btr: Handle zero-sized buffers gracefully.

[paraslash.git] / buffer_tree.c

/*
 * Copyright (C) 2009-2012 Andre Noll <maan@systemlinux.org>
 *
 * Licensed under the GPL v2. For licencing details see COPYING.
 */

/** \file buffer_tree.c Buffer tree and buffer pool implementations. */
#include <regex.h>

#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 malloced 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 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 = para_malloc(sizeof(*btrp));
        btrp->area_start = para_malloc(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 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
 * btr_pool_new().
 */
size_t btr_pool_size(struct btr_pool *btrp)
{
        return btrp->area_end - btrp->area_start;
}

static size_t btr_pool_filled(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. Use btr_pool_available() to obtain the length of
 * the largest contiguous buffer that can currently be allocated from the
 * buffer pool.
 */
size_t btr_pool_unused(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(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(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 scatter array.
 *
 * \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 \a iov are initialized.
 */
int btr_pool_get_buffers(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
 * 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 btr_free_node() after the node has been removed from the
 * buffer tree via btr_remove_node().
 */
struct btr_node *btr_new_node(struct btr_node_description *bnd)
{
        struct btr_node *btrn = para_malloc(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 refcount and will
 * not be associated with a btr pool.
 */
static struct btr_buffer *new_btrb(char *buf, size_t size)
{
        struct btr_buffer *btrb = para_calloc(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(struct btr_node *btrn)
{
        if (list_empty(&btrn->input_queue))
                return NULL;
        return list_first_entry(&btrn->input_queue,
                struct btr_buffer_reference, node);
}

/*
 * Deallocate the reference, release the resources if refcount drops to zero.
 */
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) {
                dealloc_buffer(btrb);
                free(btrb);
        }
}

static void 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;
        FOR_EACH_CHILD(ch, btrn) {
                struct btr_buffer_reference *br = para_calloc(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;
        }
}

/**
 * Insert a malloced buffer into the buffer tree.
 *
 * \param buf The buffer to insert.
 * \param size The size of \a buf in bytes.
 * \param btrn Position in the buffer tree to create the output.
 *
 * This creates references to \a buf and adds these references to each child of
 * \a btrn. 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 \a buf was obtained from a
 * buffer pool. Use 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 btr_add_output() but additionally sets the \p dont_free
 * flag on \a buf. If the refcount for the buffer drops to zero, \a buf will
 * not be deallocated if this flag is set.
 *
 * The \p dont_free bit also prevents the children of \a btrn from modifying
 * the buffer contents inplace. Specifically, \ref btr_inplace_ok() returns
 * false if there is any buffer in the input queue with the \p 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;
        add_btrb_to_children(btrb, btrn, 0);
}

/**
 * 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 \a btrn. 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;
        add_btrb_to_children(btrb, btrn, 0);
}

/**
 * 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);
        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(struct btr_node *btrn)
{
        return list_empty(&btrn->children);
}

/**
 * Find out whether a node is an orphan node.
 *
 * \param btrn The buffer tree node.
 *
 * \return True if \a btrn has no parent.
 *
 * This function will always return true for the root node.  However in case
 * nodes have been removed from the tree, other nodes may become orphans too.
 */
bool btr_no_parent(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 \a btrn has no siblings), a
 * 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_select call.
 *
 * \return True if \a btrn has no siblings.
 */
bool btr_inplace_ok(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(struct btr_buffer_reference *br)
{
        return br->btrb->size - br->consumed;
}

static size_t btr_get_buffer_by_reference(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 \p 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 btr_next_buffer() to get a reference to the buffer obtained
 * by skipping the given number of bytes. Skipped input bytes are not consumed.
 *
 * With a zero \a omit argument, this function is equivalent to \ref
 * btr_next_buffer().
 *
 * \return Number of bytes in \a bufp. If there are less than or equal to \a
 * omit many bytes available in the input queue of the buffer tree node pointed
 * to by \a btrn, the function returns zero and the value of \a bufp is
 * undefined.
 */
size_t btr_next_buffer_omit(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 by 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 \a omit value of zero.
 */
size_t btr_next_buffer(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.
 *
 * Note that \a numbytes may be smaller than the buffer size. In this case the
 * buffer is not deallocated and subsequent calls to 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);
}

/**
 * Remove a node from a buffer tree.
 *
 * \param btrnp Determines the node to remove.
 *
 * This orphans all children of the node given by \a btrnp and removes this
 * node from the child list of its parent. Moreover, the input queue is flushed
 * and the node pointer given by \a btrp is set to \p 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);
        free(btrn->name);
        free(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(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;
}

/**
 * Remove a node from the buffer tree, reconnecting parent and children.
 *
 * \param btrn 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 btr_remove_node(), calling
 * btr_splice_out_node() does not split the tree into disconnected components
 * but reconnects the buffer tree by making all child nodes of \a btrn children
 * of the parent of \a btrn.
 */
void btr_splice_out_node(struct btr_node *btrn)
{
        struct btr_node *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);
        }
        assert(list_empty(&btrn->children));
        btrn->parent = 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(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 \a btrn upwards and looks for
 * the first node that understands \a command. On this node \a command is
 * executed, and the result is stored in \a value_result.
 *
 * \return \p -ENOTSUP if no parent node of \a btrn understands \a command.
 * Otherwise the return value of the command handler is returned.
 *
 * \sa \ref receiver::execute, filter::execute, writer::execute.
 */
int btr_exec_up(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 \a context address specified at node creation time.
 *
 * \sa btr_new_node(), struct \ref btr_node_description.
 */
void *btr_context(struct btr_node *btrn)
{
        return btrn->context;
}

static bool need_buffer_pool_merge(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 memcpy() 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 = para_malloc(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 = para_calloc(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 = para_malloc(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 = para_calloc(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 \a dest_size. This function is rather cheap in case the parent node
 * uses buffer pools and rather expensive otherwise.
 *
 * Note that if less than \a dest_size bytes are available in total, this
 * function does nothing and subsequent calls to btr_next_buffer() will still
 * return a buffer size less than \a dest_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(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(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(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 \a name is
 * \p NULL, the function returns \a root. If there is no node with the given
 * name, \p 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;
}

/** 640K 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 supposed type of \a btrn.
 *
 * Most users of the buffer tree subsystem call this function from both
 * their pre_select and the post_select 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 \p -E_BTR_EOF. Similarly, if a non-leaf node has no children, \p
 * -E_BTR_NO_CHILD is returned.
 *
 * - If less than \a min_iqs many bytes are available in the input queue and no
 * EOF condition was detected, the function returns zero.
 *
 * - If there's plenty of data left in the input queue of the children of \a
 * btrn, the function also returns zero in order to bound the memory usage of
 * the buffer tree.
 */
int btr_node_status(struct btr_node *btrn, size_t min_iqs,
        enum btr_node_type type)
{
        size_t iqs;

        assert(btrn);
        if (type != BTR_NT_LEAF) {
                if (btr_no_children(btrn))
                        return -E_BTR_NO_CHILD;
                if (btr_get_output_queue_size(btrn) > BTRN_MAX_PENDING)
                        return 0;
        }
        if (type != BTR_NT_ROOT) {
                if (btr_eof(btrn))
                        return -E_BTR_EOF;
                iqs = btr_get_input_queue_size(btrn);
                if (iqs == 0) /* we have a parent, because not eof */
                        return 0;
                if (iqs < min_iqs && !btr_no_parent(btrn))
                        return 0;
        }
        return 1;
}

/**
 * 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(struct btr_node *btrn, struct timeval *tv)
{
        *tv = btrn->start;
}

struct btr_node *btr_parent(struct btr_node *btrn)
{
        return btrn->parent;
}