/*------------------------------------------------------------------------- * * dsm_impl.c * manage dynamic shared memory segments * * This file provides low-level APIs for creating and destroying shared * memory segments using several different possible techniques. We refer * to these segments as dynamic because they can be created, altered, and * destroyed at any point during the server life cycle. This is unlike * the main shared memory segment, of which there is always exactly one * and which is always mapped at a fixed address in every PostgreSQL * background process. * * Because not all systems provide the same primitives in this area, nor * do all primitives behave the same way on all systems, we provide * several implementations of this facility. Many systems implement * POSIX shared memory (shm_open etc.), which is well-suited to our needs * in this area, with the exception that shared memory identifiers live * in a flat system-wide namespace, raising the uncomfortable prospect of * name collisions with other processes (including other copies of * PostgreSQL) running on the same system. Some systems only support * the older System V shared memory interface (shmget etc.) which is * also usable; however, the default allocation limits are often quite * small, and the namespace is even more restricted. * * We also provide an mmap-based shared memory implementation. This may * be useful on systems that provide shared memory via a special-purpose * filesystem; by opting for this implementation, the user can even * control precisely where their shared memory segments are placed. It * can also be used as a fallback for systems where shm_open and shmget * are not available or can't be used for some reason. Of course, * mapping a file residing on an actual spinning disk is a fairly poor * approximation for shared memory because writeback may hurt performance * substantially, but there should be few systems where we must make do * with such poor tools. * * As ever, Windows requires its own implementation. * * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * * IDENTIFICATION * src/backend/storage/ipc/dsm_impl.c * *------------------------------------------------------------------------- */ #include "postgres.h" #include #include #ifndef WIN32 #include #endif #include #ifdef HAVE_SYS_IPC_H #include #endif #ifdef HAVE_SYS_SHM_H #include #endif #include "common/file_perm.h" #include "miscadmin.h" #include "pgstat.h" #include "portability/mem.h" #include "postmaster/postmaster.h" #include "storage/dsm_impl.h" #include "storage/fd.h" #include "utils/guc.h" #include "utils/memutils.h" #ifdef USE_DSM_POSIX static bool dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel); static int dsm_impl_posix_resize(int fd, off_t size); #endif #ifdef USE_DSM_SYSV static bool dsm_impl_sysv(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel); #endif #ifdef USE_DSM_WINDOWS static bool dsm_impl_windows(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel); #endif #ifdef USE_DSM_MMAP static bool dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel); #endif static int errcode_for_dynamic_shared_memory(void); const struct config_enum_entry dynamic_shared_memory_options[] = { #ifdef USE_DSM_POSIX {"posix", DSM_IMPL_POSIX, false}, #endif #ifdef USE_DSM_SYSV {"sysv", DSM_IMPL_SYSV, false}, #endif #ifdef USE_DSM_WINDOWS {"windows", DSM_IMPL_WINDOWS, false}, #endif #ifdef USE_DSM_MMAP {"mmap", DSM_IMPL_MMAP, false}, #endif {NULL, 0, false} }; /* Implementation selector. */ int dynamic_shared_memory_type; /* Size of buffer to be used for zero-filling. */ #define ZBUFFER_SIZE 8192 #define SEGMENT_NAME_PREFIX "Global/PostgreSQL" /*------ * Perform a low-level shared memory operation in a platform-specific way, * as dictated by the selected implementation. Each implementation is * required to implement the following primitives. * * DSM_OP_CREATE. Create a segment whose size is the request_size and * map it. * * DSM_OP_ATTACH. Map the segment, whose size must be the request_size. * * DSM_OP_DETACH. Unmap the segment. * * DSM_OP_DESTROY. Unmap the segment, if it is mapped. Destroy the * segment. * * Arguments: * op: The operation to be performed. * handle: The handle of an existing object, or for DSM_OP_CREATE, the * a new handle the caller wants created. * request_size: For DSM_OP_CREATE, the requested size. Otherwise, 0. * impl_private: Private, implementation-specific data. Will be a pointer * to NULL for the first operation on a shared memory segment within this * backend; thereafter, it will point to the value to which it was set * on the previous call. * mapped_address: Pointer to start of current mapping; pointer to NULL * if none. Updated with new mapping address. * mapped_size: Pointer to size of current mapping; pointer to 0 if none. * Updated with new mapped size. * elevel: Level at which to log errors. * * Return value: true on success, false on failure. When false is returned, * a message should first be logged at the specified elevel, except in the * case where DSM_OP_CREATE experiences a name collision, which should * silently return false. *----- */ bool dsm_impl_op(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel) { Assert(op == DSM_OP_CREATE || request_size == 0); Assert((op != DSM_OP_CREATE && op != DSM_OP_ATTACH) || (*mapped_address == NULL && *mapped_size == 0)); switch (dynamic_shared_memory_type) { #ifdef USE_DSM_POSIX case DSM_IMPL_POSIX: return dsm_impl_posix(op, handle, request_size, impl_private, mapped_address, mapped_size, elevel); #endif #ifdef USE_DSM_SYSV case DSM_IMPL_SYSV: return dsm_impl_sysv(op, handle, request_size, impl_private, mapped_address, mapped_size, elevel); #endif #ifdef USE_DSM_WINDOWS case DSM_IMPL_WINDOWS: return dsm_impl_windows(op, handle, request_size, impl_private, mapped_address, mapped_size, elevel); #endif #ifdef USE_DSM_MMAP case DSM_IMPL_MMAP: return dsm_impl_mmap(op, handle, request_size, impl_private, mapped_address, mapped_size, elevel); #endif default: elog(ERROR, "unexpected dynamic shared memory type: %d", dynamic_shared_memory_type); return false; } } #ifdef USE_DSM_POSIX /* * Operating system primitives to support POSIX shared memory. * * POSIX shared memory segments are created and attached using shm_open() * and shm_unlink(); other operations, such as sizing or mapping the * segment, are performed as if the shared memory segments were files. * * Indeed, on some platforms, they may be implemented that way. While * POSIX shared memory segments seem intended to exist in a flat namespace, * some operating systems may implement them as files, even going so far * to treat a request for /xyz as a request to create a file by that name * in the root directory. Users of such broken platforms should select * a different shared memory implementation. */ static bool dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel) { char name[64]; int flags; int fd; char *address; snprintf(name, 64, "/PostgreSQL.%u", handle); /* Handle teardown cases. */ if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY) { if (*mapped_address != NULL && munmap(*mapped_address, *mapped_size) != 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not unmap shared memory segment \"%s\": %m", name))); return false; } *mapped_address = NULL; *mapped_size = 0; if (op == DSM_OP_DESTROY && shm_unlink(name) != 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not remove shared memory segment \"%s\": %m", name))); return false; } return true; } /* * Create new segment or open an existing one for attach. * * Even though we're not going through fd.c, we should be safe against * running out of file descriptors, because of NUM_RESERVED_FDS. We're * only opening one extra descriptor here, and we'll close it before * returning. */ flags = O_RDWR | (op == DSM_OP_CREATE ? O_CREAT | O_EXCL : 0); if ((fd = shm_open(name, flags, PG_FILE_MODE_OWNER)) == -1) { if (errno != EEXIST) ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not open shared memory segment \"%s\": %m", name))); return false; } /* * If we're attaching the segment, determine the current size; if we are * creating the segment, set the size to the requested value. */ if (op == DSM_OP_ATTACH) { struct stat st; if (fstat(fd, &st) != 0) { int save_errno; /* Back out what's already been done. */ save_errno = errno; close(fd); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not stat shared memory segment \"%s\": %m", name))); return false; } request_size = st.st_size; } else if (dsm_impl_posix_resize(fd, request_size) != 0) { int save_errno; /* Back out what's already been done. */ save_errno = errno; close(fd); shm_unlink(name); errno = save_errno; /* * If we received a query cancel or termination signal, we will have * EINTR set here. If the caller said that errors are OK here, check * for interrupts immediately. */ if (errno == EINTR && elevel >= ERROR) CHECK_FOR_INTERRUPTS(); ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not resize shared memory segment \"%s\" to %zu bytes: %m", name, request_size))); return false; } /* Map it. */ address = mmap(NULL, request_size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0); if (address == MAP_FAILED) { int save_errno; /* Back out what's already been done. */ save_errno = errno; close(fd); if (op == DSM_OP_CREATE) shm_unlink(name); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not map shared memory segment \"%s\": %m", name))); return false; } *mapped_address = address; *mapped_size = request_size; close(fd); return true; } /* * Set the size of a virtual memory region associated with a file descriptor. * If necessary, also ensure that virtual memory is actually allocated by the * operating system, to avoid nasty surprises later. * * Returns non-zero if either truncation or allocation fails, and sets errno. */ static int dsm_impl_posix_resize(int fd, off_t size) { int rc; /* Truncate (or extend) the file to the requested size. */ rc = ftruncate(fd, size); /* * On Linux, a shm_open fd is backed by a tmpfs file. After resizing with * ftruncate, the file may contain a hole. Accessing memory backed by a * hole causes tmpfs to allocate pages, which fails with SIGBUS if there * is no more tmpfs space available. So we ask tmpfs to allocate pages * here, so we can fail gracefully with ENOSPC now rather than risking * SIGBUS later. */ #if defined(HAVE_POSIX_FALLOCATE) && defined(__linux__) if (rc == 0) { /* * We may get interrupted. If so, just retry unless there is an * interrupt pending. This avoids the possibility of looping forever * if another backend is repeatedly trying to interrupt us. */ do { rc = posix_fallocate(fd, 0, size); } while (rc == EINTR && !(ProcDiePending || QueryCancelPending)); /* * The caller expects errno to be set, but posix_fallocate() doesn't * set it. Instead it returns error numbers directly. So set errno, * even though we'll also return rc to indicate success or failure. */ errno = rc; } #endif /* HAVE_POSIX_FALLOCATE && __linux__ */ return rc; } #endif /* USE_DSM_POSIX */ #ifdef USE_DSM_SYSV /* * Operating system primitives to support System V shared memory. * * System V shared memory segments are manipulated using shmget(), shmat(), * shmdt(), and shmctl(). As the default allocation limits for System V * shared memory are usually quite low, the POSIX facilities may be * preferable; but those are not supported everywhere. */ static bool dsm_impl_sysv(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel) { key_t key; int ident; char *address; char name[64]; int *ident_cache; /* * POSIX shared memory and mmap-based shared memory identify segments with * names. To avoid needless error message variation, we use the handle as * the name. */ snprintf(name, 64, "%u", handle); /* * The System V shared memory namespace is very restricted; names are of * type key_t, which is expected to be some sort of integer data type, but * not necessarily the same one as dsm_handle. Since we use dsm_handle to * identify shared memory segments across processes, this might seem like * a problem, but it's really not. If dsm_handle is bigger than key_t, * the cast below might truncate away some bits from the handle the * user-provided, but it'll truncate exactly the same bits away in exactly * the same fashion every time we use that handle, which is all that * really matters. Conversely, if dsm_handle is smaller than key_t, we * won't use the full range of available key space, but that's no big deal * either. * * We do make sure that the key isn't negative, because that might not be * portable. */ key = (key_t) handle; if (key < 1) /* avoid compiler warning if type is unsigned */ key = -key; /* * There's one special key, IPC_PRIVATE, which can't be used. If we end * up with that value by chance during a create operation, just pretend it * already exists, so that caller will retry. If we run into it anywhere * else, the caller has passed a handle that doesn't correspond to * anything we ever created, which should not happen. */ if (key == IPC_PRIVATE) { if (op != DSM_OP_CREATE) elog(DEBUG4, "System V shared memory key may not be IPC_PRIVATE"); errno = EEXIST; return false; } /* * Before we can do anything with a shared memory segment, we have to map * the shared memory key to a shared memory identifier using shmget(). To * avoid repeated lookups, we store the key using impl_private. */ if (*impl_private != NULL) { ident_cache = *impl_private; ident = *ident_cache; } else { int flags = IPCProtection; size_t segsize; /* * Allocate the memory BEFORE acquiring the resource, so that we don't * leak the resource if memory allocation fails. */ ident_cache = MemoryContextAlloc(TopMemoryContext, sizeof(int)); /* * When using shmget to find an existing segment, we must pass the * size as 0. Passing a non-zero size which is greater than the * actual size will result in EINVAL. */ segsize = 0; if (op == DSM_OP_CREATE) { flags |= IPC_CREAT | IPC_EXCL; segsize = request_size; } if ((ident = shmget(key, segsize, flags)) == -1) { if (errno != EEXIST) { int save_errno = errno; pfree(ident_cache); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not get shared memory segment: %m"))); } return false; } *ident_cache = ident; *impl_private = ident_cache; } /* Handle teardown cases. */ if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY) { pfree(ident_cache); *impl_private = NULL; if (*mapped_address != NULL && shmdt(*mapped_address) != 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not unmap shared memory segment \"%s\": %m", name))); return false; } *mapped_address = NULL; *mapped_size = 0; if (op == DSM_OP_DESTROY && shmctl(ident, IPC_RMID, NULL) < 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not remove shared memory segment \"%s\": %m", name))); return false; } return true; } /* If we're attaching it, we must use IPC_STAT to determine the size. */ if (op == DSM_OP_ATTACH) { struct shmid_ds shm; if (shmctl(ident, IPC_STAT, &shm) != 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not stat shared memory segment \"%s\": %m", name))); return false; } request_size = shm.shm_segsz; } /* Map it. */ address = shmat(ident, NULL, PG_SHMAT_FLAGS); if (address == (void *) -1) { int save_errno; /* Back out what's already been done. */ save_errno = errno; if (op == DSM_OP_CREATE) shmctl(ident, IPC_RMID, NULL); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not map shared memory segment \"%s\": %m", name))); return false; } *mapped_address = address; *mapped_size = request_size; return true; } #endif #ifdef USE_DSM_WINDOWS /* * Operating system primitives to support Windows shared memory. * * Windows shared memory implementation is done using file mapping * which can be backed by either physical file or system paging file. * Current implementation uses system paging file as other effects * like performance are not clear for physical file and it is used in similar * way for main shared memory in windows. * * A memory mapping object is a kernel object - they always get deleted when * the last reference to them goes away, either explicitly via a CloseHandle or * when the process containing the reference exits. */ static bool dsm_impl_windows(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel) { char *address; HANDLE hmap; char name[64]; MEMORY_BASIC_INFORMATION info; /* * Storing the shared memory segment in the Global\ namespace, can allow * any process running in any session to access that file mapping object * provided that the caller has the required access rights. But to avoid * issues faced in main shared memory, we are using the naming convention * similar to main shared memory. We can change here once issue mentioned * in GetSharedMemName is resolved. */ snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle); /* * Handle teardown cases. Since Windows automatically destroys the object * when no references remain, we can treat it the same as detach. */ if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY) { if (*mapped_address != NULL && UnmapViewOfFile(*mapped_address) == 0) { _dosmaperr(GetLastError()); ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not unmap shared memory segment \"%s\": %m", name))); return false; } if (*impl_private != NULL && CloseHandle(*impl_private) == 0) { _dosmaperr(GetLastError()); ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not remove shared memory segment \"%s\": %m", name))); return false; } *impl_private = NULL; *mapped_address = NULL; *mapped_size = 0; return true; } /* Create new segment or open an existing one for attach. */ if (op == DSM_OP_CREATE) { DWORD size_high; DWORD size_low; DWORD errcode; /* Shifts >= the width of the type are undefined. */ #ifdef _WIN64 size_high = request_size >> 32; #else size_high = 0; #endif size_low = (DWORD) request_size; /* CreateFileMapping might not clear the error code on success */ SetLastError(0); hmap = CreateFileMapping(INVALID_HANDLE_VALUE, /* Use the pagefile */ NULL, /* Default security attrs */ PAGE_READWRITE, /* Memory is read/write */ size_high, /* Upper 32 bits of size */ size_low, /* Lower 32 bits of size */ name); errcode = GetLastError(); if (errcode == ERROR_ALREADY_EXISTS || errcode == ERROR_ACCESS_DENIED) { /* * On Windows, when the segment already exists, a handle for the * existing segment is returned. We must close it before * returning. However, if the existing segment is created by a * service, then it returns ERROR_ACCESS_DENIED. We don't do * _dosmaperr here, so errno won't be modified. */ if (hmap) CloseHandle(hmap); return false; } if (!hmap) { _dosmaperr(errcode); ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not create shared memory segment \"%s\": %m", name))); return false; } } else { hmap = OpenFileMapping(FILE_MAP_WRITE | FILE_MAP_READ, FALSE, /* do not inherit the name */ name); /* name of mapping object */ if (!hmap) { _dosmaperr(GetLastError()); ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not open shared memory segment \"%s\": %m", name))); return false; } } /* Map it. */ address = MapViewOfFile(hmap, FILE_MAP_WRITE | FILE_MAP_READ, 0, 0, 0); if (!address) { int save_errno; _dosmaperr(GetLastError()); /* Back out what's already been done. */ save_errno = errno; CloseHandle(hmap); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not map shared memory segment \"%s\": %m", name))); return false; } /* * VirtualQuery gives size in page_size units, which is 4K for Windows. We * need size only when we are attaching, but it's better to get the size * when creating new segment to keep size consistent both for * DSM_OP_CREATE and DSM_OP_ATTACH. */ if (VirtualQuery(address, &info, sizeof(info)) == 0) { int save_errno; _dosmaperr(GetLastError()); /* Back out what's already been done. */ save_errno = errno; UnmapViewOfFile(address); CloseHandle(hmap); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not stat shared memory segment \"%s\": %m", name))); return false; } *mapped_address = address; *mapped_size = info.RegionSize; *impl_private = hmap; return true; } #endif #ifdef USE_DSM_MMAP /* * Operating system primitives to support mmap-based shared memory. * * Calling this "shared memory" is somewhat of a misnomer, because what * we're really doing is creating a bunch of files and mapping them into * our address space. The operating system may feel obliged to * synchronize the contents to disk even if nothing is being paged out, * which will not serve us well. The user can relocate the pg_dynshmem * directory to a ramdisk to avoid this problem, if available. */ static bool dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size, void **impl_private, void **mapped_address, Size *mapped_size, int elevel) { char name[64]; int flags; int fd; char *address; snprintf(name, 64, PG_DYNSHMEM_DIR "/" PG_DYNSHMEM_MMAP_FILE_PREFIX "%u", handle); /* Handle teardown cases. */ if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY) { if (*mapped_address != NULL && munmap(*mapped_address, *mapped_size) != 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not unmap shared memory segment \"%s\": %m", name))); return false; } *mapped_address = NULL; *mapped_size = 0; if (op == DSM_OP_DESTROY && unlink(name) != 0) { ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not remove shared memory segment \"%s\": %m", name))); return false; } return true; } /* Create new segment or open an existing one for attach. */ flags = O_RDWR | (op == DSM_OP_CREATE ? O_CREAT | O_EXCL : 0); if ((fd = OpenTransientFile(name, flags)) == -1) { if (errno != EEXIST) ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not open shared memory segment \"%s\": %m", name))); return false; } /* * If we're attaching the segment, determine the current size; if we are * creating the segment, set the size to the requested value. */ if (op == DSM_OP_ATTACH) { struct stat st; if (fstat(fd, &st) != 0) { int save_errno; /* Back out what's already been done. */ save_errno = errno; CloseTransientFile(fd); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not stat shared memory segment \"%s\": %m", name))); return false; } request_size = st.st_size; } else { /* * Allocate a buffer full of zeros. * * Note: palloc zbuffer, instead of just using a local char array, to * ensure it is reasonably well-aligned; this may save a few cycles * transferring data to the kernel. */ char *zbuffer = (char *) palloc0(ZBUFFER_SIZE); uint32 remaining = request_size; bool success = true; /* * Zero-fill the file. We have to do this the hard way to ensure that * all the file space has really been allocated, so that we don't * later seg fault when accessing the memory mapping. This is pretty * pessimal. */ while (success && remaining > 0) { Size goal = remaining; if (goal > ZBUFFER_SIZE) goal = ZBUFFER_SIZE; pgstat_report_wait_start(WAIT_EVENT_DSM_FILL_ZERO_WRITE); if (write(fd, zbuffer, goal) == goal) remaining -= goal; else success = false; pgstat_report_wait_end(); } if (!success) { int save_errno; /* Back out what's already been done. */ save_errno = errno; CloseTransientFile(fd); unlink(name); errno = save_errno ? save_errno : ENOSPC; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not resize shared memory segment \"%s\" to %zu bytes: %m", name, request_size))); return false; } } /* Map it. */ address = mmap(NULL, request_size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0); if (address == MAP_FAILED) { int save_errno; /* Back out what's already been done. */ save_errno = errno; CloseTransientFile(fd); if (op == DSM_OP_CREATE) unlink(name); errno = save_errno; ereport(elevel, (errcode_for_dynamic_shared_memory(), errmsg("could not map shared memory segment \"%s\": %m", name))); return false; } *mapped_address = address; *mapped_size = request_size; if (CloseTransientFile(fd) != 0) { ereport(elevel, (errcode_for_file_access(), errmsg("could not close shared memory segment \"%s\": %m", name))); return false; } return true; } #endif /* * Implementation-specific actions that must be performed when a segment is to * be preserved even when no backend has it attached. * * Except on Windows, we don't need to do anything at all. But since Windows * cleans up segments automatically when no references remain, we duplicate * the segment handle into the postmaster process. The postmaster needn't * do anything to receive the handle; Windows transfers it automatically. */ void dsm_impl_pin_segment(dsm_handle handle, void *impl_private, void **impl_private_pm_handle) { switch (dynamic_shared_memory_type) { #ifdef USE_DSM_WINDOWS case DSM_IMPL_WINDOWS: { HANDLE hmap; if (!DuplicateHandle(GetCurrentProcess(), impl_private, PostmasterHandle, &hmap, 0, FALSE, DUPLICATE_SAME_ACCESS)) { char name[64]; snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle); _dosmaperr(GetLastError()); ereport(ERROR, (errcode_for_dynamic_shared_memory(), errmsg("could not duplicate handle for \"%s\": %m", name))); } /* * Here, we remember the handle that we created in the * postmaster process. This handle isn't actually usable in * any process other than the postmaster, but that doesn't * matter. We're just holding onto it so that, if the segment * is unpinned, dsm_impl_unpin_segment can close it. */ *impl_private_pm_handle = hmap; break; } #endif default: break; } } /* * Implementation-specific actions that must be performed when a segment is no * longer to be preserved, so that it will be cleaned up when all backends * have detached from it. * * Except on Windows, we don't need to do anything at all. For Windows, we * close the extra handle that dsm_impl_pin_segment created in the * postmaster's process space. */ void dsm_impl_unpin_segment(dsm_handle handle, void **impl_private) { switch (dynamic_shared_memory_type) { #ifdef USE_DSM_WINDOWS case DSM_IMPL_WINDOWS: { if (*impl_private && !DuplicateHandle(PostmasterHandle, *impl_private, NULL, NULL, 0, FALSE, DUPLICATE_CLOSE_SOURCE)) { char name[64]; snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle); _dosmaperr(GetLastError()); ereport(ERROR, (errcode_for_dynamic_shared_memory(), errmsg("could not duplicate handle for \"%s\": %m", name))); } *impl_private = NULL; break; } #endif default: break; } } static int errcode_for_dynamic_shared_memory(void) { if (errno == EFBIG || errno == ENOMEM) return errcode(ERRCODE_OUT_OF_MEMORY); else return errcode_for_file_access(); }