proc.h

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * proc.h
 *    per-process shared memory data structures
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/storage/proc.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef _PROC_H_
#define _PROC_H_

#include "access/clog.h"
#include "access/xlogdefs.h"
#include "lib/ilist.h"
#include "storage/latch.h"
#include "storage/lock.h"
#include "storage/pg_sema.h"
#include "storage/proclist_types.h"

/*
 * Each backend advertises up to PGPROC_MAX_CACHED_SUBXIDS TransactionIds
 * for non-aborted subtransactions of its current top transaction.  These
 * have to be treated as running XIDs by other backends.
 *
 * We also keep track of whether the cache overflowed (ie, the transaction has
 * generated at least one subtransaction that didn't fit in the cache).
 * If none of the caches have overflowed, we can assume that an XID that's not
 * listed anywhere in the PGPROC array is not a running transaction.  Else we
 * have to look at pg_subtrans.
 */
#define PGPROC_MAX_CACHED_SUBXIDS 64    /* XXX guessed-at value */

typedef struct XidCacheStatus
{
    /* number of cached subxids, never more than PGPROC_MAX_CACHED_SUBXIDS */
    uint8   count;
    /* has PGPROC->subxids overflowed */
    bool    overflowed;
} XidCacheStatus;

struct XidCache
{
    TransactionId xids[PGPROC_MAX_CACHED_SUBXIDS];
};

/*
 * Flags for PGPROC->statusFlags and PROC_HDR->statusFlags[]
 */
#define     PROC_IS_AUTOVACUUM  0x01    /* is it an autovac worker? */
#define     PROC_IN_VACUUM      0x02    /* currently running lazy vacuum */
#define     PROC_IN_SAFE_IC     0x04    /* currently running CREATE INDEX
                                         * CONCURRENTLY or REINDEX
                                         * CONCURRENTLY on non-expressional,
                                         * non-partial index */
#define     PROC_VACUUM_FOR_WRAPAROUND  0x08    /* set by autovac only */
#define     PROC_IN_LOGICAL_DECODING    0x10    /* currently doing logical
                                                 * decoding outside xact */

/* flags reset at EOXact */
#define     PROC_VACUUM_STATE_MASK \
    (PROC_IN_VACUUM | PROC_IN_SAFE_IC | PROC_VACUUM_FOR_WRAPAROUND)

/*
 * We allow a small number of "weak" relation locks (AccessShareLock,
 * RowShareLock, RowExclusiveLock) to be recorded in the PGPROC structure
 * rather than the main lock table.  This eases contention on the lock
 * manager LWLocks.  See storage/lmgr/README for additional details.
 */
#define     FP_LOCK_SLOTS_PER_BACKEND 16

/*
 * An invalid pgprocno.  Must be larger than the maximum number of PGPROC
 * structures we could possibly have.  See comments for MAX_BACKENDS.
 */
#define INVALID_PGPROCNO        PG_INT32_MAX

typedef enum
{
    PROC_WAIT_STATUS_OK,
    PROC_WAIT_STATUS_WAITING,
    PROC_WAIT_STATUS_ERROR,
} ProcWaitStatus;

/*
 * Each backend has a PGPROC struct in shared memory.  There is also a list of
 * currently-unused PGPROC structs that will be reallocated to new backends.
 *
 * links: list link for any list the PGPROC is in.  When waiting for a lock,
 * the PGPROC is linked into that lock's waitProcs queue.  A recycled PGPROC
 * is linked into ProcGlobal's freeProcs list.
 *
 * Note: twophase.c also sets up a dummy PGPROC struct for each currently
 * prepared transaction.  These PGPROCs appear in the ProcArray data structure
 * so that the prepared transactions appear to be still running and are
 * correctly shown as holding locks.  A prepared transaction PGPROC can be
 * distinguished from a real one at need by the fact that it has pid == 0.
 * The semaphore and lock-activity fields in a prepared-xact PGPROC are unused,
 * but its myProcLocks[] lists are valid.
 *
 * We allow many fields of this struct to be accessed without locks, such as
 * delayChkpt and isBackgroundWorker. However, keep in mind that writing
 * mirrored ones (see below) requires holding ProcArrayLock or XidGenLock in
 * at least shared mode, so that pgxactoff does not change concurrently.
 *
 * Mirrored fields:
 *
 * Some fields in PGPROC (see "mirrored in ..." comment) are mirrored into an
 * element of more densely packed ProcGlobal arrays. These arrays are indexed
 * by PGPROC->pgxactoff. Both copies need to be maintained coherently.
 *
 * NB: The pgxactoff indexed value can *never* be accessed without holding
 * locks.
 *
 * See PROC_HDR for details.
 */
struct PGPROC
{
    /* proc->links MUST BE FIRST IN STRUCT (see ProcSleep,ProcWakeup,etc) */
    SHM_QUEUE   links;          /* list link if process is in a list */
    PGPROC    **procgloballist; /* procglobal list that owns this PGPROC */

    PGSemaphore sem;            /* ONE semaphore to sleep on */
    ProcWaitStatus waitStatus;

    Latch       procLatch;      /* generic latch for process */


    TransactionId xid;          /* id of top-level transaction currently being
                                 * executed by this proc, if running and XID
                                 * is assigned; else InvalidTransactionId.
                                 * mirrored in ProcGlobal->xids[pgxactoff] */

    TransactionId xmin;         /* minimal running XID as it was when we were
                                 * starting our xact, excluding LAZY VACUUM:
                                 * vacuum must not remove tuples deleted by
                                 * xid >= xmin ! */

    LocalTransactionId lxid;    /* local id of top-level transaction currently
                                 * being executed by this proc, if running;
                                 * else InvalidLocalTransactionId */
    int         pid;            /* Backend's process ID; 0 if prepared xact */

    int         pgxactoff;      /* offset into various ProcGlobal->arrays
                                 * with data mirrored from this PGPROC */
    int         pgprocno;

    /* These fields are zero while a backend is still starting up: */
    BackendId   backendId;      /* This backend's backend ID (if assigned) */
    Oid         databaseId;     /* OID of database this backend is using */
    Oid         roleId;         /* OID of role using this backend */

    Oid         tempNamespaceId;    /* OID of temp schema this backend is
                                     * using */

    bool        isBackgroundWorker; /* true if background worker. */

    /*
     * While in hot standby mode, shows that a conflict signal has been sent
     * for the current transaction. Set/cleared while holding ProcArrayLock,
     * though not required. Accessed without lock, if needed.
     */
    bool        recoveryConflictPending;

    /* Info about LWLock the process is currently waiting for, if any. */
    bool        lwWaiting;      /* true if waiting for an LW lock */
    uint8       lwWaitMode;     /* lwlock mode being waited for */
    proclist_node lwWaitLink;   /* position in LW lock wait list */

    /* Support for condition variables. */
    proclist_node cvWaitLink;   /* position in CV wait list */

    /* Info about lock the process is currently waiting for, if any. */
    /* waitLock and waitProcLock are NULL if not currently waiting. */
    LOCK       *waitLock;       /* Lock object we're sleeping on ... */
    PROCLOCK   *waitProcLock;   /* Per-holder info for awaited lock */
    LOCKMODE    waitLockMode;   /* type of lock we're waiting for */
    LOCKMASK    heldLocks;      /* bitmask for lock types already held on this
                                 * lock object by this backend */

    bool        delayChkpt;     /* true if this proc delays checkpoint start */

    uint8       statusFlags;    /* this backend's status flags, see PROC_*
                                 * above. mirrored in
                                 * ProcGlobal->statusFlags[pgxactoff] */

    /*
     * Info to allow us to wait for synchronous replication, if needed.
     * waitLSN is InvalidXLogRecPtr if not waiting; set only by user backend.
     * syncRepState must not be touched except by owning process or WALSender.
     * syncRepLinks used only while holding SyncRepLock.
     */
    XLogRecPtr  waitLSN;        /* waiting for this LSN or higher */
    int         syncRepState;   /* wait state for sync rep */
    SHM_QUEUE   syncRepLinks;   /* list link if process is in syncrep queue */

    /*
     * All PROCLOCK objects for locks held or awaited by this backend are
     * linked into one of these lists, according to the partition number of
     * their lock.
     */
    SHM_QUEUE   myProcLocks[NUM_LOCK_PARTITIONS];

    XidCacheStatus subxidStatus; /* mirrored with
                                  * ProcGlobal->subxidStates[i] */
    struct XidCache subxids;    /* cache for subtransaction XIDs */

    /* Support for group XID clearing. */
    /* true, if member of ProcArray group waiting for XID clear */
    bool        procArrayGroupMember;
    /* next ProcArray group member waiting for XID clear */
    pg_atomic_uint32 procArrayGroupNext;

    /*
     * latest transaction id among the transaction's main XID and
     * subtransactions
     */
    TransactionId procArrayGroupMemberXid;

    uint32      wait_event_info;    /* proc's wait information */

    /* Support for group transaction status update. */
    bool        clogGroupMember;    /* true, if member of clog group */
    pg_atomic_uint32 clogGroupNext; /* next clog group member */
    TransactionId clogGroupMemberXid;   /* transaction id of clog group member */
    XidStatus   clogGroupMemberXidStatus;   /* transaction status of clog
                                             * group member */
    int         clogGroupMemberPage;    /* clog page corresponding to
                                         * transaction id of clog group member */
    XLogRecPtr  clogGroupMemberLsn; /* WAL location of commit record for clog
                                     * group member */

    /* Lock manager data, recording fast-path locks taken by this backend. */
    LWLock      fpInfoLock;     /* protects per-backend fast-path state */
    uint64      fpLockBits;     /* lock modes held for each fast-path slot */
    Oid         fpRelId[FP_LOCK_SLOTS_PER_BACKEND]; /* slots for rel oids */
    bool        fpVXIDLock;     /* are we holding a fast-path VXID lock? */
    LocalTransactionId fpLocalTransactionId;    /* lxid for fast-path VXID
                                                 * lock */

    /*
     * Support for lock groups.  Use LockHashPartitionLockByProc on the group
     * leader to get the LWLock protecting these fields.
     */
    PGPROC     *lockGroupLeader;    /* lock group leader, if I'm a member */
    dlist_head  lockGroupMembers;   /* list of members, if I'm a leader */
    dlist_node  lockGroupLink;  /* my member link, if I'm a member */
};

/* NOTE: "typedef struct PGPROC PGPROC" appears in storage/lock.h. */


extern PGDLLIMPORT PGPROC *MyProc;

/*
 * There is one ProcGlobal struct for the whole database cluster.
 *
 * Adding/Removing an entry into the procarray requires holding *both*
 * ProcArrayLock and XidGenLock in exclusive mode (in that order). Both are
 * needed because the dense arrays (see below) are accessed from
 * GetNewTransactionId() and GetSnapshotData(), and we don't want to add
 * further contention by both using the same lock. Adding/Removing a procarray
 * entry is much less frequent.
 *
 * Some fields in PGPROC are mirrored into more densely packed arrays (e.g.
 * xids), with one entry for each backend. These arrays only contain entries
 * for PGPROCs that have been added to the shared array with ProcArrayAdd()
 * (in contrast to PGPROC array which has unused PGPROCs interspersed).
 *
 * The dense arrays are indexed by PGPROC->pgxactoff. Any concurrent
 * ProcArrayAdd() / ProcArrayRemove() can lead to pgxactoff of a procarray
 * member to change.  Therefore it is only safe to use PGPROC->pgxactoff to
 * access the dense array while holding either ProcArrayLock or XidGenLock.
 *
 * As long as a PGPROC is in the procarray, the mirrored values need to be
 * maintained in both places in a coherent manner.
 *
 * The denser separate arrays are beneficial for three main reasons: First, to
 * allow for as tight loops accessing the data as possible. Second, to prevent
 * updates of frequently changing data (e.g. xmin) from invalidating
 * cachelines also containing less frequently changing data (e.g. xid,
 * statusFlags). Third to condense frequently accessed data into as few
 * cachelines as possible.
 *
 * There are two main reasons to have the data mirrored between these dense
 * arrays and PGPROC. First, as explained above, a PGPROC's array entries can
 * only be accessed with either ProcArrayLock or XidGenLock held, whereas the
 * PGPROC entries do not require that (obviously there may still be locking
 * requirements around the individual field, separate from the concerns
 * here). That is particularly important for a backend to efficiently checks
 * it own values, which it often can safely do without locking.  Second, the
 * PGPROC fields allow to avoid unnecessary accesses and modification to the
 * dense arrays. A backend's own PGPROC is more likely to be in a local cache,
 * whereas the cachelines for the dense array will be modified by other
 * backends (often removing it from the cache for other cores/sockets). At
 * commit/abort time a check of the PGPROC value can avoid accessing/dirtying
 * the corresponding array value.
 *
 * Basically it makes sense to access the PGPROC variable when checking a
 * single backend's data, especially when already looking at the PGPROC for
 * other reasons already.  It makes sense to look at the "dense" arrays if we
 * need to look at many / most entries, because we then benefit from the
 * reduced indirection and better cross-process cache-ability.
 *
 * When entering a PGPROC for 2PC transactions with ProcArrayAdd(), the data
 * in the dense arrays is initialized from the PGPROC while it already holds
 * ProcArrayLock.
 */
typedef struct PROC_HDR
{
    /* Array of PGPROC structures (not including dummies for prepared txns) */
    PGPROC     *allProcs;

    /* Array mirroring PGPROC.xid for each PGPROC currently in the procarray */
    TransactionId *xids;

    /*
     * Array mirroring PGPROC.subxidStatus for each PGPROC currently in the
     * procarray.
     */
    XidCacheStatus *subxidStates;

    /*
     * Array mirroring PGPROC.statusFlags for each PGPROC currently in the
     * procarray.
     */
    uint8      *statusFlags;

    /* Length of allProcs array */
    uint32      allProcCount;
    /* Head of list of free PGPROC structures */
    PGPROC     *freeProcs;
    /* Head of list of autovacuum's free PGPROC structures */
    PGPROC     *autovacFreeProcs;
    /* Head of list of bgworker free PGPROC structures */
    PGPROC     *bgworkerFreeProcs;
    /* Head of list of walsender free PGPROC structures */
    PGPROC     *walsenderFreeProcs;
    /* First pgproc waiting for group XID clear */
    pg_atomic_uint32 procArrayGroupFirst;
    /* First pgproc waiting for group transaction status update */
    pg_atomic_uint32 clogGroupFirst;
    /* WALWriter process's latch */
    Latch      *walwriterLatch;
    /* Checkpointer process's latch */
    Latch      *checkpointerLatch;
    /* Current shared estimate of appropriate spins_per_delay value */
    int         spins_per_delay;
    /* The proc of the Startup process, since not in ProcArray */
    PGPROC     *startupProc;
    int         startupProcPid;
    /* Buffer id of the buffer that Startup process waits for pin on, or -1 */
    int         startupBufferPinWaitBufId;
} PROC_HDR;

extern PGDLLIMPORT PROC_HDR *ProcGlobal;

extern PGPROC *PreparedXactProcs;

/* Accessor for PGPROC given a pgprocno. */
#define GetPGProcByNumber(n) (&ProcGlobal->allProcs[(n)])

/*
 * We set aside some extra PGPROC structures for auxiliary processes,
 * ie things that aren't full-fledged backends but need shmem access.
 *
 * Background writer, checkpointer and WAL writer run during normal operation.
 * Startup process and WAL receiver also consume 2 slots, but WAL writer is
 * launched only after startup has exited, so we only need 4 slots.
 */
#define NUM_AUXILIARY_PROCS     4

/* configurable options */
extern PGDLLIMPORT int DeadlockTimeout;
extern PGDLLIMPORT int StatementTimeout;
extern PGDLLIMPORT int LockTimeout;
extern PGDLLIMPORT int IdleInTransactionSessionTimeout;
extern PGDLLIMPORT int IdleSessionTimeout;
extern bool log_lock_waits;


/*
 * Function Prototypes
 */
extern int  ProcGlobalSemas(void);
extern Size ProcGlobalShmemSize(void);
extern void InitProcGlobal(void);
extern void InitProcess(void);
extern void InitProcessPhase2(void);
extern void InitAuxiliaryProcess(void);

extern void PublishStartupProcessInformation(void);
extern void SetStartupBufferPinWaitBufId(int bufid);
extern int  GetStartupBufferPinWaitBufId(void);

extern bool HaveNFreeProcs(int n);
extern void ProcReleaseLocks(bool isCommit);

extern void ProcQueueInit(PROC_QUEUE *queue);
extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable);
extern PGPROC *ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
extern void CheckDeadLockAlert(void);
extern bool IsWaitingForLock(void);
extern void LockErrorCleanup(void);

extern void ProcWaitForSignal(uint32 wait_event_info);
extern void ProcSendSignal(int pid);

extern PGPROC *AuxiliaryPidGetProc(int pid);

extern void BecomeLockGroupLeader(void);
extern bool BecomeLockGroupMember(PGPROC *leader, int pid);

#endif                          /* _PROC_H_ */