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This section introduces the PROCLOCK structure in PostgreSQL Locks, translated from the README file.

I. PROCLOCK Struct/* * We may have several different backends holding or awaiting locks * on the same lockable object. We need to store some per-holder/waiter * information for each such holder (or would-be holder). This is kept in * a PROCLOCK struct. * * PROCLOCKTAG is the key information needed to look up a PROCLOCK item in the * proclock hashtable. A PROCLOCKTAG value uniquely identifies the combination * of a lockable object and a holder/waiter for that object. (We can use * pointers here because the PROCLOCKTAG need only be unique for the lifespan * of the PROCLOCK, and it will never outlive the lock or the proc.) * * Internally to a backend, it is possible for the same lock to be held * for different purposes: the backend tracks transaction locks separately * from session locks. However, this is not reflected in the shared-memory * state: we only track which backend(s) hold the lock. This is OK since a * backend can never block itself. * * The holdMask field shows the already-granted locks represented by this * proclock. Note that there will be a proclock object, possibly with * zero holdMask, for any lock that the process is currently waiting on. * Otherwise, proclock objects whose holdMasks are zero are recycled * as soon as convenient. * * releaseMask is workspace for LockReleaseAll(): it shows the locks due * to be released during the current call. This must only be examined or * set by the backend owning the PROCLOCK. * * Each PROCLOCK object is linked into lists for both the associated LOCK * object and the owning PGPROC object. Note that the PROCLOCK is entered * into these lists as soon as it is created, even if no lock has yet been * granted. A PGPROC that is waiting for a lock to be granted will also be * linked into the lock's waitProcs queue. */typedef struct PROCLOCKTAG{ /* NB: we assume this struct contains no padding! */ LOCK *myLock; /* link to per-lockable-object information */ PGPROC *myProc; /* link to PGPROC of owning backend */} PROCLOCKTAG;typedef struct PROCLOCK{ /* tag */ PROCLOCKTAG tag; /* unique identifier of proclock object */ /* data */ PGPROC *groupLeader; /* proc's lock group leader, or proc itself */ LOCKMASK holdMask; /* bitmask for lock types currently held */ LOCKMASK releaseMask; /* bitmask for lock types to be released */ SHM_QUEUE lockLink; /* list link in LOCK's list of proclocks */ SHM_QUEUE procLink; /* list link in PGPROC's list of proclocks */} PROCLOCK;#define PROCLOCK_LOCKMETHOD(proclock) \ LOCK_LOCKMETHOD(*((proclock).tag.myLock))---------------------------------------------------------------------------The lock manager's PROCLOCK objects contain:tag - The key fields that are used for hashing entries in the shared memory PROCLOCK hash table. This is declared as a separate struct to ensure that we always zero out the correct number of bytes. It is critical that any alignment-padding bytes the compiler might insert in the struct be zeroed out, else the hash computation will be random. (Currently, we are careful to define struct PROCLOCKTAG so that there are no padding bytes.) tag.myLock Pointer to the shared LOCK object this PROCLOCK is for. tag.myProc Pointer to the PGPROC of backend process that owns this PROCLOCK. Note: it's OK to use pointers here because a PROCLOCK never outlives either its lock or its proc. The tag is therefore unique for as long as it needs to be, even though the same tag values might mean something else at other times.tag - Similar to the tag field in the LOCK struct, it identifies the corresponding entry in the PROCLOCK hash table in shared memory. tag.mylock Pointer to the shared LOCK object represented by PROCLOCK tag.myProc Points to the PGROC daemon holding the PROCLOCK holdMask - A bitmask for the lock modes successfully acquired by this PROCLOCK. This should be a subset of the LOCK object's grantMask, and also a subset of the PGPROC object's heldLocks mask (if the PGPROC is currently waiting for another lock mode on this lock).holdMask - The PROCLOCK request obtains the lock pattern bitmask. Expected a subset of LOCK's grantMask mask and also a subset of PGPOC's heldlocks mask.releaseMask - A bitmask for the lock modes due to be released during LockReleaseAll. This must be a subset of the holdMask. Note that it is modified without taking the partition LWLock, and therefore it is unsafe for any backend except the one owning the PROCLOCK to examine/change it.releaseMask - The lock pattern bitmask to be released during LockReleaseAll execution. Expected subset of holdMask bitmask. Note that updating this value does not require holding partition LWLock, so in addition to its own PROCLOCK Other daemons are unsafe.lockLink - List link for shared memory queue of all the PROCLOCK objects for the same LOCK.lockLink - PROCLOCK object queue linked list of the same LOCK in shared memory.procLink -LOCK List link for shared memory queue of all the PROCLOCK objects for the same backend.procLink - A linked list of all PROCLOCK objects in the same daemon in shared memory. II. References

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