stashpersist.c

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/*-------------------------------------------------------------------------
 *
 * stashpersist.c
 *    Persistence support for pg_stash_advice.
 *
 * Copyright (c) 2016-2026, PostgreSQL Global Development Group
 *
 *    contrib/pg_stash_advice/stashpersist.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <sys/stat.h>
 
#include "common/hashfn.h"
#include "miscadmin.h"
#include "pg_stash_advice.h"
#include "postmaster/bgworker.h"
#include "postmaster/interrupt.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "utils/backend_status.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/timestamp.h"
 
typedef struct pgsa_writer_context
{
    char        pathname[MAXPGPATH];
    FILE       *file;
    pgsa_stash_name_table_hash *nhash;
    StringInfoData buf;
    int         entries_written;
} pgsa_writer_context;
 
/*
 * A parsed entry line, with pointers into the slurp buffer.
 */
typedef struct pgsa_saved_entry
{
    char       *stash_name;
    int64       queryId;
    char       *advice_string;
} pgsa_saved_entry;
 
/*
 * simplehash for detecting duplicate stash names during parsing.
 * Keyed by stash name (char *), pointing into the slurp buffer.
 */
typedef struct pgsa_saved_stash
{
    uint32      status;
    char       *name;
} pgsa_saved_stash;
 
#define SH_PREFIX pgsa_saved_stash_table
#define SH_ELEMENT_TYPE pgsa_saved_stash
#define SH_KEY_TYPE char *
#define SH_KEY name
#define SH_HASH_KEY(tb, key) hash_bytes((const unsigned char *) (key), strlen(key))
#define SH_EQUAL(tb, a, b) (strcmp(a, b) == 0)
#define SH_SCOPE static inline
#define SH_DEFINE
#define SH_DECLARE
#include "lib/simplehash.h"
 
extern PGDLLEXPORT void pg_stash_advice_worker_main(Datum main_arg);
static void pgsa_append_tsv_escaped_string(StringInfo buf, const char *str);
static void pgsa_detach_shmem(int code, Datum arg);
static char *pgsa_next_tsv_field(char **cursor);
static void pgsa_read_from_disk(void);
static void pgsa_restore_entries(pgsa_saved_entry *entries, int num_entries);
static void pgsa_restore_stashes(pgsa_saved_stash_table_hash *saved_stashes);
static void pgsa_unescape_tsv_field(char *str, const char *filename,
                                    unsigned lineno);
static void pgsa_write_entries(pgsa_writer_context *wctx);
pg_noreturn static void pgsa_write_error(pgsa_writer_context *wctx);
static void pgsa_write_stashes(pgsa_writer_context *wctx);
static void pgsa_write_to_disk(void);
 
/*
 * Background worker entry point for pg_stash_advice persistence.
 *
 * On startup, if load_from_disk_pending is set, we load previously saved
 * stash data from disk.  Then we enter a loop, periodically checking whether
 * any changes have been made (via the change_count atomic counter) and
 * writing them to disk.  On shutdown, we perform a final write.
 */
PGDLLEXPORT void
pg_stash_advice_worker_main(Datum main_arg)
{
    uint64      last_change_count;
    TimestampTz last_write_time = 0;
 
    /* Establish signal handlers; once that's done, unblock signals. */
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    BackgroundWorkerUnblockSignals();
 
    /* Log a debug message */
    ereport(DEBUG1,
            errmsg("pg_stash_advice worker started"));
 
    /* Set up session user so pgstat can report it. */
    InitializeSessionUserIdStandalone();
 
    /* Report this worker in pg_stat_activity. */
    pgstat_beinit();
    pgstat_bestart_initial();
    pgstat_bestart_final();
 
    /* Attach to shared memory structures. */
    pgsa_attach();
 
    /* Set on-detach hook so that our PID will be cleared on exit. */
    before_shmem_exit(pgsa_detach_shmem, 0);
 
    /*
     * Store our PID in shared memory, unless there's already another worker
     * running, in which case just exit.
     */
    LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
    if (pgsa_state->bgworker_pid != InvalidPid)
    {
        LWLockRelease(&pgsa_state->lock);
        ereport(LOG,
                (errmsg("pg_stash_advice worker is already running under PID %d",
                        (int) pgsa_state->bgworker_pid)));
        return;
    }
    pgsa_state->bgworker_pid = MyProcPid;
    LWLockRelease(&pgsa_state->lock);
 
    /*
     * If pg_stash_advice.persist was set to true during
     * process_shared_preload_libraries() and the data has not yet been
     * successfully loaded, load it now.
     */
    if (pg_atomic_unlocked_test_flag(&pgsa_state->stashes_ready))
    {
        pgsa_read_from_disk();
        pg_atomic_test_set_flag(&pgsa_state->stashes_ready);
    }
 
    /* Note the current change count so we can detect future changes. */
    last_change_count = pg_atomic_read_u64(&pgsa_state->change_count);
 
    /* Periodically write to disk until terminated. */
    while (!ShutdownRequestPending)
    {
        /* In case of a SIGHUP, just reload the configuration. */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }
 
        if (pg_stash_advice_persist_interval <= 0)
        {
            /* Only writing at shutdown, so just wait forever. */
            (void) WaitLatch(MyLatch,
                             WL_LATCH_SET | WL_EXIT_ON_PM_DEATH,
                             -1L,
                             PG_WAIT_EXTENSION);
        }
        else
        {
            TimestampTz next_write_time;
            long        delay_in_ms;
            uint64      current_change_count;
 
            /* Compute when the next write should happen. */
            next_write_time =
                TimestampTzPlusMilliseconds(last_write_time,
                                            pg_stash_advice_persist_interval * 1000);
            delay_in_ms =
                TimestampDifferenceMilliseconds(GetCurrentTimestamp(),
                                                next_write_time);
 
            /*
             * When we reach next_write_time, we always update last_write_time
             * (which is really the time at which we last considered writing),
             * but we only actually write to disk if something has changed.
             */
            if (delay_in_ms <= 0)
            {
                current_change_count =
                    pg_atomic_read_u64(&pgsa_state->change_count);
                if (current_change_count != last_change_count)
                {
                    pgsa_write_to_disk();
                    last_change_count = current_change_count;
                }
                last_write_time = GetCurrentTimestamp();
                continue;
            }
 
            /* Sleep until the next write time. */
            (void) WaitLatch(MyLatch,
                             WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                             delay_in_ms,
                             PG_WAIT_EXTENSION);
        }
 
        ResetLatch(MyLatch);
    }
 
    /* Write one last time before exiting. */
    pgsa_write_to_disk();
}
 
/*
 * Clear our PID from shared memory on exit.
 */
static void
pgsa_detach_shmem(int code, Datum arg)
{
    LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
    if (pgsa_state->bgworker_pid == MyProcPid)
        pgsa_state->bgworker_pid = InvalidPid;
    LWLockRelease(&pgsa_state->lock);
}
 
/*
 * Load advice stash data from a dump file on disk, if there is one.
 */
static void
pgsa_read_from_disk(void)
{
    struct stat statbuf;
    FILE       *file;
    char       *filebuf;
    size_t      nread;
    char       *p;
    unsigned    lineno;
    pgsa_saved_stash_table_hash *saved_stashes;
    int         num_stashes = 0;
    pgsa_saved_entry *entries;
    int         num_entries = 0;
    int         max_entries = 64;
    MemoryContext tmpcxt;
    MemoryContext oldcxt;
 
    Assert(pgsa_entry_dshash != NULL);
 
    /*
     * Clear any existing shared memory state.
     *
     * Normally, there won't be any, but if this function was called before
     * and failed after beginning to apply changes to shared memory, then we
     * need to get rid of any entries created at that time before trying
     * again.
     */
    LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
    pgsa_reset_all_stashes();
    LWLockRelease(&pgsa_state->lock);
 
    /* Open the dump file. If it doesn't exist, we're done. */
    file = AllocateFile(PGSA_DUMP_FILE, "r");
    if (!file)
    {
        if (errno == ENOENT)
            return;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", PGSA_DUMP_FILE)));
    }
 
    /* Use a temporary context for all parse-phase allocations. */
    tmpcxt = AllocSetContextCreate(CurrentMemoryContext,
                                   "pg_stash_advice load",
                                   ALLOCSET_DEFAULT_SIZES);
    oldcxt = MemoryContextSwitchTo(tmpcxt);
 
    /* Figure out how long the file is. */
    if (fstat(fileno(file), &statbuf) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not stat file \"%s\": %m", PGSA_DUMP_FILE)));
 
    /*
     * Slurp the entire file into memory all at once.
     *
     * We could avoid this by reading the file incrementally and applying
     * changes to pgsa_stash_dshash and pgsa_entry_dshash as we go. Given the
     * lockout mechanism implemented by stashes_ready, that shouldn't have any
     * user-visible behavioral consequences, but it would consume shared
     * memory to no benefit. It seems better to buffer everything in private
     * memory first, and then only apply the changes once the file has been
     * successfully parsed in its entirety.
     *
     * That also has the advantage of possibly being more future-proof: if we
     * decide to remove the stashes_ready mechanism in the future, or say
     * allow for multiple save files, fully validating the file before
     * applying any changes will become much more important.
     *
     * Of course, this approach does have one major disadvantage, which is
     * that we'll temporarily use about twice as much memory as we're
     * ultimately going to need, but that seems like it shouldn't be a problem
     * in practice. If there's so much stashed advice that parsing the disk
     * file runs us out of memory, something has gone terribly wrong. In that
     * situation, there probably also isn't enough free memory for the
     * workload that the advice is attempting to manipulate to run
     * successfully.
     */
    filebuf = palloc_extended(statbuf.st_size + 1, MCXT_ALLOC_HUGE);
    nread = fread(filebuf, 1, statbuf.st_size, file);
    if (ferror(file))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not read file \"%s\": %m", PGSA_DUMP_FILE)));
    FreeFile(file);
    filebuf[nread] = '\0';
 
    /* Initial memory allocations. */
    saved_stashes = pgsa_saved_stash_table_create(tmpcxt, 64, NULL);
    entries = palloc(max_entries * sizeof(pgsa_saved_entry));
 
    /*
     * For memory and CPU efficiency, we parse the file in place. The end of
     * each line gets replaced with a NUL byte, and then the end of each field
     * within a line gets the same treatment. The advice string is unescaped
     * in place, and stash names and query IDs can't contain any special
     * characters. All of the resulting pointers point right back into the
     * buffer; we only need additional memory to grow the 'entries' array and
     * the 'saved_stashes' hash table.
     */
    for (p = filebuf, lineno = 1; *p != '\0'; lineno++)
    {
        char       *cursor = p;
        char       *eol;
        char       *line_type;
 
        /* Find end of line and NUL-terminate. */
        eol = strchr(p, '\n');
        if (eol != NULL)
        {
            *eol = '\0';
            p = eol + 1;
            if (eol > cursor && eol[-1] == '\r')
                eol[-1] = '\0';
        }
        else
            p += strlen(p);
 
        /* Skip empty lines. */
        if (*cursor == '\0')
            continue;
 
        /* First field is the type of line, either "stash" or "entry". */
        line_type = pgsa_next_tsv_field(&cursor);
        if (strcmp(line_type, "stash") == 0)
        {
            char       *name;
            bool        found;
 
            /* Second field should be the stash name. */
            name = pgsa_next_tsv_field(&cursor);
            if (name == NULL || *name == '\0')
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: expected stash name",
                                PGSA_DUMP_FILE, lineno)));
 
            /* No further fields are expected. */
            if (*cursor != '\0')
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: expected end of line",
                                PGSA_DUMP_FILE, lineno)));
 
            /* Duplicate check. */
            (void) pgsa_saved_stash_table_insert(saved_stashes, name, &found);
            if (found)
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: duplicate stash name \"%s\"",
                                PGSA_DUMP_FILE, lineno, name)));
            num_stashes++;
        }
        else if (strcmp(line_type, "entry") == 0)
        {
            char       *stash_name;
            char       *queryid_str;
            char       *advice_str;
            char       *endptr;
            int64       queryId;
 
            /* Second field should be the stash name. */
            stash_name = pgsa_next_tsv_field(&cursor);
            if (stash_name == NULL)
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: expected stash name",
                                PGSA_DUMP_FILE, lineno)));
 
            /* Third field should be the query ID. */
            queryid_str = pgsa_next_tsv_field(&cursor);
            if (queryid_str == NULL)
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: expected query ID",
                                PGSA_DUMP_FILE, lineno)));
 
            /* Fourth field should be the advice string. */
            advice_str = pgsa_next_tsv_field(&cursor);
            if (advice_str == NULL)
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: expected advice string",
                                PGSA_DUMP_FILE, lineno)));
 
            /* No further fields are expected. */
            if (*cursor != '\0')
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: expected end of line",
                                PGSA_DUMP_FILE, lineno)));
 
            /* Make sure the stash is one we've actually seen. */
            if (pgsa_saved_stash_table_lookup(saved_stashes,
                                              stash_name) == NULL)
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: unknown stash \"%s\"",
                                PGSA_DUMP_FILE, lineno, stash_name)));
 
            /* Parse the query ID. */
            errno = 0;
            queryId = strtoll(queryid_str, &endptr, 10);
            if (*endptr != '\0' || errno != 0 || queryid_str == endptr ||
                queryId == 0)
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: invalid query ID \"%s\"",
                                PGSA_DUMP_FILE, lineno, queryid_str)));
 
            /* Unescape the advice string. */
            pgsa_unescape_tsv_field(advice_str, PGSA_DUMP_FILE, lineno);
 
            /* Append to the entry array. */
            if (num_entries >= max_entries)
            {
                max_entries *= 2;
                entries = repalloc(entries,
                                   max_entries * sizeof(pgsa_saved_entry));
            }
            entries[num_entries].stash_name = stash_name;
            entries[num_entries].queryId = queryId;
            entries[num_entries].advice_string = advice_str;
            num_entries++;
        }
        else
        {
            ereport(ERROR,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("syntax error in file \"%s\" line %u: unrecognized line type",
                            PGSA_DUMP_FILE, lineno)));
        }
    }
 
    /*
     * Parsing succeeded. Apply everything to shared memory.
     *
     * At this point, we know that the file we just read is fully valid, but
     * it's still possible for this to fail if, for example, DSA memory cannot
     * be allocated. If that happens, the worker will die, the postmaster will
     * eventually restart it, and we'll try again after clearing any data that
     * we did manage to put into shared memory. (Note that we call
     * pgsa_reset_all_stashes() at the top of this function.)
     */
    pgsa_restore_stashes(saved_stashes);
    pgsa_restore_entries(entries, num_entries);
 
    /* Hooray, it worked! Notify the user. */
    ereport(LOG,
            (errmsg("loaded %d advice stashes and %d entries from \"%s\"",
                    num_stashes, num_entries, PGSA_DUMP_FILE)));
 
    /* Clean up. */
    MemoryContextSwitchTo(oldcxt);
    MemoryContextDelete(tmpcxt);
}
 
/*
 * Write all advice stash data to disk.
 *
 * The file format is a simple TSV with a line-type prefix:
 *   stash\tstash_name
 *   entry\tstash_name\tquery_id\tadvice_string
 */
static void
pgsa_write_to_disk(void)
{
    pgsa_writer_context wctx = {0};
    MemoryContext tmpcxt;
    MemoryContext oldcxt;
 
    Assert(pgsa_entry_dshash != NULL);
 
    /* Use a temporary context so all allocations are freed at the end. */
    tmpcxt = AllocSetContextCreate(CurrentMemoryContext,
                                   "pg_stash_advice dump",
                                   ALLOCSET_DEFAULT_SIZES);
    oldcxt = MemoryContextSwitchTo(tmpcxt);
 
    /* Set up the writer context. */
    snprintf(wctx.pathname, MAXPGPATH, "%s.tmp", PGSA_DUMP_FILE);
    wctx.file = AllocateFile(wctx.pathname, "w");
    if (!wctx.file)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", wctx.pathname)));
    wctx.nhash = pgsa_stash_name_table_create(tmpcxt, 64, NULL);
    initStringInfo(&wctx.buf);
 
    /* Write stash lines, then entry lines. */
    pgsa_write_stashes(&wctx);
    pgsa_write_entries(&wctx);
 
    /*
     * If nothing was written, remove both the temp file and any existing dump
     * file rather than installing a zero-length file.
     */
    if (wctx.nhash->members == 0)
    {
        ereport(DEBUG1,
                errmsg("there are no advice stashes to save"));
        FreeFile(wctx.file);
        unlink(wctx.pathname);
        if (unlink(PGSA_DUMP_FILE) == 0)
            ereport(DEBUG1,
                    errmsg("removed \"%s\"", PGSA_DUMP_FILE));
    }
    else
    {
        if (FreeFile(wctx.file) != 0)
        {
            int         save_errno = errno;
 
            unlink(wctx.pathname);
            errno = save_errno;
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not close file \"%s\": %m",
                            wctx.pathname)));
        }
        (void) durable_rename(wctx.pathname, PGSA_DUMP_FILE, ERROR);
 
        ereport(LOG,
                errmsg("saved %d advice stashes and %d entries to \"%s\"",
                       (int) wctx.nhash->members, wctx.entries_written,
                       PGSA_DUMP_FILE));
    }
 
    MemoryContextSwitchTo(oldcxt);
    MemoryContextDelete(tmpcxt);
}
 
/*
 * Append the TSV-escaped form of str to buf.
 *
 * Backslash, tab, newline, and carriage return are escaped with backslash
 * sequences.  All other characters are passed through unchanged.
 */
static void
pgsa_append_tsv_escaped_string(StringInfo buf, const char *str)
{
    for (const char *p = str; *p != '\0'; p++)
    {
        switch (*p)
        {
            case '\\':
                appendStringInfoString(buf, "\\\\");
                break;
            case '\t':
                appendStringInfoString(buf, "\\t");
                break;
            case '\n':
                appendStringInfoString(buf, "\\n");
                break;
            case '\r':
                appendStringInfoString(buf, "\\r");
                break;
            default:
                appendStringInfoChar(buf, *p);
                break;
        }
    }
}
 
/*
 * Extract the next tab-delimited field from *cursor.
 *
 * The tab delimiter is replaced with '\0' and *cursor is advanced past it.
 * If *cursor already points to '\0' (no more fields), returns NULL.
 */
static char *
pgsa_next_tsv_field(char **cursor)
{
    char       *start = *cursor;
    char       *p = start;
 
    if (*p == '\0')
        return NULL;
 
    while (*p != '\0' && *p != '\t')
        p++;
 
    if (*p == '\t')
        *p++ = '\0';
 
    *cursor = p;
    return start;
}
 
/*
 * Insert entries into shared memory from the parsed entry array.
 */
static void
pgsa_restore_entries(pgsa_saved_entry *entries, int num_entries)
{
    LWLockAcquire(&pgsa_state->lock, LW_SHARED);
    for (int i = 0; i < num_entries; i++)
    {
        ereport(DEBUG2,
                errmsg("restoring advice stash entry for \"%s\", query ID %" PRId64,
                       entries[i].stash_name, entries[i].queryId));
        pgsa_set_advice_string(entries[i].stash_name,
                               entries[i].queryId,
                               entries[i].advice_string);
    }
    LWLockRelease(&pgsa_state->lock);
}
 
/*
 * Create stashes in shared memory from the parsed stash hash table.
 */
static void
pgsa_restore_stashes(pgsa_saved_stash_table_hash *saved_stashes)
{
    pgsa_saved_stash_table_iterator iter;
    pgsa_saved_stash *s;
 
    LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
    pgsa_saved_stash_table_start_iterate(saved_stashes, &iter);
    while ((s = pgsa_saved_stash_table_iterate(saved_stashes,
                                               &iter)) != NULL)
    {
        ereport(DEBUG2,
                errmsg("restoring advice stash \"%s\"", s->name));
        pgsa_create_stash(s->name);
    }
    LWLockRelease(&pgsa_state->lock);
}
 
/*
 * Unescape a TSV field in place.
 *
 * Recognized escape sequences are \\, \t, \n, and \r.  A trailing backslash
 * or an unrecognized escape sequence is a syntax error.
 */
static void
pgsa_unescape_tsv_field(char *str, const char *filename, unsigned lineno)
{
    char       *src = str;
    char       *dst = str;
 
    while (*src != '\0')
    {
        /* Just pass through anything that's not a backslash-escape. */
        if (likely(*src != '\\'))
        {
            *dst++ = *src++;
            continue;
        }
 
        /* Check what sort of escape we've got. */
        switch (src[1])
        {
            case '\\':
                *dst++ = '\\';
                break;
            case 't':
                *dst++ = '\t';
                break;
            case 'n':
                *dst++ = '\n';
                break;
            case 'r':
                *dst++ = '\r';
                break;
            case '\0':
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: trailing backslash",
                                filename, lineno)));
                break;
            default:
                ereport(ERROR,
                        (errcode(ERRCODE_DATA_CORRUPTED),
                         errmsg("syntax error in file \"%s\" line %u: unrecognized escape \"\\%c\"",
                                filename, lineno, src[1])));
                break;
        }
 
        /* We consumed the backslash and the following character. */
        src += 2;
    }
    *dst = '\0';
}
 
/*
 * Write an entry line for each advice entry.
 */
static void
pgsa_write_entries(pgsa_writer_context *wctx)
{
    dshash_seq_status iter;
    pgsa_entry *entry;
 
    dshash_seq_init(&iter, pgsa_entry_dshash, false);
    while ((entry = dshash_seq_next(&iter)) != NULL)
    {
        pgsa_stash_name *n;
        char       *advice_string;
 
        if (entry->advice_string == InvalidDsaPointer)
            continue;
 
        n = pgsa_stash_name_table_lookup(wctx->nhash,
                                         entry->key.pgsa_stash_id);
        if (n == NULL)
            continue;           /* orphan entry, skip */
 
        advice_string = dsa_get_address(pgsa_dsa_area, entry->advice_string);
 
        resetStringInfo(&wctx->buf);
        appendStringInfo(&wctx->buf, "entry\t%s\t%" PRId64 "\t",
                         n->name, entry->key.queryId);
        pgsa_append_tsv_escaped_string(&wctx->buf, advice_string);
        appendStringInfoChar(&wctx->buf, '\n');
        fwrite(wctx->buf.data, 1, wctx->buf.len, wctx->file);
        if (ferror(wctx->file))
            pgsa_write_error(wctx);
        wctx->entries_written++;
    }
    dshash_seq_term(&iter);
}
 
/*
 * Clean up and report a write error.  Does not return.
 */
static void
pgsa_write_error(pgsa_writer_context *wctx)
{
    int         save_errno = errno;
 
    FreeFile(wctx->file);
    unlink(wctx->pathname);
    errno = save_errno;
    ereport(ERROR,
            (errcode_for_file_access(),
             errmsg("could not write to file \"%s\": %m", wctx->pathname)));
}
 
/*
 * Write a stash line for each advice stash, and populate the ID-to-name
 * hash table for use by pgsa_write_entries.
 */
static void
pgsa_write_stashes(pgsa_writer_context *wctx)
{
    dshash_seq_status iter;
    pgsa_stash *stash;
 
    dshash_seq_init(&iter, pgsa_stash_dshash, false);
    while ((stash = dshash_seq_next(&iter)) != NULL)
    {
        pgsa_stash_name *n;
        bool        found;
 
        n = pgsa_stash_name_table_insert(wctx->nhash, stash->pgsa_stash_id,
                                         &found);
        Assert(!found);
        n->name = pstrdup(stash->name);
 
        resetStringInfo(&wctx->buf);
        appendStringInfo(&wctx->buf, "stash\t%s\n", n->name);
        fwrite(wctx->buf.data, 1, wctx->buf.len, wctx->file);
        if (ferror(wctx->file))
            pgsa_write_error(wctx);
    }
    dshash_seq_term(&iter);
}