test_custom_var_stats.c

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/*------------------------------------------------------------------------------------
 *
 * test_custom_var_stats.c
 *      Test module for variable-sized custom pgstats
 *
 * Copyright (c) 2025-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      src/test/modules/test_custom_var_stats/test_custom_var_stats.c
 *
 * ------------------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "common/hashfn.h"
#include "funcapi.h"
#include "storage/dsm_registry.h"
#include "utils/builtins.h"
#include "utils/pgstat_internal.h"
 
PG_MODULE_MAGIC_EXT(
                    .name = "test_custom_var_stats",
                    .version = PG_VERSION
);
 
#define TEST_CUSTOM_VAR_MAGIC_NUMBER (0xBEEFBEEF)
 
/*--------------------------------------------------------------------------
 * Macros and constants
 *--------------------------------------------------------------------------
 */
 
/*
 * Kind ID for test_custom_var_stats statistics.
 */
#define PGSTAT_KIND_TEST_CUSTOM_VAR_STATS 25
 
/* File paths for auxiliary data serialization */
#define TEST_CUSTOM_AUX_DATA_DESC "pg_stat/test_custom_var_stats_desc.stats"
 
/*
 * Hash statistic name to generate entry index for pgstat lookup.
 */
#define PGSTAT_CUSTOM_VAR_STATS_IDX(name) hash_bytes_extended((const unsigned char *) name, strlen(name), 0)
 
/*--------------------------------------------------------------------------
 * Type definitions
 *--------------------------------------------------------------------------
 */
 
/* Backend-local pending statistics before flush to shared memory */
typedef struct PgStat_StatCustomVarEntry
{
    PgStat_Counter numcalls;    /* times statistic was incremented */
} PgStat_StatCustomVarEntry;
 
/* Shared memory statistics entry visible to all backends */
typedef struct PgStatShared_CustomVarEntry
{
    PgStatShared_Common header; /* standard pgstat entry header */
    PgStat_StatCustomVarEntry stats;    /* custom statistics data */
    dsa_pointer description;    /* pointer to description string in DSA */
} PgStatShared_CustomVarEntry;
 
/*--------------------------------------------------------------------------
 * Global Variables
 *--------------------------------------------------------------------------
 */
 
/* File handle for auxiliary data serialization */
static FILE *fd_description = NULL;
 
/* Current write offset in fd_description file */
static pgoff_t fd_description_offset = 0;
 
/* DSA area for storing variable-length description strings */
static dsa_area *custom_stats_description_dsa = NULL;
 
/*--------------------------------------------------------------------------
 * Function prototypes
 *--------------------------------------------------------------------------
 */
 
/* Flush callback: merge pending stats into shared memory */
static bool test_custom_stats_var_flush_pending_cb(PgStat_EntryRef *entry_ref,
                                                   bool nowait);
 
/* Serialization callback: write auxiliary entry data */
static void test_custom_stats_var_to_serialized_data(const PgStat_HashKey *key,
                                                     const PgStatShared_Common *header,
                                                     FILE *statfile);
 
/* Deserialization callback: read auxiliary entry data */
static bool test_custom_stats_var_from_serialized_data(const PgStat_HashKey *key,
                                                       PgStatShared_Common *header,
                                                       FILE *statfile);
 
/* Finish callback: end of statistics file operations */
static void test_custom_stats_var_finish(PgStat_StatsFileOp status);
 
/*--------------------------------------------------------------------------
 * Custom kind configuration
 *--------------------------------------------------------------------------
 */
 
static const PgStat_KindInfo custom_stats = {
    .name = "test_custom_var_stats",
    .fixed_amount = false,      /* variable number of entries */
    .write_to_file = true,      /* persist across restarts */
    .track_entry_count = true,  /* count active entries */
    .accessed_across_databases = true,  /* global statistics */
    .shared_size = sizeof(PgStatShared_CustomVarEntry),
    .shared_data_off = offsetof(PgStatShared_CustomVarEntry, stats),
    .shared_data_len = sizeof(((PgStatShared_CustomVarEntry *) 0)->stats),
    .pending_size = sizeof(PgStat_StatCustomVarEntry),
    .flush_pending_cb = test_custom_stats_var_flush_pending_cb,
    .to_serialized_data = test_custom_stats_var_to_serialized_data,
    .from_serialized_data = test_custom_stats_var_from_serialized_data,
    .finish = test_custom_stats_var_finish,
};
 
/*--------------------------------------------------------------------------
 * Module initialization
 *--------------------------------------------------------------------------
 */
 
void
_PG_init(void)
{
    /* Must be loaded via shared_preload_libraries */
    if (!process_shared_preload_libraries_in_progress)
        return;
 
    /* Register custom statistics kind */
    pgstat_register_kind(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS, &custom_stats);
}
 
/*--------------------------------------------------------------------------
 * Statistics callback functions
 *--------------------------------------------------------------------------
 */
 
/*
 * test_custom_stats_var_flush_pending_cb
 *      Merge pending backend statistics into shared memory
 *
 * Called by pgstat collector to flush accumulated local statistics
 * to shared memory where other backends can read them.
 *
 * Returns false only if nowait=true and lock acquisition fails.
 */
static bool
test_custom_stats_var_flush_pending_cb(PgStat_EntryRef *entry_ref, bool nowait)
{
    PgStat_StatCustomVarEntry *pending_entry;
    PgStatShared_CustomVarEntry *shared_entry;
 
    pending_entry = (PgStat_StatCustomVarEntry *) entry_ref->pending;
    shared_entry = (PgStatShared_CustomVarEntry *) entry_ref->shared_stats;
 
    if (!pgstat_lock_entry(entry_ref, nowait))
        return false;
 
    /* Add pending counts to shared totals */
    shared_entry->stats.numcalls += pending_entry->numcalls;
 
    pgstat_unlock_entry(entry_ref);
 
    return true;
}
 
/*
 * test_custom_stats_var_to_serialized_data() -
 *
 * Serialize auxiliary data (descriptions) for custom statistics entries
 * to a secondary statistics file. This is called while writing the statistics
 * to disk.
 *
 * This callback writes a mix of data within the main pgstats file and a
 * secondary statistics file.  The following data is written to the main file for
 * each entry:
 * - An arbitrary magic number.
 * - An offset.  This is used to know the location we need to look at
 * to retrieve the information from the second file.
 *
 * The following data is written to the secondary statistics file:
 * - The entry key, cross-checked with the data from the main file
 * when reloaded.
 * - The length of the description.
 * - The description data itself.
 */
static void
test_custom_stats_var_to_serialized_data(const PgStat_HashKey *key,
                                         const PgStatShared_Common *header,
                                         FILE *statfile)
{
    char       *description;
    size_t      len;
    const PgStatShared_CustomVarEntry *entry = (const PgStatShared_CustomVarEntry *) header;
    bool        found;
    uint32      magic_number = TEST_CUSTOM_VAR_MAGIC_NUMBER;
 
    /*
     * First mark the main file with a magic number, keeping a trace that some
     * auxiliary data will exist in the secondary statistics file.
     */
    pgstat_write_chunk_s(statfile, &magic_number);
 
    /* Open statistics file for writing. */
    if (!fd_description)
    {
        fd_description = AllocateFile(TEST_CUSTOM_AUX_DATA_DESC, PG_BINARY_W);
        if (fd_description == NULL)
        {
            ereport(LOG,
                    (errcode_for_file_access(),
                     errmsg("could not open statistics file \"%s\" for writing: %m",
                            TEST_CUSTOM_AUX_DATA_DESC)));
            return;
        }
 
        /* Initialize offset for secondary statistics file. */
        fd_description_offset = 0;
    }
 
    /* Write offset to the main data file */
    pgstat_write_chunk_s(statfile, &fd_description_offset);
 
    /*
     * First write the entry key to the secondary statistics file.  This will
     * be cross-checked with the key read from main stats file at loading
     * time.
     */
    pgstat_write_chunk_s(fd_description, (PgStat_HashKey *) key);
    fd_description_offset += sizeof(PgStat_HashKey);
 
    if (!custom_stats_description_dsa)
        custom_stats_description_dsa = GetNamedDSA("test_custom_stat_dsa", &found);
 
    /* Handle entries without descriptions */
    if (!DsaPointerIsValid(entry->description) || !custom_stats_description_dsa)
    {
        /* length to description file */
        len = 0;
        pgstat_write_chunk_s(fd_description, &len);
        fd_description_offset += sizeof(size_t);
        return;
    }
 
    /*
     * Retrieve description from DSA, then write the length followed by the
     * description.
     */
    description = dsa_get_address(custom_stats_description_dsa,
                                  entry->description);
    len = strlen(description) + 1;
    pgstat_write_chunk_s(fd_description, &len);
    pgstat_write_chunk(fd_description, description, len);
 
    /*
     * Update offset for next entry, counting for the length (size_t) of the
     * description and the description contents.
     */
    fd_description_offset += len + sizeof(size_t);
}
 
/*
 * test_custom_stats_var_from_serialized_data() -
 *
 * Read auxiliary data (descriptions) for custom statistics entries from
 * the secondary statistics file.  This is called while loading the statistics
 * at startup.
 *
 * See the top of test_custom_stats_var_to_serialized_data() for a
 * detailed description of the data layout read here.
 */
static bool
test_custom_stats_var_from_serialized_data(const PgStat_HashKey *key,
                                           PgStatShared_Common *header,
                                           FILE *statfile)
{
    PgStatShared_CustomVarEntry *entry;
    dsa_pointer dp;
    size_t      len;
    pgoff_t     offset;
    char       *buffer;
    bool        found;
    uint32      magic_number = 0;
    PgStat_HashKey file_key;
 
    /* Check the magic number first, in the main file. */
    if (!pgstat_read_chunk_s(statfile, &magic_number))
    {
        elog(WARNING, "failed to read magic number from statistics file");
        return false;
    }
 
    if (magic_number != TEST_CUSTOM_VAR_MAGIC_NUMBER)
    {
        elog(WARNING, "found magic number %u from statistics file, should be %u",
             magic_number, TEST_CUSTOM_VAR_MAGIC_NUMBER);
        return false;
    }
 
    /*
     * Read the offset from the main stats file, to be able to read the
     * auxiliary data from the secondary statistics file.
     */
    if (!pgstat_read_chunk_s(statfile, &offset))
    {
        elog(WARNING, "failed to read metadata offset from statistics file");
        return false;
    }
 
    /* Open statistics file for reading if not already open */
    if (!fd_description)
    {
        fd_description = AllocateFile(TEST_CUSTOM_AUX_DATA_DESC, PG_BINARY_R);
        if (fd_description == NULL)
        {
            if (errno != ENOENT)
                ereport(LOG,
                        (errcode_for_file_access(),
                         errmsg("could not open statistics file \"%s\" for reading: %m",
                                TEST_CUSTOM_AUX_DATA_DESC)));
            pgstat_reset_of_kind(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS);
            return false;
        }
    }
 
    /* Read data from the secondary statistics file, at the specified offset */
    if (fseeko(fd_description, offset, SEEK_SET) != 0)
    {
        elog(WARNING, "could not seek in file \"%s\": %m",
             TEST_CUSTOM_AUX_DATA_DESC);
        return false;
    }
 
    /* Read the hash key from the secondary statistics file */
    if (!pgstat_read_chunk_s(fd_description, &file_key))
    {
        elog(WARNING, "failed to read hash key from file");
        return false;
    }
 
    /* Check key consistency */
    if (file_key.kind != key->kind ||
        file_key.dboid != key->dboid ||
        file_key.objid != key->objid)
    {
        elog(WARNING, "found entry key %u/%u/%" PRIu64 " not matching with %u/%u/%" PRIu64,
             file_key.kind, file_key.dboid, file_key.objid,
             key->kind, key->dboid, key->objid);
        return false;
    }
 
    entry = (PgStatShared_CustomVarEntry *) header;
 
    /* Read the description length and its data */
    if (!pgstat_read_chunk_s(fd_description, &len))
    {
        elog(WARNING, "failed to read metadata length from statistics file");
        return false;
    }
 
    /* Handle empty descriptions */
    if (len == 0)
    {
        entry->description = InvalidDsaPointer;
        return true;
    }
 
    /* Initialize DSA if needed */
    if (!custom_stats_description_dsa)
        custom_stats_description_dsa = GetNamedDSA("test_custom_stat_dsa", &found);
 
    if (!custom_stats_description_dsa)
    {
        elog(WARNING, "could not access DSA for custom statistics descriptions");
        return false;
    }
 
    buffer = palloc(len);
    if (!pgstat_read_chunk(fd_description, buffer, len))
    {
        pfree(buffer);
        elog(WARNING, "failed to read description from file");
        return false;
    }
 
    /* Allocate space in DSA and copy the description */
    dp = dsa_allocate(custom_stats_description_dsa, len);
    memcpy(dsa_get_address(custom_stats_description_dsa, dp), buffer, len);
    entry->description = dp;
    pfree(buffer);
 
    return true;
}
 
/*
 * test_custom_stats_var_finish() -
 *
 * Cleanup function called at the end of statistics file operations.
 * Handles closing files and cleanup based on the operation type.
 */
static void
test_custom_stats_var_finish(PgStat_StatsFileOp status)
{
    switch (status)
    {
        case STATS_WRITE:
            if (!fd_description)
                return;
 
            fd_description_offset = 0;
 
            /* Check for write errors and cleanup if necessary */
            if (ferror(fd_description))
            {
                ereport(LOG,
                        (errcode_for_file_access(),
                         errmsg("could not write to file \"%s\": %m",
                                TEST_CUSTOM_AUX_DATA_DESC)));
                FreeFile(fd_description);
                unlink(TEST_CUSTOM_AUX_DATA_DESC);
            }
            else if (FreeFile(fd_description) < 0)
            {
                ereport(LOG,
                        (errcode_for_file_access(),
                         errmsg("could not close file \"%s\": %m",
                                TEST_CUSTOM_AUX_DATA_DESC)));
                unlink(TEST_CUSTOM_AUX_DATA_DESC);
            }
            break;
 
        case STATS_READ:
            if (fd_description)
                FreeFile(fd_description);
 
            /* Remove the file after reading */
            elog(DEBUG2, "removing file \"%s\"", TEST_CUSTOM_AUX_DATA_DESC);
            unlink(TEST_CUSTOM_AUX_DATA_DESC);
            break;
 
        case STATS_DISCARD:
            {
                int         ret;
 
                /* Attempt to remove the file */
                ret = unlink(TEST_CUSTOM_AUX_DATA_DESC);
                if (ret != 0)
                {
                    if (errno == ENOENT)
                        elog(LOG,
                             "didn't need to unlink file \"%s\" - didn't exist",
                             TEST_CUSTOM_AUX_DATA_DESC);
                    else
                        ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not unlink file \"%s\": %m",
                                        TEST_CUSTOM_AUX_DATA_DESC)));
                }
                else
                {
                    ereport(LOG,
                            (errmsg_internal("unlinked file \"%s\"",
                                             TEST_CUSTOM_AUX_DATA_DESC)));
                }
            }
            break;
    }
 
    fd_description = NULL;
}
 
/*--------------------------------------------------------------------------
 * Helper functions
 *--------------------------------------------------------------------------
 */
 
/*
 * test_custom_stats_var_fetch_entry
 *      Look up custom statistic by name
 *
 * Returns statistics entry from shared memory, or NULL if not found.
 */
static PgStat_StatCustomVarEntry *
test_custom_stats_var_fetch_entry(const char *stat_name)
{
    /* Fetch entry by hashed name */
    return (PgStat_StatCustomVarEntry *)
        pgstat_fetch_entry(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS,
                           InvalidOid,
                           PGSTAT_CUSTOM_VAR_STATS_IDX(stat_name));
}
 
/*--------------------------------------------------------------------------
 * SQL-callable functions
 *--------------------------------------------------------------------------
 */
 
/*
 * test_custom_stats_var_create
 *      Create new custom statistic entry
 *
 * Initializes a statistics entry with the given name and description.
 */
PG_FUNCTION_INFO_V1(test_custom_stats_var_create);
Datum
test_custom_stats_var_create(PG_FUNCTION_ARGS)
{
    PgStat_EntryRef *entry_ref;
    PgStatShared_CustomVarEntry *shared_entry;
    char       *stat_name = text_to_cstring(PG_GETARG_TEXT_PP(0));
    char       *description = text_to_cstring(PG_GETARG_TEXT_PP(1));
    dsa_pointer dp = InvalidDsaPointer;
    bool        found;
 
    /* Validate name length first */
    if (strlen(stat_name) >= NAMEDATALEN)
        ereport(ERROR,
                (errcode(ERRCODE_NAME_TOO_LONG),
                 errmsg("custom statistic name \"%s\" is too long", stat_name),
                 errdetail("Name must be less than %d characters.", NAMEDATALEN)));
 
    /* Initialize DSA and description provided */
    if (!custom_stats_description_dsa)
        custom_stats_description_dsa = GetNamedDSA("test_custom_stat_dsa", &found);
 
    if (!custom_stats_description_dsa)
        ereport(ERROR,
                (errmsg("could not access DSA for custom statistics descriptions")));
 
    /* Allocate space in DSA and copy description */
    dp = dsa_allocate(custom_stats_description_dsa, strlen(description) + 1);
    memcpy(dsa_get_address(custom_stats_description_dsa, dp),
           description,
           strlen(description) + 1);
 
    /* Create or get existing entry */
    entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS, InvalidOid,
                                            PGSTAT_CUSTOM_VAR_STATS_IDX(stat_name), true);
 
    if (!entry_ref)
        PG_RETURN_VOID();
 
    shared_entry = (PgStatShared_CustomVarEntry *) entry_ref->shared_stats;
 
    /* Zero-initialize statistics */
    memset(&shared_entry->stats, 0, sizeof(shared_entry->stats));
 
    /* Store description pointer */
    shared_entry->description = dp;
 
    pgstat_unlock_entry(entry_ref);
 
    PG_RETURN_VOID();
}
 
/*
 * test_custom_stats_var_update
 *      Increment custom statistic counter
 *
 * Increments call count in backend-local memory.  Changes are flushed
 * to shared memory by the statistics collector.
 */
PG_FUNCTION_INFO_V1(test_custom_stats_var_update);
Datum
test_custom_stats_var_update(PG_FUNCTION_ARGS)
{
    char       *stat_name = text_to_cstring(PG_GETARG_TEXT_PP(0));
    PgStat_EntryRef *entry_ref;
    PgStat_StatCustomVarEntry *pending_entry;
 
    /* Get pending entry in local memory */
    entry_ref = pgstat_prep_pending_entry(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS, InvalidOid,
                                          PGSTAT_CUSTOM_VAR_STATS_IDX(stat_name), NULL);
 
    pending_entry = (PgStat_StatCustomVarEntry *) entry_ref->pending;
    pending_entry->numcalls++;
 
    PG_RETURN_VOID();
}
 
/*
 * test_custom_stats_var_drop
 *      Remove custom statistic entry
 *
 * Drops the named statistic from shared memory.
 */
PG_FUNCTION_INFO_V1(test_custom_stats_var_drop);
Datum
test_custom_stats_var_drop(PG_FUNCTION_ARGS)
{
    char       *stat_name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
    /* Drop entry and request GC if the entry could not be freed */
    if (!pgstat_drop_entry(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS, InvalidOid,
                           PGSTAT_CUSTOM_VAR_STATS_IDX(stat_name)))
        pgstat_request_entry_refs_gc();
 
    PG_RETURN_VOID();
}
 
/*
 * test_custom_stats_var_report
 *      Retrieve custom statistic values
 *
 * Returns single row with statistic name, call count, and description if the
 * statistic exists, otherwise returns no rows.
 */
PG_FUNCTION_INFO_V1(test_custom_stats_var_report);
Datum
test_custom_stats_var_report(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    char       *stat_name;
    PgStat_StatCustomVarEntry *stat_entry;
 
    if (SRF_IS_FIRSTCALL())
    {
        TupleDesc   tupdesc;
        MemoryContext oldcontext;
 
        /* Initialize SRF context */
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
 
        /* Get composite return type */
        if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
            elog(ERROR, "test_custom_stats_var_report: return type is not composite");
 
        funcctx->tuple_desc = BlessTupleDesc(tupdesc);
        funcctx->max_calls = 1; /* single row result */
 
        MemoryContextSwitchTo(oldcontext);
    }
 
    funcctx = SRF_PERCALL_SETUP();
 
    if (funcctx->call_cntr < funcctx->max_calls)
    {
        Datum       values[3];
        bool        nulls[3] = {false, false, false};
        HeapTuple   tuple;
        PgStat_EntryRef *entry_ref;
        PgStatShared_CustomVarEntry *shared_entry;
        char       *description = NULL;
        bool        found;
 
        stat_name = text_to_cstring(PG_GETARG_TEXT_PP(0));
        stat_entry = test_custom_stats_var_fetch_entry(stat_name);
 
        /* Return row only if entry exists */
        if (stat_entry)
        {
            /* Get entry ref to access shared entry */
            entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_TEST_CUSTOM_VAR_STATS, InvalidOid,
                                             PGSTAT_CUSTOM_VAR_STATS_IDX(stat_name), false, NULL);
 
            if (entry_ref)
            {
                shared_entry = (PgStatShared_CustomVarEntry *) entry_ref->shared_stats;
 
                /* Get description from DSA if available */
                if (DsaPointerIsValid(shared_entry->description))
                {
                    if (!custom_stats_description_dsa)
                        custom_stats_description_dsa = GetNamedDSA("test_custom_stat_dsa", &found);
 
                    if (custom_stats_description_dsa)
                        description = dsa_get_address(custom_stats_description_dsa, shared_entry->description);
                }
            }
 
            values[0] = PointerGetDatum(cstring_to_text(stat_name));
            values[1] = Int64GetDatum(stat_entry->numcalls);
 
            if (description)
                values[2] = PointerGetDatum(cstring_to_text(description));
            else
                nulls[2] = true;
 
            tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
            SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
        }
    }
 
    SRF_RETURN_DONE(funcctx);
}