dsm_registry.c

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/*-------------------------------------------------------------------------
 *
 * dsm_registry.c
 *    Functions for interfacing with the dynamic shared memory registry.
 *
 * This provides a way for libraries to use shared memory without needing
 * to request it at startup time via a shmem_request_hook.  The registry
 * stores dynamic shared memory (DSM) segment handles keyed by a
 * library-specified string.
 *
 * The registry is accessed by calling GetNamedDSMSegment().  If a segment
 * with the provided name does not yet exist, it is created and initialized
 * with the provided init_callback callback function.  Otherwise,
 * GetNamedDSMSegment() simply ensures that the segment is attached to the
 * current backend.  This function guarantees that only one backend
 * initializes the segment and that all other backends just attach it.
 *
 * A DSA can be created in or retrieved from the registry by calling
 * GetNamedDSA().  As with GetNamedDSMSegment(), if a DSA with the provided
 * name does not yet exist, it is created.  Otherwise, GetNamedDSA()
 * ensures the DSA is attached to the current backend.  This function
 * guarantees that only one backend initializes the DSA and that all other
 * backends just attach it.
 *
 * A dshash table can be created in or retrieved from the registry by
 * calling GetNamedDSHash().  As with GetNamedDSMSegment(), if a hash
 * table with the provided name does not yet exist, it is created.
 * Otherwise, GetNamedDSHash() ensures the hash table is attached to the
 * current backend.  This function guarantees that only one backend
 * initializes the table and that all other backends just attach it.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/dsm_registry.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "funcapi.h"
#include "lib/dshash.h"
#include "storage/dsm_registry.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
 
typedef struct DSMRegistryCtxStruct
{
    dsa_handle  dsah;
    dshash_table_handle dshh;
} DSMRegistryCtxStruct;
 
static DSMRegistryCtxStruct *DSMRegistryCtx;
 
typedef struct NamedDSMState
{
    dsm_handle  handle;
    size_t      size;
} NamedDSMState;
 
typedef struct NamedDSAState
{
    dsa_handle  handle;
    int         tranche;
} NamedDSAState;
 
typedef struct NamedDSHState
{
    dsa_handle dsa_handle;
    dshash_table_handle dsh_handle;
    int         tranche;
} NamedDSHState;
 
typedef enum DSMREntryType
{
    DSMR_ENTRY_TYPE_DSM,
    DSMR_ENTRY_TYPE_DSA,
    DSMR_ENTRY_TYPE_DSH,
} DSMREntryType;
 
static const char *const DSMREntryTypeNames[] =
{
    [DSMR_ENTRY_TYPE_DSM] = "segment",
    [DSMR_ENTRY_TYPE_DSA] = "area",
    [DSMR_ENTRY_TYPE_DSH] = "hash",
};
 
typedef struct DSMRegistryEntry
{
    char        name[NAMEDATALEN];
    DSMREntryType type;
    union
    {
        NamedDSMState dsm;
        NamedDSAState dsa;
        NamedDSHState dsh;
    };
} DSMRegistryEntry;
 
static const dshash_parameters dsh_params = {
    offsetof(DSMRegistryEntry, type),
    sizeof(DSMRegistryEntry),
    dshash_strcmp,
    dshash_strhash,
    dshash_strcpy,
    LWTRANCHE_DSM_REGISTRY_HASH
};
 
static dsa_area *dsm_registry_dsa;
static dshash_table *dsm_registry_table;
 
Size
DSMRegistryShmemSize(void)
{
    return MAXALIGN(sizeof(DSMRegistryCtxStruct));
}
 
void
DSMRegistryShmemInit(void)
{
    bool        found;
 
    DSMRegistryCtx = (DSMRegistryCtxStruct *)
        ShmemInitStruct("DSM Registry Data",
                        DSMRegistryShmemSize(),
                        &found);
 
    if (!found)
    {
        DSMRegistryCtx->dsah = DSA_HANDLE_INVALID;
        DSMRegistryCtx->dshh = DSHASH_HANDLE_INVALID;
    }
}
 
/*
 * Initialize or attach to the dynamic shared hash table that stores the DSM
 * registry entries, if not already done.  This must be called before accessing
 * the table.
 */
static void
init_dsm_registry(void)
{
    /* Quick exit if we already did this. */
    if (dsm_registry_table)
        return;
 
    /* Otherwise, use a lock to ensure only one process creates the table. */
    LWLockAcquire(DSMRegistryLock, LW_EXCLUSIVE);
 
    if (DSMRegistryCtx->dshh == DSHASH_HANDLE_INVALID)
    {
        /* Initialize dynamic shared hash table for registry. */
        dsm_registry_dsa = dsa_create(LWTRANCHE_DSM_REGISTRY_DSA);
        dsm_registry_table = dshash_create(dsm_registry_dsa, &dsh_params, NULL);
 
        dsa_pin(dsm_registry_dsa);
        dsa_pin_mapping(dsm_registry_dsa);
 
        /* Store handles in shared memory for other backends to use. */
        DSMRegistryCtx->dsah = dsa_get_handle(dsm_registry_dsa);
        DSMRegistryCtx->dshh = dshash_get_hash_table_handle(dsm_registry_table);
    }
    else
    {
        /* Attach to existing dynamic shared hash table. */
        dsm_registry_dsa = dsa_attach(DSMRegistryCtx->dsah);
        dsa_pin_mapping(dsm_registry_dsa);
        dsm_registry_table = dshash_attach(dsm_registry_dsa, &dsh_params,
                                           DSMRegistryCtx->dshh, NULL);
    }
 
    LWLockRelease(DSMRegistryLock);
}
 
/*
 * Initialize or attach a named DSM segment.
 *
 * This routine returns the address of the segment.  init_callback is called to
 * initialize the segment when it is first created.
 */
void *
GetNamedDSMSegment(const char *name, size_t size,
                   void (*init_callback) (void *ptr), bool *found)
{
    DSMRegistryEntry *entry;
    MemoryContext oldcontext;
    void       *ret;
    NamedDSMState *state;
    dsm_segment *seg;
 
    Assert(found);
 
    if (!name || *name == '\0')
        ereport(ERROR,
                (errmsg("DSM segment name cannot be empty")));
 
    if (strlen(name) >= offsetof(DSMRegistryEntry, type))
        ereport(ERROR,
                (errmsg("DSM segment name too long")));
 
    if (size == 0)
        ereport(ERROR,
                (errmsg("DSM segment size must be nonzero")));
 
    /* Be sure any local memory allocated by DSM/DSA routines is persistent. */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 
    /* Connect to the registry. */
    init_dsm_registry();
 
    entry = dshash_find_or_insert(dsm_registry_table, name, found);
    state = &entry->dsm;
    if (!(*found))
    {
        entry->type = DSMR_ENTRY_TYPE_DSM;
        state->handle = DSM_HANDLE_INVALID;
        state->size = size;
    }
    else if (entry->type != DSMR_ENTRY_TYPE_DSM)
        ereport(ERROR,
                (errmsg("requested DSM segment does not match type of existing entry")));
    else if (state->size != size)
        ereport(ERROR,
                (errmsg("requested DSM segment size does not match size of existing segment")));
 
    if (state->handle == DSM_HANDLE_INVALID)
    {
        *found = false;
 
        /* Initialize the segment. */
        seg = dsm_create(size, 0);
 
        if (init_callback)
            (*init_callback) (dsm_segment_address(seg));
 
        dsm_pin_segment(seg);
        dsm_pin_mapping(seg);
        state->handle = dsm_segment_handle(seg);
    }
    else
    {
        /* If the existing segment is not already attached, attach it now. */
        seg = dsm_find_mapping(state->handle);
        if (seg == NULL)
        {
            seg = dsm_attach(state->handle);
            if (seg == NULL)
                elog(ERROR, "could not map dynamic shared memory segment");
 
            dsm_pin_mapping(seg);
        }
    }
 
    ret = dsm_segment_address(seg);
    dshash_release_lock(dsm_registry_table, entry);
    MemoryContextSwitchTo(oldcontext);
 
    return ret;
}
 
/*
 * Initialize or attach a named DSA.
 *
 * This routine returns a pointer to the DSA.  A new LWLock tranche ID will be
 * generated if needed.  Note that the lock tranche will be registered with the
 * provided name.  Also note that this should be called at most once for a
 * given DSA in each backend.
 */
dsa_area *
GetNamedDSA(const char *name, bool *found)
{
    DSMRegistryEntry *entry;
    MemoryContext oldcontext;
    dsa_area   *ret;
    NamedDSAState *state;
 
    Assert(found);
 
    if (!name || *name == '\0')
        ereport(ERROR,
                (errmsg("DSA name cannot be empty")));
 
    if (strlen(name) >= offsetof(DSMRegistryEntry, type))
        ereport(ERROR,
                (errmsg("DSA name too long")));
 
    /* Be sure any local memory allocated by DSM/DSA routines is persistent. */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 
    /* Connect to the registry. */
    init_dsm_registry();
 
    entry = dshash_find_or_insert(dsm_registry_table, name, found);
    state = &entry->dsa;
    if (!(*found))
    {
        entry->type = DSMR_ENTRY_TYPE_DSA;
        state->handle = DSA_HANDLE_INVALID;
        state->tranche = -1;
    }
    else if (entry->type != DSMR_ENTRY_TYPE_DSA)
        ereport(ERROR,
                (errmsg("requested DSA does not match type of existing entry")));
 
    if (state->tranche == -1)
    {
        *found = false;
 
        /* Initialize the LWLock tranche for the DSA. */
        state->tranche = LWLockNewTrancheId(name);
    }
 
    if (state->handle == DSA_HANDLE_INVALID)
    {
        *found = false;
 
        /* Initialize the DSA. */
        ret = dsa_create(state->tranche);
        dsa_pin(ret);
        dsa_pin_mapping(ret);
 
        /* Store handle for other backends to use. */
        state->handle = dsa_get_handle(ret);
    }
    else if (dsa_is_attached(state->handle))
        ereport(ERROR,
                (errmsg("requested DSA already attached to current process")));
    else
    {
        /* Attach to existing DSA. */
        ret = dsa_attach(state->handle);
        dsa_pin_mapping(ret);
    }
 
    dshash_release_lock(dsm_registry_table, entry);
    MemoryContextSwitchTo(oldcontext);
 
    return ret;
}
 
/*
 * Initialize or attach a named dshash table.
 *
 * This routine returns the address of the table.  The tranche_id member of
 * params is ignored; a new LWLock tranche ID will be generated if needed.
 * Note that the lock tranche will be registered with the provided name.  Also
 * note that this should be called at most once for a given table in each
 * backend.
 */
dshash_table *
GetNamedDSHash(const char *name, const dshash_parameters *params, bool *found)
{
    DSMRegistryEntry *entry;
    MemoryContext oldcontext;
    dshash_table *ret;
    NamedDSHState *dsh_state;
 
    Assert(params);
    Assert(found);
 
    if (!name || *name == '\0')
        ereport(ERROR,
                (errmsg("DSHash name cannot be empty")));
 
    if (strlen(name) >= offsetof(DSMRegistryEntry, type))
        ereport(ERROR,
                (errmsg("DSHash name too long")));
 
    /* Be sure any local memory allocated by DSM/DSA routines is persistent. */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 
    /* Connect to the registry. */
    init_dsm_registry();
 
    entry = dshash_find_or_insert(dsm_registry_table, name, found);
    dsh_state = &entry->dsh;
    if (!(*found))
    {
        entry->type = DSMR_ENTRY_TYPE_DSH;
        dsh_state->dsa_handle = DSA_HANDLE_INVALID;
        dsh_state->dsh_handle = DSHASH_HANDLE_INVALID;
        dsh_state->tranche = -1;
    }
    else if (entry->type != DSMR_ENTRY_TYPE_DSH)
        ereport(ERROR,
                (errmsg("requested DSHash does not match type of existing entry")));
 
    if (dsh_state->tranche == -1)
    {
        *found = false;
 
        /* Initialize the LWLock tranche for the hash table. */
        dsh_state->tranche = LWLockNewTrancheId(name);
    }
 
    if (dsh_state->dsa_handle == DSA_HANDLE_INVALID)
    {
        dshash_parameters params_copy;
        dsa_area   *dsa;
 
        *found = false;
 
        /* Initialize the DSA for the hash table. */
        dsa = dsa_create(dsh_state->tranche);
 
        /* Initialize the dshash table. */
        memcpy(&params_copy, params, sizeof(dshash_parameters));
        params_copy.tranche_id = dsh_state->tranche;
        ret = dshash_create(dsa, &params_copy, NULL);
 
        dsa_pin(dsa);
        dsa_pin_mapping(dsa);
 
        /* Store handles for other backends to use. */
        dsh_state->dsa_handle = dsa_get_handle(dsa);
        dsh_state->dsh_handle = dshash_get_hash_table_handle(ret);
    }
    else if (dsa_is_attached(dsh_state->dsa_handle))
        ereport(ERROR,
                (errmsg("requested DSHash already attached to current process")));
    else
    {
        dsa_area   *dsa;
 
        /* XXX: Should we verify params matches what table was created with? */
 
        /* Attach to existing DSA for the hash table. */
        dsa = dsa_attach(dsh_state->dsa_handle);
        dsa_pin_mapping(dsa);
 
        /* Attach to existing dshash table. */
        ret = dshash_attach(dsa, params, dsh_state->dsh_handle, NULL);
    }
 
    dshash_release_lock(dsm_registry_table, entry);
    MemoryContextSwitchTo(oldcontext);
 
    return ret;
}
 
Datum
pg_get_dsm_registry_allocations(PG_FUNCTION_ARGS)
{
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    DSMRegistryEntry *entry;
    MemoryContext oldcontext;
    dshash_seq_status status;
 
    InitMaterializedSRF(fcinfo, MAT_SRF_USE_EXPECTED_DESC);
 
    /* Be sure any local memory allocated by DSM/DSA routines is persistent. */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
    init_dsm_registry();
    MemoryContextSwitchTo(oldcontext);
 
    dshash_seq_init(&status, dsm_registry_table, false);
    while ((entry = dshash_seq_next(&status)) != NULL)
    {
        Datum       vals[3];
        bool        nulls[3] = {0};
 
        /* Do not show partially-initialized entries. */
        if (entry->type == DSMR_ENTRY_TYPE_DSM &&
            entry->dsm.handle == DSM_HANDLE_INVALID)
            continue;
        if (entry->type == DSMR_ENTRY_TYPE_DSA &&
            entry->dsa.handle == DSA_HANDLE_INVALID)
            continue;
        if (entry->type == DSMR_ENTRY_TYPE_DSH &&
            entry->dsh.dsa_handle == DSA_HANDLE_INVALID)
            continue;
 
        vals[0] = CStringGetTextDatum(entry->name);
        vals[1] = CStringGetTextDatum(DSMREntryTypeNames[entry->type]);
 
        /*
         * Since we can't know the size of DSA/dshash entries without first
         * attaching to them, return NULL for those.
         */
        if (entry->type == DSMR_ENTRY_TYPE_DSM)
            vals[2] = Int64GetDatum(entry->dsm.size);
        else
            nulls[2] = true;
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, vals, nulls);
    }
    dshash_seq_term(&status);
 
    return (Datum) 0;
}