provsql_shmem.c

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/**
 * @file provsql_shmem.c
 * @brief Shared-memory segment lifecycle and LWLock management.
 *
 * Implements the functions declared in @c provsql_shmem.h:
 * - @c provsql_shmem_startup(): allocates (or attaches to) the
 *   @c provsqlSharedState segment, initialises the @c LWLock, and
 *   calls @c pipe(2) to create the two inter-process pipes.  Chains
 *   to @c prev_shmem_startup if set.
 * - @c provsql_memsize(): returns the size of the shared segment.
 * - @c provsql_shmem_request(): requests shared memory on PG ≥ 15.
 * - @c provsql_shmem_lock_exclusive() / @c provsql_shmem_lock_shared() /
 *   @c provsql_shmem_unlock(): thin wrappers around
 *   @c LWLockAcquire() / @c LWLockRelease().
 */
#include <unistd.h>
 
#include "postgres.h"
#include "storage/shmem.h"
 
#include "provsql_shmem.h"
#include "provsql_mmap.h"
 
shmem_startup_hook_type prev_shmem_startup = NULL;
#if (PG_VERSION_NUM >= 150000)
shmem_request_hook_type prev_shmem_request = NULL;
#endif
 
provsqlSharedState *provsql_shared_state = NULL;
 
void provsql_shmem_startup(void)
{
  bool found;
  int pipes_b_to_m[2];
  int pipes_m_to_b[2];
 
  if(prev_shmem_startup)
    prev_shmem_startup();
 
  // Reset in case of restart
  provsql_shared_state = NULL;
 
  LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
 
  provsql_shared_state = ShmemInitStruct(
    "provsql",
    sizeof(provsql_shared_state),
    &found);
 
  if(!found) {
    provsql_shared_state->lock =&(GetNamedLWLockTranche("provsql"))->lock;
  }
 
  LWLockRelease(AddinShmemInitLock);
 
  // Already initialized
  if(found)
    return;
 
  if(pipe(pipes_b_to_m) || pipe(pipes_m_to_b))
    provsql_error("Cannot create pipe to communicate with MMap worker");
 
  provsql_shared_state->pipebmr=pipes_b_to_m[0];
  provsql_shared_state->pipebmw=pipes_b_to_m[1];
  provsql_shared_state->pipembr=pipes_m_to_b[0];
  provsql_shared_state->pipembw=pipes_m_to_b[1];
}
 
Size provsql_memsize(void)
{
  return MAXALIGN(sizeof(provsqlSharedState));
}
 
void provsql_shmem_request(void)
{
#if (PG_VERSION_NUM >= 150000)
  if (prev_shmem_request)
    prev_shmem_request();
#endif
 
  RequestAddinShmemSpace(provsql_memsize());
 
  RequestNamedLWLockTranche("provsql", 1);
}
 
void provsql_shmem_lock_exclusive(void)
{
  LWLockAcquire(provsql_shared_state->lock, LW_EXCLUSIVE);
}
 
void provsql_shmem_lock_shared(void)
{
  LWLockAcquire(provsql_shared_state->lock, LW_SHARED);
}
 
void provsql_shmem_unlock(void)
{
  LWLockRelease(provsql_shared_state->lock);
}