next_buffer()
 * return the remaining part of the buffer.
 */
void btr_consume(struct btr_node *btrn, size_t numbytes)
{
	struct btr_buffer_reference *br, *tmp;
	size_t sz;

	if (numbytes == 0)
		return;
	br = get_first_input_br(btrn);
	assert(br);

	if (br->wrap_count == 0) {
		/*
		 * No wrap buffer. Drop buffer references whose buffer
		 * has been fully used. */
		FOR_EACH_BUFFER_REF_SAFE(br, tmp, btrn) {
			if (br->consumed + numbytes <= br->btrb->size) {
				br->consumed += numbytes;
				if (br->consumed == br->btrb->size)
					btr_drop_buffer_reference(br);
				return;
			}
			numbytes -= br->btrb->size - br->consumed;
			btr_drop_buffer_reference(br);
		}
		assert(false);
	}
	/*
	 * We have a wrap buffer, consume from it. If in total, i.e. including
	 * previous calls to brt_consume(), less than wrap_count has been
	 * consumed, there's nothing more we can do.
	 *
	 * Otherwise we drop the wrap buffer and consume from subsequent
	 * buffers of the input queue the correct amount of bytes. This is the
	 * total number of bytes that have been consumed from the wrap buffer.
	 */
	PARA_DEBUG_LOG("consuming %zu/%zu bytes from wrap buffer\n", numbytes,
		br_available_bytes(br));

	assert(numbytes <= br_available_bytes(br));
	if (br->consumed + numbytes < br->wrap_count) {
		br->consumed += numbytes;
		return;
	}
	PARA_DEBUG_LOG("dropping wrap buffer (%zu bytes)\n", br->btrb->size);
	/* get rid of the wrap buffer */
	sz = br->consumed + numbytes;
	btr_drop_buffer_reference(br);
	return btr_consume(btrn, sz);
}

/**
 * Clear the input queue of a buffer tree node.
 *
 * \param btrn The node whose input queue should be cleared.
 */
void btr_drain(struct btr_node *btrn)
{
	struct btr_buffer_reference *br, *tmp;

	FOR_EACH_BUFFER_REF_SAFE(br, tmp, btrn)
		btr_drop_buffer_reference(br);
}

static void btr_free_node(struct btr_node *btrn)
{
	free(btrn->name);
	free(btrn);
}

/**
 * Remove a node from a buffer tree.
 *
 * \param btrnp Determines the node to remove.
 *
 * This orphans all children of the given buffer tree node and removes this
 * node from the child list of its parent. Moreover, the input queue is
 * flushed and the given node pointer given is set to NULL.
 *
 * \sa \ref btr_splice_out_node().
 */
void btr_remove_node(struct btr_node **btrnp)
{
	struct btr_node *ch;
	struct btr_node *btrn;

	if (!btrnp)
		return;
	btrn = *btrnp;
	if (!btrn)
		goto out;
	PARA_INFO_LOG("removing btr node %s from buffer tree\n", btrn->name);
	FOR_EACH_CHILD(ch, btrn)
		ch->parent = NULL;
	btr_drain(btrn);
	if (btrn->parent)
		list_del(&btrn->node);
	btr_free_node(btrn);
out:
	*btrnp = NULL;
}

/**
 * Return the amount of available input bytes of a buffer tree node.
 *
 * \param btrn The node whose input size should be computed.
 *
 * \return The total number of bytes available in the node's input
 * queue.
 *
 * This simply iterates over all buffer references in the input queue and
 * returns the sum of the sizes of all references.
 */
size_t btr_get_input_queue_size(const struct btr_node *btrn)
{
	struct btr_buffer_reference *br;
	size_t size = 0, wrap_consumed = 0;

	FOR_EACH_BUFFER_REF(br, btrn) {
		if (br->wrap_count != 0) {
			wrap_consumed = br->consumed;
			continue;
		}
		size += br_available_bytes(br);
	}
	assert(wrap_consumed <= size);
	size -= wrap_consumed;
	return size;
}

static bool min_iqs_available(size_t min_iqs, const struct btr_node *btrn)
{
	struct btr_buffer_reference *br;
	size_t have = 0, wrap_consumed = 0;

	FOR_EACH_BUFFER_REF(br, btrn) {
		if (br->wrap_count != 0) {
			wrap_consumed = br->consumed;
			continue;
		}
		have += br_available_bytes(br);
		if (have > wrap_consumed + min_iqs)
			return true;
	}
	return false;
}
/**
 * Remove a node from the buffer tree, reconnecting parent and children.
 *
 * \param btrnp The node to splice out.
 *
 * This function is used by buffer tree nodes that do not exist during the
 * whole lifetime of the buffer tree. Unlike \ref btr_remove_node(), calling
 * \ref btr_splice_out_node() does not split the tree into disconnected
 * components but reconnects the buffer tree by making all child nodes of
 * of the given node children of its parent.
 */
void btr_splice_out_node(struct btr_node **btrnp)
{
	struct btr_node *btrn = *btrnp, *ch, *tmp;

	assert(btrn);
	PARA_NOTICE_LOG("splicing out %s\n", btrn->name);
	btr_pushdown(btrn);
	if (btrn->parent)
		list_del(&btrn->node);
	FOR_EACH_CHILD_SAFE(ch, tmp, btrn) {
		PARA_INFO_LOG("parent(%s): %s\n", ch->name,
			btrn->parent? btrn->parent->name : "NULL");
		ch->parent = btrn->parent;
		if (btrn->parent)
			list_move(&ch->node, &btrn->parent->children);
		else
			list_del(&ch->node);
	}
	assert(list_empty(&btrn->children));
	btr_free_node(btrn);
	*btrnp = NULL;
}

/**
 * Return number of queued output bytes of a buffer tree node.
 *
 * \param btrn The node whose output queue size should be computed.
 *
 * \return This function iterates over all children of the given node and
 * returns the size of the largest input queue.
 */
size_t btr_get_output_queue_size(const struct btr_node *btrn)
{
	size_t max_size = 0;
	struct btr_node *ch;

	FOR_EACH_CHILD(ch, btrn) {
		size_t size = btr_get_input_queue_size(ch);
		max_size = PARA_MAX(max_size, size);
	}
	return max_size;
}

/**
 * Execute an inter-node command on the given node or on a parent node.
 *
 * \param btrn The node to start looking.
 * \param command The command to execute.
 * \param value_result Additional arguments and result value.
 *
 * This function traverses the buffer tree from the given buffer tree node
 * upwards and looks for the first node that understands the given command. On
 * this node the command is executed, and the result is returned via the
 * value/result pointer.
 *
 * \return -ENOTSUP if no parent node understands the command. Otherwise the
 * return value of the command handler is returned.
 *
 * \sa \ref receiver::execute(), \ref filter::execute()
 */
int btr_exec_up(const struct btr_node *btrn, const char *command, char **value_result)
{
	int ret;

	for (; btrn; btrn = btrn->parent) {
		if (!btrn->execute)
			continue;
		PARA_INFO_LOG("executing %s on %s\n", command, btrn->name);
		ret = btrn->execute(btrn, command, value_result);
		if (ret == -ERRNO_TO_PARA_ERROR(ENOTSUP))
			continue;
		if (ret < 0)
			return ret;
		if (value_result && *value_result)
			PARA_INFO_LOG("%s(%s): %s\n", command, btrn->name,
				*value_result);
		return 1;
	}
	return -ERRNO_TO_PARA_ERROR(ENOTSUP);
}

/**
 * Obtain the context of a buffer node tree.
 *
 * \param btrn The node whose output queue size should be computed.
 *
 * \return A pointer to the context as specified at node creation time.
 *
 * \sa \ref btr_new_node(), struct \ref btr_node_description.
 */
void *btr_context(const struct btr_node *btrn)
{
	return btrn->context;
}

static bool need_buffer_pool_merge(const struct btr_node *btrn)
{
	struct btr_buffer_reference *br = get_first_input_br(btrn);

	if (!br)
		return false;
	if (br->wrap_count != 0)
		return true;
	if (br->btrb->pool)
		return true;
	return false;
}

static void merge_input_pool(struct btr_node *btrn, size_t dest_size)
{
	struct btr_buffer_reference *br, *wbr = NULL;
	int num_refs; /* including wrap buffer */
	char *buf, *buf1 = NULL, *buf2 = NULL;
	size_t sz, sz1 = 0, sz2 = 0, wb_consumed = 0;

	br = get_first_input_br(btrn);
	if (!br || br_available_bytes(br) >= dest_size)
		return;
	num_refs = 0;
	FOR_EACH_BUFFER_REF(br, btrn) {
		num_refs++;
		sz = btr_get_buffer_by_reference(br, &buf);
		if (sz == 0)
			break;
		if (br->wrap_count != 0) {
			assert(!wbr);
			assert(num_refs == 1);
			wbr = br;
			if (sz >= dest_size)
				return;
			wb_consumed = br->consumed;
			continue;
		}
		if (!buf1) {
			buf1 = buf;
			sz1 = sz;
			goto next;
		}
		if (buf1 + sz1 == buf) {
			sz1 += sz;
			goto next;
		}
		if (!buf2) {
			buf2 = buf;
			sz2 = sz;
			goto next;
		}
		assert(buf2 + sz2 == buf);
		sz2 += sz;
next:
		if (sz1 + sz2 >= dest_size + wb_consumed)
			break;
	}
	if (!buf2) /* nothing to do */
		return;
	assert(buf1 && sz2 > 0);
	/*
	 * If the second buffer is large, we only take the first part of it
	 * to avoid having to copy huge buffers.
	 */
	sz2 = PARA_MIN(sz2, (size_t)(64 * 1024));
	if (!wbr) {
		/* Make a new wrap buffer combining buf1 and buf2. */
		sz = sz1 + sz2;
		buf = alloc(sz);
		PARA_DEBUG_LOG("merging input buffers: (%p:%zu, %p:%zu) -> %p:%zu\n",
			buf1, sz1, buf2, sz2, buf, sz);
		memcpy(buf, buf1, sz1);
		memcpy(buf + sz1, buf2, sz2);
		br = zalloc(sizeof(*br));
		br->btrb = new_btrb(buf, sz);
		br->btrb->refcount = 1;
		br->consumed = 0;
		/* This is a wrap buffer */
		br->wrap_count = sz1;
		para_list_add(&br->node, &btrn->input_queue);
		return;
	}
	/*
	 * We already have a wrap buffer, but it is too small. It might be
	 * partially used.
	 */
	if (wbr->wrap_count == sz1 && wbr->btrb->size >= sz1 + sz2) /* nothing we can do about it */
		return;
	sz = sz1 + sz2 - wbr->btrb->size; /* amount of new data */
	PARA_DEBUG_LOG("increasing wrap buffer %zu -> %zu\n", wbr->btrb->size,
		wbr->btrb->size + sz);
	wbr->btrb->size += sz;
	wbr->btrb->buf = para_realloc(wbr->btrb->buf, wbr->btrb->size);
	/* copy the new data to the end of the reallocated buffer */
	assert(sz2 >= sz);
	memcpy(wbr->btrb->buf + wbr->btrb->size - sz, buf2 + sz2 - sz, sz);
}

/**
 * Merge the first two input buffers into one.
 *
 * This is a quite expensive operation.
 *
 * \return The number of buffers that have been available (zero, one or two).
 */
static int merge_input(struct btr_node *btrn)
{
	struct btr_buffer_reference *brs[2], *br;
	char *bufs[2], *buf;
	size_t szs[2], sz;
	int i;

	if (list_empty(&btrn->input_queue))
		return 0;
	if (list_is_singular(&btrn->input_queue))
		return 1;
	i = 0;
	/* get references to the first two buffers */
	FOR_EACH_BUFFER_REF(br, btrn) {
		brs[i] = br;
		szs[i] = btr_get_buffer_by_reference(brs[i], bufs + i);
		i++;
		if (i == 2)
			break;
	}
	assert(i == 2);
	/* make a new btrb that combines the two buffers and a br to it. */
	sz = szs[0] + szs[1];
	buf = alloc(sz);
	PARA_DEBUG_LOG("%s: memory merging input buffers: (%zu, %zu) -> %zu\n",
		btrn->name, szs[0], szs[1], sz);
	memcpy(buf, bufs[0], szs[0]);
	memcpy(buf + szs[0], bufs[1], szs[1]);

	br = zalloc(sizeof(*br));
	br->btrb = new_btrb(buf, sz);
	br->btrb->refcount = 1;

	/* replace the first two refs by the new one */
	btr_drop_buffer_reference(brs[0]);
	btr_drop_buffer_reference(brs[1]);
	para_list_add(&br->node, &btrn->input_queue);
	return 2;
}

/**
 * Combine input queue buffers.
 *
 * \param btrn The buffer tree node whose input should be merged.
 * \param dest_size Stop merging if a buffer of at least this size exists.
 *
 * Used to combine as many buffers as needed into a single buffer whose size
 * is at least the destination size. This function is rather cheap in case
 * the parent node uses buffer pools and rather expensive otherwise.
 *
 * Note that if less than the destination size bytes are available in total,
 * this function does nothing and subsequent calls to \ref btr_next_buffer()
 * will still return a buffer whose size is less than the destination size.
 */
void btr_merge(struct btr_node *btrn, size_t dest_size)
{
	if (need_buffer_pool_merge(btrn))
		return merge_input_pool(btrn, dest_size);
	for (;;) {
		char *buf;
		size_t len = btr_next_buffer(btrn, &buf);
		if (len >= dest_size)
			return;
		PARA_DEBUG_LOG("input size = %zu < %zu = dest\n", len, dest_size);
		if (merge_input(btrn) < 2)
			return;
	}
}

static bool btr_eof(const struct btr_node *btrn)
{
	char *buf;
	size_t len = btr_next_buffer(btrn, &buf);

	return (len == 0 && btr_no_parent(btrn));
}

static void log_tree_recursively(const struct btr_node *btrn, int loglevel, int depth)
{
	struct btr_node *ch;
	const char spaces[] = "                 ", *space = spaces + 16 - depth;

	if (depth > 16)
		return;
	para_log(loglevel, "%s%s\n", space, btrn->name);
	FOR_EACH_CHILD(ch, btrn)
		log_tree_recursively(ch, loglevel, depth + 1);
}

/**
 * Write the current buffer (sub-)tree to the log.
 *
 * \param btrn Start logging at this node.
 * \param loglevel Set severity with which the tree should be logged.
 */
void btr_log_tree(const struct btr_node *btrn, int loglevel)
{
	return log_tree_recursively(btrn, loglevel, 0);
}

/**
 * Find the node with the given name in the buffer tree.
 *
 * \param name The name of the node to search.
 * \param root Where to start the search.
 *
 * \return A pointer to the node with the given name on success. If the name
 * is NULL, the function returns the given root node. If there is no node
 * with the given name, NULL is returned.
 */
struct btr_node *btr_search_node(const char *name, struct btr_node *root)
{
	struct btr_node *ch;

	if (!name)
		return root;
	if (!strcmp(root->name, name))
		return root;
	FOR_EACH_CHILD(ch, root) {
		struct btr_node *result = btr_search_node(name, ch);
		if (result)
			return result;
	}
	return NULL;
}

/* 96K ought to be enough for everybody ;) */
#define BTRN_MAX_PENDING (96 * 1024)

/**
 * Return the current state of a buffer tree node.
 *
 * \param btrn The node whose state should be queried.
 * \param min_iqs The minimal input queue size.
 * \param type The type of the buffer tree node.
 *
 * Most users of the buffer tree subsystem call this function from both
 * their ->pre_monitor() and ->post_monitor() methods.
 *
 * \return Negative if an error condition was detected, zero if there
 * is nothing to do and positive otherwise.
 *
 * Examples:
 *
 * - If a non-root node has no parent and an empty input queue, the function
 * returns -E_BTR_EOF. Similarly, if a non-leaf node has no children,
 * -E_BTR_NO_CHILD is returned.
 *
 * - If less bytes than the minimal input queue size are available in the
 * input queue and no EOF condition was detected, the function returns zero.
 *
 * - If there is plenty of data left in the input queue of the children of
 * the given buffer tree node, the function also returns zero in order to
 * bound the memory usage of the buffer tree.
 */
int btr_node_status(const struct btr_node *btrn, size_t min_iqs,
	enum btr_node_type type)
{
	if (type != BTR_NT_LEAF && btr_no_children(btrn))
		return -E_BTR_NO_CHILD;
	if (type != BTR_NT_ROOT && btr_eof(btrn))
		return -E_EOF;

	if (btr_get_output_queue_size(btrn) > BTRN_MAX_PENDING)
		return 0;
	if (type == BTR_NT_ROOT)
		return 1;
	if (min_iqs_available(min_iqs, btrn))
		return 1;
	return btr_no_parent(btrn);
}

/**
 * Get the time of the first I/O for a buffer tree node.
 *
 * \param btrn The node whose I/O time should be obtained.
 * \param tv Result pointer.
 *
 * Mainly useful for the time display of para_audiod.
 */
void btr_get_node_start(const struct btr_node *btrn, struct timeval *tv)
{
	*tv = btrn->start;
}

/**
 * Get the parent node of a buffer tree node.
 *
 * \param btrn The node whose parent should be returned. Must not be NULL.
 *
 * \return The parent node, or NULL if the given node is the root of the
 * buffer tree.
 */
struct btr_node *btr_parent(const struct btr_node *btrn)
{
	return btrn->parent;
}