filemap.c

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/*-------------------------------------------------------------------------
 *
 * filemap.c
 *    A data structure for keeping track of files that have changed.
 *
 * This source file contains the logic to decide what to do with different
 * kinds of files, and the data structure to support it.  Before modifying
 * anything, pg_rewind collects information about all the files and their
 * attributes in the target and source data directories.  It also scans the
 * WAL log in the target, and collects information about data blocks that
 * were changed.  All this information is stored in a hash table, using the
 * file path relative to the root of the data directory as the key.
 *
 * After collecting all the information required, the decide_file_actions()
 * function scans the hash table and decides what action needs to be taken
 * for each file.  Finally, it sorts the array to the final order that the
 * actions should be executed in.
 *
 * Copyright (c) 2013-2025, PostgreSQL Global Development Group
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres_fe.h"
 
#include <sys/stat.h>
#include <unistd.h>
 
#include "access/xlog_internal.h"
#include "catalog/pg_tablespace_d.h"
#include "common/file_utils.h"
#include "common/hashfn_unstable.h"
#include "common/string.h"
#include "datapagemap.h"
#include "filemap.h"
#include "pg_rewind.h"
 
/*
 * Define a hash table which we can use to store information about the files
 * appearing in source and target systems.
 */
#define SH_PREFIX               filehash
#define SH_ELEMENT_TYPE         file_entry_t
#define SH_KEY_TYPE             const char *
#define SH_KEY                  path
#define SH_HASH_KEY(tb, key)    hash_string(key)
#define SH_EQUAL(tb, a, b)      (strcmp(a, b) == 0)
#define SH_SCOPE                static inline
#define SH_RAW_ALLOCATOR        pg_malloc0
#define SH_DECLARE
#define SH_DEFINE
#include "lib/simplehash.h"
 
#define FILEHASH_INITIAL_SIZE   1000
 
static filehash_hash *filehash;
 
static file_content_type_t getFileContentType(const char *path);
static char *datasegpath(RelFileLocator rlocator, ForkNumber forknum,
                         BlockNumber segno);
 
static file_entry_t *insert_filehash_entry(const char *path);
static file_entry_t *lookup_filehash_entry(const char *path);
 
/*
 * A separate hash table which tracks WAL files that must not be deleted.
 */
typedef struct keepwal_entry
{
    const char *path;
    uint32      status;
} keepwal_entry;
 
#define SH_PREFIX               keepwal
#define SH_ELEMENT_TYPE         keepwal_entry
#define SH_KEY_TYPE             const char *
#define SH_KEY                  path
#define SH_HASH_KEY(tb, key)    hash_string(key)
#define SH_EQUAL(tb, a, b)      (strcmp(a, b) == 0)
#define SH_SCOPE                static inline
#define SH_RAW_ALLOCATOR        pg_malloc0
#define SH_DECLARE
#define SH_DEFINE
#include "lib/simplehash.h"
 
#define KEEPWAL_INITIAL_SIZE    1000
 
 
static keepwal_hash *keepwal = NULL;
static bool keepwal_entry_exists(const char *path);
 
static int  final_filemap_cmp(const void *a, const void *b);
 
static bool check_file_excluded(const char *path, bool is_source);
 
/*
 * Definition of one element part of an exclusion list, used to exclude
 * contents when rewinding.  "name" is the name of the file or path to
 * check for exclusion.  If "match_prefix" is true, any items matching
 * the name as prefix are excluded.
 */
struct exclude_list_item
{
    const char *name;
    bool        match_prefix;
};
 
/*
 * The contents of these directories are removed or recreated during server
 * start so they are not included in data processed by pg_rewind.
 *
 * Note: those lists should be kept in sync with what basebackup.c provides.
 * Some of the values, contrary to what basebackup.c uses, are hardcoded as
 * they are defined in backend-only headers.  So this list is maintained
 * with a best effort in mind.
 */
static const char *const excludeDirContents[] =
{
    /*
     * Skip temporary statistics files. PG_STAT_TMP_DIR must be skipped
     * because extensions like pg_stat_statements store data there.
     */
    "pg_stat_tmp",              /* defined as PG_STAT_TMP_DIR */
 
    /*
     * It is generally not useful to backup the contents of this directory
     * even if the intention is to restore to another primary. See backup.sgml
     * for a more detailed description.
     */
    "pg_replslot",              /* defined as PG_REPLSLOT_DIR */
 
    /* Contents removed on startup, see dsm_cleanup_for_mmap(). */
    "pg_dynshmem",              /* defined as PG_DYNSHMEM_DIR */
 
    /* Contents removed on startup, see AsyncShmemInit(). */
    "pg_notify",
 
    /*
     * Old contents are loaded for possible debugging but are not required for
     * normal operation, see SerialInit().
     */
    "pg_serial",
 
    /* Contents removed on startup, see DeleteAllExportedSnapshotFiles(). */
    "pg_snapshots",
 
    /* Contents zeroed on startup, see StartupSUBTRANS(). */
    "pg_subtrans",
 
    /* end of list */
    NULL
};
 
/*
 * List of files excluded from filemap processing.   Files are excluded
 * if their prefix match.
 */
static const struct exclude_list_item excludeFiles[] =
{
    /* Skip auto conf temporary file. */
    {"postgresql.auto.conf.tmp", false},    /* defined as PG_AUTOCONF_FILENAME */
 
    /* Skip current log file temporary file */
    {"current_logfiles.tmp", false},    /* defined as
                                         * LOG_METAINFO_DATAFILE_TMP */
 
    /* Skip relation cache because it is rebuilt on startup */
    {"pg_internal.init", true}, /* defined as RELCACHE_INIT_FILENAME */
 
    /*
     * If there is a backup_label or tablespace_map file, it indicates that a
     * recovery failed and this cluster probably can't be rewound, but exclude
     * them anyway if they are found.
     */
    {"backup_label", false},    /* defined as BACKUP_LABEL_FILE */
    {"tablespace_map", false},  /* defined as TABLESPACE_MAP */
 
    /*
     * If there's a backup_manifest, it belongs to a backup that was used to
     * start this server. It is *not* correct for this backup. Our
     * backup_manifest is injected into the backup separately if users want
     * it.
     */
    {"backup_manifest", false},
 
    {"postmaster.pid", false},
    {"postmaster.opts", false},
 
    /* end of list */
    {NULL, false}
};
 
/*
 * Initialize the hash table for the file map.
 */
void
filehash_init(void)
{
    filehash = filehash_create(FILEHASH_INITIAL_SIZE, NULL);
}
 
/* Look up entry for 'path', creating a new one if it doesn't exist */
static file_entry_t *
insert_filehash_entry(const char *path)
{
    file_entry_t *entry;
    bool        found;
 
    entry = filehash_insert(filehash, path, &found);
    if (!found)
    {
        entry->path = pg_strdup(path);
        entry->content_type = getFileContentType(path);
 
        entry->target_exists = false;
        entry->target_type = FILE_TYPE_UNDEFINED;
        entry->target_size = 0;
        entry->target_link_target = NULL;
        entry->target_pages_to_overwrite.bitmap = NULL;
        entry->target_pages_to_overwrite.bitmapsize = 0;
 
        entry->source_exists = false;
        entry->source_type = FILE_TYPE_UNDEFINED;
        entry->source_size = 0;
        entry->source_link_target = NULL;
 
        entry->action = FILE_ACTION_UNDECIDED;
    }
 
    return entry;
}
 
static file_entry_t *
lookup_filehash_entry(const char *path)
{
    return filehash_lookup(filehash, path);
}
 
/*
 * Initialize a hash table to store WAL file names that must be kept.
 */
void
keepwal_init(void)
{
    /* An initial hash size out of thin air */
    keepwal = keepwal_create(KEEPWAL_INITIAL_SIZE, NULL);
}
 
/* Mark the given file to prevent its removal */
void
keepwal_add_entry(const char *path)
{
    keepwal_entry *entry;
    bool        found;
 
    /* Should only be called with keepwal initialized */
    Assert(keepwal != NULL);
 
    entry = keepwal_insert(keepwal, path, &found);
 
    if (!found)
        entry->path = pg_strdup(path);
}
 
/* Return true if file is marked as not to be removed, false otherwise */
static bool
keepwal_entry_exists(const char *path)
{
    return keepwal_lookup(keepwal, path) != NULL;
}
 
/*
 * Callback for processing source file list.
 *
 * This is called once for every file in the source server.  We record the
 * type and size of the file, so that decide_file_action() can later decide what
 * to do with it.
 */
void
process_source_file(const char *path, file_type_t type, size_t size,
                    const char *link_target)
{
    file_entry_t *entry;
 
    /*
     * Pretend that pg_wal is a directory, even if it's really a symlink. We
     * don't want to mess with the symlink itself, nor complain if it's a
     * symlink in source but not in target or vice versa.
     */
    if (strcmp(path, "pg_wal") == 0 && type == FILE_TYPE_SYMLINK)
        type = FILE_TYPE_DIRECTORY;
 
    /*
     * sanity check: a filename that looks like a data file better be a
     * regular file
     */
    if (type != FILE_TYPE_REGULAR && getFileContentType(path) == FILE_CONTENT_TYPE_RELATION)
        pg_fatal("data file \"%s\" in source is not a regular file", path);
 
    /* Remember this source file */
    entry = insert_filehash_entry(path);
    if (entry->source_exists)
        pg_fatal("duplicate source file \"%s\"", path);
    entry->source_exists = true;
    entry->source_type = type;
    entry->source_size = size;
    entry->source_link_target = link_target ? pg_strdup(link_target) : NULL;
}
 
/*
 * Callback for processing target file list.
 *
 * Record the type and size of the file, like process_source_file() does.
 */
void
process_target_file(const char *path, file_type_t type, size_t size,
                    const char *link_target)
{
    file_entry_t *entry;
 
    /*
     * Do not apply any exclusion filters here.  This has advantage to remove
     * from the target data folder all paths which have been filtered out from
     * the source data folder when processing the source files.
     */
 
    /*
     * Like in process_source_file, pretend that pg_wal is always a directory.
     */
    if (strcmp(path, "pg_wal") == 0 && type == FILE_TYPE_SYMLINK)
        type = FILE_TYPE_DIRECTORY;
 
    /* Remember this target file */
    entry = insert_filehash_entry(path);
    if (entry->target_exists)
        pg_fatal("duplicate source file \"%s\"", path);
    entry->target_exists = true;
    entry->target_type = type;
    entry->target_size = size;
    entry->target_link_target = link_target ? pg_strdup(link_target) : NULL;
}
 
/*
 * This callback gets called while we read the WAL in the target, for every
 * block that has changed in the target system.  It decides if the given
 * 'blkno' in the target relfile needs to be overwritten from the source, and
 * if so, records it in 'target_pages_to_overwrite' bitmap.
 *
 * NOTE: All the files on both systems must have already been added to the
 * hash table!
 */
void
process_target_wal_block_change(ForkNumber forknum, RelFileLocator rlocator,
                                BlockNumber blkno)
{
    char       *path;
    file_entry_t *entry;
    BlockNumber blkno_inseg;
    int         segno;
 
    segno = blkno / RELSEG_SIZE;
    blkno_inseg = blkno % RELSEG_SIZE;
 
    path = datasegpath(rlocator, forknum, segno);
    entry = lookup_filehash_entry(path);
    pfree(path);
 
    /*
     * If the block still exists in both systems, remember it. Otherwise we
     * can safely ignore it.
     *
     * If the block is beyond the EOF in the source system, or the file
     * doesn't exist in the source at all, we're going to truncate/remove it
     * away from the target anyway. Likewise, if it doesn't exist in the
     * target anymore, we will copy it over with the "tail" from the source
     * system, anyway.
     *
     * It is possible to find WAL for a file that doesn't exist on either
     * system anymore. It means that the relation was dropped later in the
     * target system, and independently on the source system too, or that it
     * was created and dropped in the target system and it never existed in
     * the source. Either way, we can safely ignore it.
     */
    if (entry)
    {
        Assert(entry->content_type == FILE_CONTENT_TYPE_RELATION);
 
        if (entry->target_exists)
        {
            if (entry->target_type != FILE_TYPE_REGULAR)
                pg_fatal("unexpected page modification for non-regular file \"%s\"",
                         entry->path);
 
            if (entry->source_exists)
            {
                off_t       end_offset;
 
                end_offset = (blkno_inseg + 1) * BLCKSZ;
                if (end_offset <= entry->source_size && end_offset <= entry->target_size)
                    datapagemap_add(&entry->target_pages_to_overwrite, blkno_inseg);
            }
        }
    }
}
 
/*
 * Is this the path of file that pg_rewind can skip copying?
 */
static bool
check_file_excluded(const char *path, bool is_source)
{
    char        localpath[MAXPGPATH];
    int         excludeIdx;
    const char *filename;
 
    /*
     * Skip all temporary files, .../pgsql_tmp/... and .../pgsql_tmp.*
     */
    if (strstr(path, "/" PG_TEMP_FILE_PREFIX) != NULL ||
        strstr(path, "/" PG_TEMP_FILES_DIR "/") != NULL)
    {
        return true;
    }
 
    /* check individual files... */
    for (excludeIdx = 0; excludeFiles[excludeIdx].name != NULL; excludeIdx++)
    {
        int         cmplen = strlen(excludeFiles[excludeIdx].name);
 
        filename = last_dir_separator(path);
        if (filename == NULL)
            filename = path;
        else
            filename++;
 
        if (!excludeFiles[excludeIdx].match_prefix)
            cmplen++;
        if (strncmp(filename, excludeFiles[excludeIdx].name, cmplen) == 0)
        {
            if (is_source)
                pg_log_debug("entry \"%s\" excluded from source file list",
                             path);
            else
                pg_log_debug("entry \"%s\" excluded from target file list",
                             path);
            return true;
        }
    }
 
    /*
     * ... And check some directories.  Note that this includes any contents
     * within the directories themselves.
     */
    for (excludeIdx = 0; excludeDirContents[excludeIdx] != NULL; excludeIdx++)
    {
        snprintf(localpath, sizeof(localpath), "%s/",
                 excludeDirContents[excludeIdx]);
        if (strstr(path, localpath) == path)
        {
            if (is_source)
                pg_log_debug("entry \"%s\" excluded from source file list",
                             path);
            else
                pg_log_debug("entry \"%s\" excluded from target file list",
                             path);
            return true;
        }
    }
 
    return false;
}
 
static const char *
action_to_str(file_action_t action)
{
    switch (action)
    {
        case FILE_ACTION_NONE:
            return "NONE";
        case FILE_ACTION_COPY:
            return "COPY";
        case FILE_ACTION_TRUNCATE:
            return "TRUNCATE";
        case FILE_ACTION_COPY_TAIL:
            return "COPY_TAIL";
        case FILE_ACTION_CREATE:
            return "CREATE";
        case FILE_ACTION_REMOVE:
            return "REMOVE";
 
        default:
            return "unknown";
    }
}
 
/*
 * Calculate the totals needed for progress reports.
 */
void
calculate_totals(filemap_t *filemap)
{
    file_entry_t *entry;
    int         i;
 
    filemap->total_size = 0;
    filemap->fetch_size = 0;
 
    for (i = 0; i < filemap->nentries; i++)
    {
        entry = filemap->entries[i];
 
        if (entry->source_type != FILE_TYPE_REGULAR)
            continue;
 
        filemap->total_size += entry->source_size;
 
        if (entry->action == FILE_ACTION_COPY)
        {
            filemap->fetch_size += entry->source_size;
            continue;
        }
 
        if (entry->action == FILE_ACTION_COPY_TAIL)
            filemap->fetch_size += (entry->source_size - entry->target_size);
 
        if (entry->target_pages_to_overwrite.bitmapsize > 0)
        {
            datapagemap_iterator_t *iter;
            BlockNumber blk;
 
            iter = datapagemap_iterate(&entry->target_pages_to_overwrite);
            while (datapagemap_next(iter, &blk))
                filemap->fetch_size += BLCKSZ;
 
            pg_free(iter);
        }
    }
}
 
void
print_filemap(filemap_t *filemap)
{
    file_entry_t *entry;
    int         i;
 
    for (i = 0; i < filemap->nentries; i++)
    {
        entry = filemap->entries[i];
 
        if (entry->action != FILE_ACTION_NONE ||
            entry->content_type == FILE_CONTENT_TYPE_WAL ||
            entry->target_pages_to_overwrite.bitmapsize > 0)
        {
            pg_log_debug("%s (%s)", entry->path,
                         action_to_str(entry->action));
 
            if (entry->target_pages_to_overwrite.bitmapsize > 0)
                datapagemap_print(&entry->target_pages_to_overwrite);
        }
    }
    fflush(stdout);
}
 
/*
 * Determine what kind of file this one looks like.
 */
static file_content_type_t
getFileContentType(const char *path)
{
    RelFileLocator rlocator;
    unsigned int segNo;
    int         nmatch;
    file_content_type_t result = FILE_CONTENT_TYPE_OTHER;
 
    /* Check if it is a WAL file. */
    if (strncmp("pg_wal/", path, 7) == 0)
    {
        const char *filename = path + 7;    /* Skip "pg_wal/" */
 
        if (IsXLogFileName(filename))
            return FILE_CONTENT_TYPE_WAL;
        else
            return FILE_CONTENT_TYPE_OTHER;
    }
 
    /*----
     * Does it look like a relation data file?
     *
     * For our purposes, only files belonging to the main fork are considered
     * relation files. Other forks are always copied in toto, because we
     * cannot reliably track changes to them, because WAL only contains block
     * references for the main fork.
     *
     * Relation data files can be in one of the following directories:
     *
     * global/
     *      shared relations
     *
     * base/<db oid>/
     *      regular relations, default tablespace
     *
     * pg_tblspc/<tblspc oid>/<tblspc version>/
     *      within a non-default tablespace (the name of the directory
     *      depends on version)
     *
     * And the relation data files themselves have a filename like:
     *
     * <oid>.<segment number>
     *
     *----
     */
    rlocator.spcOid = InvalidOid;
    rlocator.dbOid = InvalidOid;
    rlocator.relNumber = InvalidRelFileNumber;
    segNo = 0;
    result = FILE_CONTENT_TYPE_OTHER;
 
    nmatch = sscanf(path, "global/%u.%u", &rlocator.relNumber, &segNo);
    if (nmatch == 1 || nmatch == 2)
    {
        rlocator.spcOid = GLOBALTABLESPACE_OID;
        rlocator.dbOid = 0;
        result = FILE_CONTENT_TYPE_RELATION;
    }
    else
    {
        nmatch = sscanf(path, "base/%u/%u.%u",
                        &rlocator.dbOid, &rlocator.relNumber, &segNo);
        if (nmatch == 2 || nmatch == 3)
        {
            rlocator.spcOid = DEFAULTTABLESPACE_OID;
            result = FILE_CONTENT_TYPE_RELATION;
        }
        else
        {
            nmatch = sscanf(path, "pg_tblspc/%u/" TABLESPACE_VERSION_DIRECTORY "/%u/%u.%u",
                            &rlocator.spcOid, &rlocator.dbOid, &rlocator.relNumber,
                            &segNo);
            if (nmatch == 3 || nmatch == 4)
                result = FILE_CONTENT_TYPE_RELATION;
        }
    }
 
    /*
     * The sscanf tests above can match files that have extra characters at
     * the end. To eliminate such cases, cross-check that GetRelationPath
     * creates the exact same filename, when passed the RelFileLocator
     * information we extracted from the filename.
     */
    if (result == FILE_CONTENT_TYPE_RELATION)
    {
        char       *check_path = datasegpath(rlocator, MAIN_FORKNUM, segNo);
 
        if (strcmp(check_path, path) != 0)
            result = FILE_CONTENT_TYPE_OTHER;
 
        pfree(check_path);
    }
 
    return result;
}
 
/*
 * A helper function to create the path of a relation file and segment.
 *
 * The returned path is palloc'd
 */
static char *
datasegpath(RelFileLocator rlocator, ForkNumber forknum, BlockNumber segno)
{
    RelPathStr  path;
    char       *segpath;
 
    path = relpathperm(rlocator, forknum);
    if (segno > 0)
    {
        segpath = psprintf("%s.%u", path.str, segno);
        return segpath;
    }
    else
        return pstrdup(path.str);
}
 
/*
 * In the final stage, the filemap is sorted so that removals come last.
 * From disk space usage point of view, it would be better to do removals
 * first, but for now, safety first. If a whole directory is deleted, all
 * files and subdirectories inside it need to removed first. On creation,
 * parent directory needs to be created before files and directories inside
 * it. To achieve that, the file_action_t enum is ordered so that we can
 * just sort on that first. Furthermore, sort REMOVE entries in reverse
 * path order, so that "foo/bar" subdirectory is removed before "foo".
 */
static int
final_filemap_cmp(const void *a, const void *b)
{
    file_entry_t *fa = *((file_entry_t **) a);
    file_entry_t *fb = *((file_entry_t **) b);
 
    if (fa->action > fb->action)
        return 1;
    if (fa->action < fb->action)
        return -1;
 
    if (fa->action == FILE_ACTION_REMOVE)
        return strcmp(fb->path, fa->path);
    else
        return strcmp(fa->path, fb->path);
}
 
/*
 * Decide what to do with a WAL segment file based on its position
 * relative to the point of divergence.
 *
 * Caller is responsible for ensuring that the file exists on both
 * source and target servers.
 */
static file_action_t
decide_wal_file_action(const char *fname, XLogSegNo last_common_segno,
                       size_t source_size, size_t target_size)
{
    TimeLineID  file_tli;
    XLogSegNo   file_segno;
 
    /* Get current WAL segment number given current segment file name */
    XLogFromFileName(fname, &file_tli, &file_segno, WalSegSz);
 
    /*
     * Avoid copying files before the last common segment.
     *
     * These files exist on the source and the target servers, so they should
     * be identical and located strictly before the segment that contains the
     * LSN where target and source servers have diverged.
     *
     * While we are on it, double-check the size of each file and copy the
     * file if they do not match, in case.
     */
    if (file_segno < last_common_segno &&
        source_size == target_size)
        return FILE_ACTION_NONE;
 
    return FILE_ACTION_COPY;
}
 
/*
 * Decide what action to perform to a file.
 */
static file_action_t
decide_file_action(file_entry_t *entry, XLogSegNo last_common_segno)
{
    const char *path = entry->path;
 
    /*
     * Don't touch the control file. It is handled specially, after copying
     * all the other files.
     */
    if (strcmp(path, XLOG_CONTROL_FILE) == 0)
        return FILE_ACTION_NONE;
 
    /* Skip macOS system files */
    if (strstr(path, ".DS_Store") != NULL)
        return FILE_ACTION_NONE;
 
    /*
     * Remove all files matching the exclusion filters in the target.
     */
    if (check_file_excluded(path, true))
    {
        if (entry->target_exists)
            return FILE_ACTION_REMOVE;
        else
            return FILE_ACTION_NONE;
    }
 
    /*
     * Handle cases where the file is missing from one of the systems.
     */
    if (!entry->target_exists && entry->source_exists)
    {
        /*
         * File exists in source, but not in target. Copy it in toto. (If it's
         * a relation data file, WAL replay after rewinding should re-create
         * it anyway. But there's no harm in copying it now.)
         */
        switch (entry->source_type)
        {
            case FILE_TYPE_DIRECTORY:
            case FILE_TYPE_SYMLINK:
                return FILE_ACTION_CREATE;
            case FILE_TYPE_REGULAR:
                return FILE_ACTION_COPY;
            case FILE_TYPE_UNDEFINED:
                pg_fatal("unknown file type for \"%s\"", entry->path);
                break;
        }
    }
    else if (entry->target_exists && !entry->source_exists)
    {
        /*
         * For files that exist in target but not in source, we check the
         * keepwal hash table; any files listed therein must not be removed.
         */
        if (keepwal_entry_exists(path))
        {
            pg_log_debug("Not removing file \"%s\" because it is required for recovery", path);
            return FILE_ACTION_NONE;
        }
        return FILE_ACTION_REMOVE;
    }
    else if (!entry->target_exists && !entry->source_exists)
    {
        /*
         * Doesn't exist in either server. Why does it have an entry in the
         * first place??
         */
        Assert(false);
        return FILE_ACTION_NONE;
    }
 
    /*
     * Otherwise, the file exists on both systems
     */
    Assert(entry->target_exists && entry->source_exists);
 
    if (entry->source_type != entry->target_type)
    {
        /* But it's a different kind of object. Strange.. */
        pg_fatal("file \"%s\" is of different type in source and target", entry->path);
    }
 
    /*
     * PG_VERSION files should be identical on both systems, but avoid
     * overwriting them for paranoia.
     */
    if (pg_str_endswith(entry->path, "PG_VERSION"))
        return FILE_ACTION_NONE;
 
    switch (entry->source_type)
    {
        case FILE_TYPE_DIRECTORY:
            return FILE_ACTION_NONE;
 
        case FILE_TYPE_SYMLINK:
 
            /*
             * XXX: Should we check if it points to the same target?
             */
            return FILE_ACTION_NONE;
 
        case FILE_TYPE_REGULAR:
            if (entry->content_type == FILE_CONTENT_TYPE_WAL)
            {
                /* Handle WAL segment file */
                const char *filename = last_dir_separator(entry->path);
 
                if (filename == NULL)
                    filename = entry->path;
                else
                    filename++; /* Skip the separator */
 
                return decide_wal_file_action(filename, last_common_segno,
                                              entry->source_size,
                                              entry->target_size);
            }
            else if (entry->content_type != FILE_CONTENT_TYPE_RELATION)
            {
                /*
                 * It's a non-data file that we have no special processing
                 * for. Copy it in toto.
                 */
                return FILE_ACTION_COPY;
            }
            else
            {
                /*
                 * It's a data file that exists in both systems.
                 *
                 * If it's larger in target, we can truncate it. There will
                 * also be a WAL record of the truncation in the source
                 * system, so WAL replay would eventually truncate the target
                 * too, but we might as well do it now.
                 *
                 * If it's smaller in the target, it means that it has been
                 * truncated in the target, or enlarged in the source, or
                 * both. If it was truncated in the target, we need to copy
                 * the missing tail from the source system. If it was enlarged
                 * in the source system, there will be WAL records in the
                 * source system for the new blocks, so we wouldn't need to
                 * copy them here. But we don't know which scenario we're
                 * dealing with, and there's no harm in copying the missing
                 * blocks now, so do it now.
                 *
                 * If it's the same size, do nothing here. Any blocks modified
                 * in the target will be copied based on parsing the target
                 * system's WAL, and any blocks modified in the source will be
                 * updated after rewinding, when the source system's WAL is
                 * replayed.
                 */
                if (entry->target_size < entry->source_size)
                    return FILE_ACTION_COPY_TAIL;
                else if (entry->target_size > entry->source_size)
                    return FILE_ACTION_TRUNCATE;
                else
                    return FILE_ACTION_NONE;
            }
            break;
 
        case FILE_TYPE_UNDEFINED:
            pg_fatal("unknown file type for \"%s\"", path);
            break;
    }
 
    /* unreachable */
    pg_fatal("could not decide what to do with file \"%s\"", path);
}
 
/*
 * Decide what to do with each file.
 *
 * Returns a 'filemap' with the entries in the order that their actions
 * should be executed.
 */
filemap_t *
decide_file_actions(XLogSegNo last_common_segno)
{
    int         i;
    filehash_iterator it;
    file_entry_t *entry;
    filemap_t  *filemap;
 
    filehash_start_iterate(filehash, &it);
    while ((entry = filehash_iterate(filehash, &it)) != NULL)
    {
        entry->action = decide_file_action(entry, last_common_segno);
    }
 
    /*
     * Turn the hash table into an array, and sort in the order that the
     * actions should be performed.
     */
    filemap = pg_malloc(offsetof(filemap_t, entries) +
                        filehash->members * sizeof(file_entry_t *));
    filemap->nentries = filehash->members;
    filehash_start_iterate(filehash, &it);
    i = 0;
    while ((entry = filehash_iterate(filehash, &it)) != NULL)
    {
        filemap->entries[i++] = entry;
    }
 
    qsort(&filemap->entries, filemap->nentries, sizeof(file_entry_t *),
          final_filemap_cmp);
 
    return filemap;
}