text: /* -------------------------------------------------------------------------
   *
   * sysv_shmem.c
   *    Implement shared memory using SysV facilities
   *
   * These routines represent a fairly thin layer on top of SysV shared
   * memory functionality.
   *
   * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * IDENTIFICATION
   *    src/common/backend/port/sysv_shmem.c
   *
   * -------------------------------------------------------------------------
   */
  #include "postgres.h"
  #include "knl/knl_variable.h"
  
  #include <signal.h>
  #include <sys/file.h>
  #include <sys/stat.h>
  #ifdef HAVE_SYS_IPC_H
  #include <sys/ipc.h>
  #endif
  #ifdef HAVE_SYS_SHM_H
  #include <sys/shm.h>
  #endif
  
  #include "miscadmin.h"
  #include "postmaster/postmaster.h"
  #include "storage/ipc.h"
  #include "storage/pg_shmem.h"
  #include "securec.h"
  
  typedef key_t IpcMemoryKey; /* shared memory key passed to shmget(2) */
  typedef int IpcMemoryId;    /* shared memory ID returned by shmget(2) */
  
  #define IPCProtection (0600) /* access/modify by user only */
  
  #ifdef SHM_SHARE_MMU /* use intimate shared memory on Solaris */
  #define PG_SHMAT_FLAGS SHM_SHARE_MMU
  #else
  #define PG_SHMAT_FLAGS 0
  #endif
  
  THR_LOCAL unsigned long UsedShmemSegID = 0;
  THR_LOCAL void* UsedShmemSegAddr = NULL;
  
  static void* InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
  static void IpcMemoryDetach(int status, Datum shmaddr);
  static void IpcMemoryDelete(int status, Datum shmId);
  static PGShmemHeader* PGSharedMemoryAttach(IpcMemoryKey key, IpcMemoryId* shmid);
  
  /*
   * InternalIpcMemoryCreate(memKey, size)
   *
   * Attempt to create a new shared memory segment with the specified key.
   * Will fail (return NULL) if such a segment already exists.  If successful,
   * attach the segment to the current process and return its attached address.
   * On success, callbacks are registered with on_shmem_exit to detach and
   * delete the segment when on_shmem_exit is called.
   *
   * If we fail with a failure code other than collision-with-existing-segment,
   * print out an error and abort.  Other types of errors are not recoverable.
   */
  static void* InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
  {
      IpcMemoryId shmid;
      void* memAddress = NULL;
  
      shmid = shmget(memKey, size, IPC_CREAT | IPC_EXCL | IPCProtection);
      if (shmid < 0) {
          /*
           * Fail quietly if error indicates a collision with existing segment.
           * One would expect EEXIST, given that we said IPC_EXCL, but perhaps
           * we could get a permission violation instead?  Also, EIDRM might
           * occur if an old seg is slated for destruction but not gone yet.
           */
          if (errno == EEXIST || errno == EACCES
  #ifdef EIDRM
              || errno == EIDRM
  #endif
          )
              return NULL;
  
          /*
           * Some BSD-derived kernels are known to return EINVAL, not EEXIST, if
           * there is an existing segment but it's smaller than "size" (this is
           * a result of poorly-thought-out ordering of error tests). To
           * distinguish between collision and invalid size in such cases, we
           * make a second try with size = 0.  These kernels do not test size
           * against SHMMIN in the preexisting-segment case, so we will not get
           * EINVAL a second time if there is such a segment.
           */
          if (errno == EINVAL) {
              int save_errno = errno;
              shmid = shmget(memKey, 0, IPC_CREAT | IPC_EXCL | IPCProtection);
              if (shmid < 0) {
                  /* As above, fail quietly if we verify a collision */
                  if (errno == EEXIST || errno == EACCES
  #ifdef EIDRM
                      || errno == EIDRM
  #endif
                  )
                      return NULL;
                  /* Otherwise, fall through to report the original error */
              } else {
                  /*
                   * On most platforms we cannot get here because SHMMIN is
                   * greater than zero.  However, if we do succeed in creating a
                   * zero-size segment, free it and then fall through to report
                   * the original error.
                   */
                  if (shmctl(shmid, IPC_RMID, NULL) < 0)
                      elog(LOG, "shmctl(%d, %d, 0) failed: %m", (int)shmid, IPC_RMID);
              }
  
              errno = save_errno;
          }
  
          if (AmPostmasterProcess()) {
              t_thrd.proc_cxt.MyPMChildSlot = 0;
          }
  
          /*
           * Else complain and abort.
           *
           * Note: at this point EINVAL should mean that either SHMMIN or SHMMAX
           * is violated.  SHMALL violation might be reported as either ENOMEM
           * (BSDen) or ENOSPC (Linux); the Single Unix Spec fails to say which
           * it should be.  SHMMNI violation is ENOSPC, per spec.  Just plain
           * not-enough-RAM is ENOMEM.
           */
          ereport(FATAL,
              (errmsg("could not create shared memory segment: %m"),
                  errdetail("Failed system call was shmget(key=%lu, size=%lu, 0%o).",
                      (unsigned long)memKey,
                      (unsigned long)size,
                      IPC_CREAT | IPC_EXCL | IPCProtection),
                  (errno == EINVAL) ? errhint("This error usually means that PostgreSQL's request for a shared memory "
                                              "segment exceeded your kernel's SHMMAX parameter.  You can either "
                                              "reduce the request size or reconfigure the kernel with larger SHMMAX.  "
                                              "To reduce the request size (currently %lu bytes), reduce "
                                              "PostgreSQL's shared memory usage, perhaps by reducing shared_buffers.\n"
                                              "If the request size is already small, it's possible that it is less than "
                                              "your kernel's SHMMIN parameter, in which case raising the request size or "
                                              "reconfiguring SHMMIN is called for.\n"
                                              "The PostgreSQL documentation contains more information about shared "
                                              "memory configuration.", (unsigned long)size)
                      : 0,
                  (errno == ENOMEM) ? errhint("This error usually means that PostgreSQL's request for a shared "
                                              "memory segment exceeded available memory or swap space, "
                                              "or exceeded your kernel's SHMALL parameter.  You can either "
                                              "reduce the request size or reconfigure the kernel with larger SHMALL.  "
                                              "To reduce the request size (currently %lu bytes), reduce "
                                              "PostgreSQL's shared memory usage, perhaps by reducing shared_buffers.\n"
                                              "The PostgreSQL documentation contains more information about shared "
                                              "memory configuration.", (unsigned long)size)
                      : 0,
                  (errno == ENOSPC) ? errhint("This error does *not* mean that you have run out of disk space.  "
                                              "It occurs either if all available shared memory IDs have been taken, "
                                              "in which case you need to raise the SHMMNI parameter in your kernel, "
                                              "or because the system's overall limit for shared memory has been "
                                              "reached.  If you cannot increase the shared memory limit, "
                                              "reduce PostgreSQL's shared memory request (currently %lu bytes), "
                                              "perhaps by reducing shared_buffers.\n"
                                              "The PostgreSQL documentation contains more information about shared "
                                              "memory configuration.", (unsigned long)size)
                      : 0));
      }
  
      /* Register on-exit routine to delete the new segment */
      on_shmem_exit(IpcMemoryDelete, Int32GetDatum(shmid));
  
      /* OK, should be able to attach to the segment */
      memAddress = shmat(shmid, NULL, PG_SHMAT_FLAGS);
  
      if (memAddress == (void*)-1)
          ereport(FATAL, (errmsg("shmat(id=%d) failed: %m", shmid)));
  
      /* Register on-exit routine to detach new segment before deleting */
      on_shmem_exit(IpcMemoryDetach, PointerGetDatum(memAddress));
  
      /*
       * Store shmem key and ID in data directory lockfile.  Format to try to
       * keep it the same length always (trailing junk in the lockfile won't
       * hurt, but might confuse humans).
       */
      {
          char line[64];
          errno_t rc = sprintf_s(line, sizeof(line), "%9lu %9lu", (unsigned long)memKey, (unsigned long)shmid);
          securec_check_ss_c(rc, "\0", "\0");
          AddToDataDirLockFile(LOCK_FILE_LINE_SHMEM_KEY, line);
      }
  
      return memAddress;
  }
  
  /****************************************************************************/
  /*	IpcMemoryDetach(status, shmaddr)	removes a shared memory segment		*/
  /*										from process' address spaceq		*/
  /*	(called as an on_shmem_exit callback, hence funny argument list)		*/
  /****************************************************************************/
  static void IpcMemoryDetach(int status, Datum shmaddr)
  {
      if (shmdt(DatumGetPointer(shmaddr)) < 0)
          ereport(LOG, (errmsg("shmdt failed")));
      return;
  }
  
  /****************************************************************************/
  /*	IpcMemoryDelete(status, shmId)		deletes a shared memory segment		*/
  /*	(called as an on_shmem_exit callback, hence funny argument list)		*/
  /****************************************************************************/
  static void IpcMemoryDelete(int status, Datum shmId)
  {
      if (shmctl(DatumGetInt32(shmId), IPC_RMID, NULL) < 0)
          elog(LOG, "shmctl(%d, %d, 0) failed: %m", DatumGetInt32(shmId), IPC_RMID);
  }
  
  /*
   * PGSharedMemoryIsInUse
   *
   * Is a previously-existing shmem segment still existing and in use?
   *
   * The point of this exercise is to detect the case where a prior postmaster
   * crashed, but it left child backends that are still running.	Therefore
   * we only care about shmem segments that are associated with the intended
   * t_thrd.proc_cxt.DataDir.  This is an important consideration since accidental matches of
   * shmem segment IDs are reasonably common.
   */
  bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
  {
      IpcMemoryId shmId = (IpcMemoryId)id2;
      struct shmid_ds shmStat;
      struct stat statbuf;
      PGShmemHeader* hdr = NULL;
  
      /*
       * We detect whether a shared memory segment is in use by seeing whether
       * it (a) exists and (b) has any processes attached to it.
       */
      if (shmctl(shmId, IPC_STAT, &shmStat) < 0) {
          /*
           * EINVAL actually has multiple possible causes documented in the
           * shmctl man page, but we assume it must mean the segment no longer
           * exists.
           */
          if (errno == EINVAL)
              return false;
  
          /*
           * EACCES implies that the segment belongs to some other userid, which
           * means it is not a Postgres shmem segment (or at least, not one that
           * is relevant to our data directory).
           */
          if (errno == EACCES)
              return false;
  
              /*
               * Some Linux kernel versions (in fact, all of them as of July 2007)
               * sometimes return EIDRM when EINVAL is correct.  The Linux kernel
               * actually does not have any internal state that would justify
               * returning EIDRM, so we can get away with assuming that EIDRM is
               * equivalent to EINVAL on that platform.
               */
  #ifdef HAVE_LINUX_EIDRM_BUG
          if (errno == EIDRM)
              return false;
  #endif
  
          /*
           * Otherwise, we had better assume that the segment is in use. The
           * only likely case is EIDRM, which implies that the segment has been
           * IPC_RMID'd but there are still processes attached to it.
           */
          return true;
      }
  
      /* If it has no attached processes, it's not in use */
      if (shmStat.shm_nattch == 0)
          return false;
  
      /*
       * Try to attach to the segment and see if it matches our data directory.
       * This avoids shmid-conflict problems on machines that are running
       * several postmasters under the same userid.
       */
      if (stat(t_thrd.proc_cxt.DataDir, &statbuf) < 0)
          return true; /* if can't stat, be conservative */
  
      hdr = (PGShmemHeader*)shmat(shmId, NULL, PG_SHMAT_FLAGS);
  
      if (hdr == (PGShmemHeader*)-1)
          return true; /* if can't attach, be conservative */
  
      if (hdr->magic != PGShmemMagic || hdr->device != statbuf.st_dev || hdr->inode != statbuf.st_ino) {
          /*
           * It's either not a Postgres segment, or not one for my data
           * directory.  In either case it poses no threat.
           */
          shmdt((void*)hdr);
          return false;
      }
  
      /* Trouble --- looks a lot like there's still live backends */
      shmdt((void*)hdr);
  
      return true;
  }
  
  /*
   * PGSharedMemoryCreate
   *
   * Create a shared memory segment of the given size and initialize its
   * standard header.  Also, register an on_shmem_exit callback to release
   * the storage.
   *
   * Dead Postgres segments are recycled if found, but we do not fail upon
   * collision with non-Postgres shmem segments.	The idea here is to detect and
   * re-use keys that may have been assigned by a crashed postmaster or backend.
   *
   * makePrivate means to always create a new segment, rather than attach to
   * or recycle any existing segment.
   *
   * The port number is passed for possible use as a key (for SysV, we use
   * it to generate the starting shmem key).	In a standalone backend,
   * zero will be passed.
   */
  PGShmemHeader* PGSharedMemoryCreate(Size size, bool makePrivate, int port)
  {
      IpcMemoryKey NextShmemSegID;
      void* memAddress = NULL;
      PGShmemHeader* hdr = NULL;
      IpcMemoryId shmid;
      struct stat statbuf;
      int retry_count = 0;
      const int max_retry_count = 3;
  
      /* Room for a header? */
      Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
  
      /* Make sure PGSharedMemoryAttach doesn't fail without need */
      UsedShmemSegAddr = NULL;
  
      /* Loop till we find a free IPC key */
      NextShmemSegID = port * 1000;
  
      for (NextShmemSegID++;; NextShmemSegID++) {
          if (retry_count > max_retry_count) {
              retry_count = 0;
              continue;
          }
  
          /* Try to create new segment */
          memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
          if (memAddress != NULL) {
              break; /* successful create and attach */
          }
  
          /* Check shared memory and possibly remove and recreate */
          if (makePrivate) { /* a standalone backend shouldn't do this */
              retry_count = 0;
              continue;
          }
  
          if ((memAddress = PGSharedMemoryAttach(NextShmemSegID, &shmid)) == NULL) {
              elog(LOG, "choose next key because the shm of key %d is not one of mine", NextShmemSegID);
              retry_count = 0;
              continue; /* can't attach, not one of mine */
          }
  
          /*
           * If I am not the creator and it belongs to an extant process,
           * continue.
           */
          hdr = (PGShmemHeader*)memAddress;
          if (hdr->creatorPID != (ThreadId)getpid()) {
              if (kill(hdr->creatorPID, SIGKILL) == 0 || errno != ESRCH) {
                  elog(DEBUG3, "shared memory that key is %d is owned by pid %lu ", NextShmemSegID, hdr->creatorPID);
                  shmdt(memAddress);
                  retry_count++;
                  NextShmemSegID--;
                  continue;
              }
          }
  
          /*
           * The segment appears to be from a dead Postgres process, or from a
           * previous cycle of life in this same process.  Zap it, if possible.
           * This probably shouldn't fail, but if it does, assume the segment
           * belongs to someone else after all, and continue quietly.
           */
          shmdt(memAddress);
          if (shmctl(shmid, IPC_RMID, NULL) < 0) {
              elog(LOG, "retry SHM key %d", NextShmemSegID);
              retry_count++;
              NextShmemSegID--;
              continue;
          }
  
          /*
           * Now try again to create the segment.
           */
          memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
          if (memAddress != NULL) {
              break; /* successful create and attach */
          } else {
              /*
               * Can only get here if some other process managed to create the same
               * shmem key before we did.  Let him have that one, loop around to try
               * next key.
               */
              elog(LOG, "retry SHM key %d", NextShmemSegID);
              retry_count++;
              NextShmemSegID--;
          }
      }
  
      /*
       * OK, we created a new segment.  Mark it as created by this process. The
       * order of assignments here is critical so that another Postgres process
       * can't see the header as valid but belonging to an invalid PID!
       */
      hdr = (PGShmemHeader*)memAddress;
      hdr->creatorPID = getpid();
      hdr->magic = PGShmemMagic;
  
      /* Fill in the data directory ID info, too */
      if (stat(t_thrd.proc_cxt.DataDir, &statbuf) < 0)
          ereport(FATAL,
              (errcode_for_file_access(), errmsg("could not stat data directory \"%s\": %m", t_thrd.proc_cxt.DataDir)));
      hdr->device = statbuf.st_dev;
      hdr->inode = statbuf.st_ino;
  
      /*
       * Initialize space allocation status for segment.
       */
      hdr->totalsize = size;
      hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
  
      /* Save info for possible future use */
      UsedShmemSegAddr = memAddress;
      UsedShmemSegID = (unsigned long)NextShmemSegID;
  
      return hdr;
  }
  
  #ifdef EXEC_BACKEND
  
  /*
   * PGSharedMemoryReAttach
   *
   * Re-attach to an already existing shared memory segment.	In the non
   * EXEC_BACKEND case this is not used, because postmaster children inherit
   * the shared memory segment attachment via fork().
   *
   * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
   * routine.  The caller must have already restored them to the postmaster's
   * values.
   */
  void PGSharedMemoryReAttach(void)
  {
      IpcMemoryId shmid;
      void* hdr = NULL;
      void* origUsedShmemSegAddr = UsedShmemSegAddr;
  
      Assert(UsedShmemSegAddr != NULL);
      Assert(IsUnderPostmaster);
  
  #ifdef __CYGWIN__
      /* cygipc (currently) appears to not detach on exec. */
      PGSharedMemoryDetach();
      UsedShmemSegAddr = origUsedShmemSegAddr;
  #endif
  
      hdr = (void*)PGSharedMemoryAttach((IpcMemoryKey)UsedShmemSegID, &shmid);
      if (hdr == NULL) {
          elog(FATAL, "could not reattach to shared memory (key=%d): %m", (int)UsedShmemSegID);
      }
      if (hdr != origUsedShmemSegAddr) {
          elog(FATAL, "reattaching to shared memory returned unexpected address");
      }
  
      UsedShmemSegAddr = hdr; /* probably redundant */
  }
  #endif /* EXEC_BACKEND */
  
  /*
   * PGSharedMemoryDetach
   *
   * Detach from the shared memory segment, if still attached.  This is not
   * intended for use by the process that originally created the segment
   * (it will have an on_shmem_exit callback registered to do that).	Rather,
   * this is for subprocesses that have inherited an attachment and want to
   * get rid of it.
   */
  void PGSharedMemoryDetach(void)
  {
      if (UsedShmemSegAddr != NULL) {
          if ((shmdt(UsedShmemSegAddr) < 0)
  #if defined(EXEC_BACKEND) && defined(__CYGWIN__)
              /* Work-around for cygipc exec bug */
              && shmdt(NULL) < 0
  #endif
          )
          ereport(LOG, (errmsg("shmdt failed: %m")));
          UsedShmemSegAddr = NULL;
      }
  }
  
  /*
   * Attach to shared memory and make sure it has a Postgres header
   *
   * Returns attach address if OK, else NULL
   */
  static PGShmemHeader* PGSharedMemoryAttach(IpcMemoryKey key, IpcMemoryId* shmid)
  {
      PGShmemHeader* hdr = NULL;
  
      if ((*shmid = shmget(key, sizeof(PGShmemHeader), 0)) < 0)
          return NULL;
  
      hdr = (PGShmemHeader*)shmat(*shmid, UsedShmemSegAddr, PG_SHMAT_FLAGS);
  
      if (hdr == (PGShmemHeader*)-1)
          return NULL; /* failed: must be some other app's */
  
      if (hdr->magic != PGShmemMagic) {
          shmdt((void*)hdr);
          return NULL; /* segment belongs to a non-Postgres app */
      }
      return hdr;
  }
  
  void cancelIpcMemoryDetach(void)
  {
      cancelShmemExit(IpcMemoryDetach, 0);
  }
  