proc.h

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * proc.h
 *    per-process shared memory data structures
 *
 *
 * Portions Copyright (c) 1996-2025, 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"
#include "storage/procnumber.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.
 *
 * See src/test/isolation/specs/subxid-overflow.spec if you change this.
 */
#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 */
#define     PROC_AFFECTS_ALL_HORIZONS   0x20    /* this proc's xmin must be
                                                 * included in vacuum horizons
                                                 * in all databases */
 
/* flags reset at EOXact */
#define     PROC_VACUUM_STATE_MASK \
    (PROC_IN_VACUUM | PROC_IN_SAFE_IC | PROC_VACUUM_FOR_WRAPAROUND)
 
/*
 * Xmin-related flags. Make sure any flags that affect how the process' Xmin
 * value is interpreted by VACUUM are included here.
 */
#define     PROC_XMIN_FLAGS (PROC_IN_VACUUM | PROC_IN_SAFE_IC)
 
/*
 * We allow a limited number of "weak" relation locks (AccessShareLock,
 * RowShareLock, RowExclusiveLock) to be recorded in the PGPROC structure
 * (or rather in shared memory referenced from PGPROC) rather than the main
 * lock table.  This eases contention on the lock manager LWLocks.  See
 * storage/lmgr/README for additional details.
 */
extern PGDLLIMPORT int FastPathLockGroupsPerBackend;
 
#define     FP_LOCK_GROUPS_PER_BACKEND_MAX  1024
#define     FP_LOCK_SLOTS_PER_GROUP     16  /* don't change */
#define     FastPathLockSlotsPerBackend() \
    (FP_LOCK_SLOTS_PER_GROUP * FastPathLockGroupsPerBackend)
 
/*
 * Flags for PGPROC.delayChkptFlags
 *
 * These flags can be used to delay the start or completion of a checkpoint
 * for short periods. A flag is in effect if the corresponding bit is set in
 * the PGPROC of any backend.
 *
 * For our purposes here, a checkpoint has three phases: (1) determine the
 * location to which the redo pointer will be moved, (2) write all the
 * data durably to disk, and (3) WAL-log the checkpoint.
 *
 * Setting DELAY_CHKPT_START prevents the system from moving from phase 1
 * to phase 2. This is useful when we are performing a WAL-logged modification
 * of data that will be flushed to disk in phase 2. By setting this flag
 * before writing WAL and clearing it after we've both written WAL and
 * performed the corresponding modification, we ensure that if the WAL record
 * is inserted prior to the new redo point, the corresponding data changes will
 * also be flushed to disk before the checkpoint can complete. (In the
 * extremely common case where the data being modified is in shared buffers
 * and we acquire an exclusive content lock on the relevant buffers before
 * writing WAL, this mechanism is not needed, because phase 2 will block
 * until we release the content lock and then flush the modified data to
 * disk.)
 *
 * Setting DELAY_CHKPT_COMPLETE prevents the system from moving from phase 2
 * to phase 3. This is useful if we are performing a WAL-logged operation that
 * might invalidate buffers, such as relation truncation. In this case, we need
 * to ensure that any buffers which were invalidated and thus not flushed by
 * the checkpoint are actually destroyed on disk. Replay can cope with a file
 * or block that doesn't exist, but not with a block that has the wrong
 * contents.
 */
#define DELAY_CHKPT_START       (1<<0)
#define DELAY_CHKPT_COMPLETE    (1<<1)
 
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
 * delayChkptFlags and isRegularBackend. 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
{
    dlist_node  links;          /* list link if process is in a list */
    dlist_head *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 ! */
 
    int         pid;            /* Backend's process ID; 0 if prepared xact */
 
    int         pgxactoff;      /* offset into various ProcGlobal->arrays with
                                 * data mirrored from this PGPROC */
 
    /*
     * Currently running top-level transaction's virtual xid. Together these
     * form a VirtualTransactionId, but we don't use that struct because this
     * is not atomically assignable as whole, and we want to enforce code to
     * consider both parts separately.  See comments at VirtualTransactionId.
     */
    struct
    {
        ProcNumber  procNumber; /* For regular backends, equal to
                                 * GetNumberFromPGProc(proc).  For prepared
                                 * xacts, ID of the original backend that
                                 * processed the transaction. For unused
                                 * PGPROC entries, INVALID_PROC_NUMBER. */
        LocalTransactionId lxid;    /* local id of top-level transaction
                                     * currently * being executed by this
                                     * proc, if running; else
                                     * InvalidLocalTransactionId */
    }           vxid;
 
    /* These fields are zero while a backend is still starting up: */
    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        isRegularBackend;   /* true if it's a regular backend. */
 
    /*
     * 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. */
    uint8       lwWaiting;      /* see LWLockWaitState */
    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 */
    pg_atomic_uint64 waitStart; /* time at which wait for lock acquisition
                                 * started */
 
    int         delayChkptFlags;    /* for DELAY_CHKPT_* flags */
 
    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 */
    dlist_node  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.
     */
    dlist_head  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 */
    int64       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;        /* 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 */
    dlist_head  freeProcs;
    /* Head of list of autovacuum & special worker free PGPROC structures */
    dlist_head  autovacFreeProcs;
    /* Head of list of bgworker free PGPROC structures */
    dlist_head  bgworkerFreeProcs;
    /* Head of list of walsender free PGPROC structures */
    dlist_head  walsenderFreeProcs;
    /* First pgproc waiting for group XID clear */
    pg_atomic_uint32 procArrayGroupFirst;
    /* First pgproc waiting for group transaction status update */
    pg_atomic_uint32 clogGroupFirst;
 
    /*
     * Current slot numbers of some auxiliary processes. There can be only one
     * of each of these running at a time.
     */
    ProcNumber  walwriterProc;
    ProcNumber  checkpointerProc;
 
    /* Current shared estimate of appropriate spins_per_delay value */
    int         spins_per_delay;
    /* Buffer id of the buffer that Startup process waits for pin on, or -1 */
    int         startupBufferPinWaitBufId;
} PROC_HDR;
 
extern PGDLLIMPORT PROC_HDR *ProcGlobal;
 
extern PGDLLIMPORT PGPROC *PreparedXactProcs;
 
/*
 * Accessors for getting PGPROC given a ProcNumber and vice versa.
 */
#define GetPGProcByNumber(n) (&ProcGlobal->allProcs[(n)])
#define GetNumberFromPGProc(proc) ((proc) - &ProcGlobal->allProcs[0])
 
/*
 * We set aside some extra PGPROC structures for "special worker" processes,
 * which are full-fledged backends (they can run transactions)
 * but are unique animals that there's never more than one of.
 * Currently there are two such processes: the autovacuum launcher
 * and the slotsync worker.
 */
#define NUM_SPECIAL_WORKER_PROCS    2
 
/*
 * We set aside some extra PGPROC structures for auxiliary processes,
 * ie things that aren't full-fledged backends (they cannot run transactions
 * or take heavyweight locks) but need shmem access.
 *
 * Background writer, checkpointer, WAL writer, WAL summarizer, and archiver
 * 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 6 slots.
 */
#define MAX_IO_WORKERS          32
#define NUM_AUXILIARY_PROCS     (6 + MAX_IO_WORKERS)
 
 
/* configurable options */
extern PGDLLIMPORT int DeadlockTimeout;
extern PGDLLIMPORT int StatementTimeout;
extern PGDLLIMPORT int LockTimeout;
extern PGDLLIMPORT int IdleInTransactionSessionTimeout;
extern PGDLLIMPORT int TransactionTimeout;
extern PGDLLIMPORT int IdleSessionTimeout;
extern PGDLLIMPORT bool log_lock_waits;
 
#ifdef EXEC_BACKEND
extern PGDLLIMPORT slock_t *ProcStructLock;
extern PGDLLIMPORT PGPROC *AuxiliaryProcs;
#endif
 
 
/*
 * 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 SetStartupBufferPinWaitBufId(int bufid);
extern int  GetStartupBufferPinWaitBufId(void);
 
extern bool HaveNFreeProcs(int n, int *nfree);
extern void ProcReleaseLocks(bool isCommit);
 
extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
                                    LockMethod lockMethodTable, bool dontWait);
extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
extern void CheckDeadLockAlert(void);
extern void LockErrorCleanup(void);
extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
                                     StringInfo lock_holders_sbuf,
                                     StringInfo lock_waiters_sbuf,
                                     int *lockHoldersNum);
 
extern void ProcWaitForSignal(uint32 wait_event_info);
extern void ProcSendSignal(ProcNumber procNumber);
 
extern PGPROC *AuxiliaryPidGetProc(int pid);
 
extern void BecomeLockGroupLeader(void);
extern bool BecomeLockGroupMember(PGPROC *leader, int pid);
 
#endif                          /* _PROC_H_ */