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Diffstat (limited to 'src/backend/storage/ipc/dsm.c')
| -rw-r--r-- | src/backend/storage/ipc/dsm.c | 972 |
1 files changed, 972 insertions, 0 deletions
diff --git a/src/backend/storage/ipc/dsm.c b/src/backend/storage/ipc/dsm.c new file mode 100644 index 00000000000..e516197bd48 --- /dev/null +++ b/src/backend/storage/ipc/dsm.c @@ -0,0 +1,972 @@ +/*------------------------------------------------------------------------- + * + * dsm.c + * manage dynamic shared memory segments + * + * This file provides a set of services to make programming with dynamic + * shared memory segments more convenient. Unlike the low-level + * facilities provided by dsm_impl.h and dsm_impl.c, mappings and segments + * created using this module will be cleaned up automatically. Mappings + * will be removed when the resource owner under which they were created + * is cleaned up, unless dsm_keep_mapping() is used, in which case they + * have session lifespan. Segments will be removed when there are no + * remaining mappings, or at postmaster shutdown in any case. After a + * hard postmaster crash, remaining segments will be removed, if they + * still exist, at the next postmaster startup. + * + * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group + * Portions Copyright (c) 1994, Regents of the University of California + * + * + * IDENTIFICATION + * src/backend/storage/ipc/dsm.c + * + *------------------------------------------------------------------------- + */ + +#include "postgres.h" + +#include <fcntl.h> +#include <string.h> +#include <unistd.h> +#ifndef WIN32 +#include <sys/mman.h> +#endif +#include <sys/stat.h> + +#include "lib/ilist.h" +#include "miscadmin.h" +#include "storage/dsm.h" +#include "storage/ipc.h" +#include "storage/lwlock.h" +#include "utils/guc.h" +#include "utils/memutils.h" +#include "utils/resowner_private.h" + +#define PG_DYNSHMEM_STATE_FILE PG_DYNSHMEM_DIR "/state" +#define PG_DYNSHMEM_NEW_STATE_FILE PG_DYNSHMEM_DIR "/state.new" +#define PG_DYNSHMEM_STATE_BUFSIZ 512 +#define PG_DYNSHMEM_CONTROL_MAGIC 0x9a503d32 + +/* + * There's no point in getting too cheap here, because the minimum allocation + * is one OS page, which is probably at least 4KB and could easily be as high + * as 64KB. Each currently sizeof(dsm_control_item), currently 8 bytes. + */ +#define PG_DYNSHMEM_FIXED_SLOTS 64 +#define PG_DYNSHMEM_SLOTS_PER_BACKEND 2 + +#define INVALID_CONTROL_SLOT ((uint32) -1) + +/* Backend-local state for a dynamic shared memory segment. */ +struct dsm_segment +{ + dlist_node node; /* List link in dsm_segment_list. */ + ResourceOwner resowner; /* Resource owner. */ + dsm_handle handle; /* Segment name. */ + uint32 control_slot; /* Slot in control segment. */ + void *impl_private; /* Implementation-specific private data. */ + void *mapped_address; /* Mapping address, or NULL if unmapped. */ + uint64 mapped_size; /* Size of our mapping. */ +}; + +/* Shared-memory state for a dynamic shared memory segment. */ +typedef struct dsm_control_item +{ + dsm_handle handle; + uint32 refcnt; /* 2+ = active, 1 = moribund, 0 = gone */ +} dsm_control_item; + +/* Layout of the dynamic shared memory control segment. */ +typedef struct dsm_control_header +{ + uint32 magic; + uint32 nitems; + uint32 maxitems; + dsm_control_item item[FLEXIBLE_ARRAY_MEMBER]; +} dsm_control_header; + +static void dsm_cleanup_using_control_segment(void); +static void dsm_cleanup_for_mmap(void); +static bool dsm_read_state_file(dsm_handle *h); +static void dsm_write_state_file(dsm_handle h); +static void dsm_postmaster_shutdown(int code, Datum arg); +static void dsm_backend_shutdown(int code, Datum arg); +static dsm_segment *dsm_create_descriptor(void); +static bool dsm_control_segment_sane(dsm_control_header *control, + uint64 mapped_size); +static uint64 dsm_control_bytes_needed(uint32 nitems); + +/* Has this backend initialized the dynamic shared memory system yet? */ +static bool dsm_init_done = false; + +/* + * List of dynamic shared memory segments used by this backend. + * + * At process exit time, we must decrement the reference count of each + * segment we have attached; this list makes it possible to find all such + * segments. + * + * This list should always be empty in the postmaster. We could probably + * allow the postmaster to map dynamic shared memory segments before it + * begins to start child processes, provided that each process adjusted + * the reference counts for those segments in the control segment at + * startup time, but there's no obvious need for such a facility, which + * would also be complex to handle in the EXEC_BACKEND case. Once the + * postmaster has begun spawning children, there's an additional problem: + * each new mapping would require an update to the control segment, + * which requires locking, in which the postmaster must not be involved. + */ +static dlist_head dsm_segment_list = DLIST_STATIC_INIT(dsm_segment_list); + +/* + * Control segment information. + * + * Unlike ordinary shared memory segments, the control segment is not + * reference counted; instead, it lasts for the postmaster's entire + * life cycle. For simplicity, it doesn't have a dsm_segment object either. + */ +static dsm_handle dsm_control_handle; +static dsm_control_header *dsm_control; +static uint64 dsm_control_mapped_size = 0; +static void *dsm_control_impl_private = NULL; + +/* + * Start up the dynamic shared memory system. + * + * This is called just once during each cluster lifetime, at postmaster + * startup time. + */ +void +dsm_postmaster_startup(void) +{ + void *dsm_control_address = NULL; + uint32 maxitems; + uint64 segsize; + + Assert(!IsUnderPostmaster); + + /* If dynamic shared memory is disabled, there's nothing to do. */ + if (dynamic_shared_memory_type == DSM_IMPL_NONE) + return; + + /* + * Check for, and remove, shared memory segments left behind by a dead + * postmaster. This isn't necessary on Windows, which always removes them + * when the last reference is gone. + */ + switch (dynamic_shared_memory_type) + { + case DSM_IMPL_POSIX: + case DSM_IMPL_SYSV: + dsm_cleanup_using_control_segment(); + break; + case DSM_IMPL_MMAP: + dsm_cleanup_for_mmap(); + break; + case DSM_IMPL_WINDOWS: + /* Nothing to do. */ + break; + default: + elog(ERROR, "unknown dynamic shared memory type: %d", + dynamic_shared_memory_type); + } + + /* Determine size for new control segment. */ + maxitems = PG_DYNSHMEM_FIXED_SLOTS + + PG_DYNSHMEM_SLOTS_PER_BACKEND * MaxBackends; + elog(DEBUG2, "dynamic shared memory system will support %u segments", + maxitems); + segsize = dsm_control_bytes_needed(maxitems); + + /* Loop until we find an unused identifier for the new control segment. */ + for (;;) + { + Assert(dsm_control_address == NULL); + Assert(dsm_control_mapped_size == 0); + dsm_control_handle = random(); + if (dsm_impl_op(DSM_OP_CREATE, dsm_control_handle, segsize, + &dsm_control_impl_private, &dsm_control_address, + &dsm_control_mapped_size, ERROR)) + break; + } + dsm_control = dsm_control_address; + on_shmem_exit(dsm_postmaster_shutdown, 0); + elog(DEBUG2, "created dynamic shared memory control segment %u (" + UINT64_FORMAT " bytes)", dsm_control_handle, segsize); + dsm_write_state_file(dsm_control_handle); + + /* Initialize control segment. */ + dsm_control->magic = PG_DYNSHMEM_CONTROL_MAGIC; + dsm_control->nitems = 0; + dsm_control->maxitems = maxitems; +} + +/* + * Determine whether the control segment from the previous postmaster + * invocation still exists. If so, remove the dynamic shared memory + * segments to which it refers, and then the control segment itself. + */ +static void +dsm_cleanup_using_control_segment(void) +{ + void *mapped_address = NULL; + void *junk_mapped_address = NULL; + void *impl_private = NULL; + void *junk_impl_private = NULL; + uint64 mapped_size = 0; + uint64 junk_mapped_size = 0; + uint32 nitems; + uint32 i; + dsm_handle old_control_handle; + dsm_control_header *old_control; + + /* + * Read the state file. If it doesn't exist or is empty, there's nothing + * more to do. + */ + if (!dsm_read_state_file(&old_control_handle)) + return; + + /* + * Try to attach the segment. If this fails, it probably just means that + * the operating system has been rebooted and the segment no longer exists, + * or an unrelated proces has used the same shm ID. So just fall out + * quietly. + */ + if (!dsm_impl_op(DSM_OP_ATTACH, old_control_handle, 0, &impl_private, + &mapped_address, &mapped_size, DEBUG1)) + return; + + /* + * We've managed to reattach it, but the contents might not be sane. + * If they aren't, we disregard the segment after all. + */ + old_control = (dsm_control_header *) mapped_address; + if (!dsm_control_segment_sane(old_control, mapped_size)) + { + dsm_impl_op(DSM_OP_DETACH, old_control_handle, 0, &impl_private, + &mapped_address, &mapped_size, LOG); + return; + } + + /* + * OK, the control segment looks basically valid, so we can get use + * it to get a list of segments that need to be removed. + */ + nitems = old_control->nitems; + for (i = 0; i < nitems; ++i) + { + dsm_handle handle; + uint32 refcnt; + + /* If the reference count is 0, the slot is actually unused. */ + refcnt = old_control->item[i].refcnt; + if (refcnt == 0) + continue; + + /* Log debugging information. */ + handle = old_control->item[i].handle; + elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u (reference count %u)", + handle, refcnt); + + /* Destroy the referenced segment. */ + dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private, + &junk_mapped_address, &junk_mapped_size, LOG); + } + + /* Destroy the old control segment, too. */ + elog(DEBUG2, + "cleaning up dynamic shared memory control segment with ID %u", + old_control_handle); + dsm_impl_op(DSM_OP_DESTROY, old_control_handle, 0, &impl_private, + &mapped_address, &mapped_size, LOG); +} + +/* + * When we're using the mmap shared memory implementation, "shared memory" + * segments might even manage to survive an operating system reboot. + * But there's no guarantee as to exactly what will survive: some segments + * may survive, and others may not, and the contents of some may be out + * of date. In particular, the control segment may be out of date, so we + * can't rely on it to figure out what to remove. However, since we know + * what directory contains the files we used as shared memory, we can simply + * scan the directory and blow everything away that shouldn't be there. + */ +static void +dsm_cleanup_for_mmap(void) +{ + DIR *dir; + struct dirent *dent; + + /* Open the directory; can't use AllocateDir in postmaster. */ + if ((dir = opendir(PG_DYNSHMEM_DIR)) == NULL) + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not open directory \"%s\": %m", + PG_DYNSHMEM_DIR))); + + /* Scan for something with a name of the correct format. */ + while ((dent = readdir(dir)) != NULL) + { + if (strncmp(dent->d_name, PG_DYNSHMEM_MMAP_FILE_PREFIX, + strlen(PG_DYNSHMEM_MMAP_FILE_PREFIX)) == 0) + { + char buf[MAXPGPATH]; + snprintf(buf, MAXPGPATH, PG_DYNSHMEM_DIR "/%s", dent->d_name); + + elog(DEBUG2, "removing file \"%s\"", buf); + + /* We found a matching file; so remove it. */ + if (unlink(buf) != 0) + { + int save_errno; + + save_errno = errno; + closedir(dir); + errno = save_errno; + + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not remove file \"%s\": %m", buf))); + } + } + } + + /* Cleanup complete. */ + closedir(dir); +} + +/* + * Read and parse the state file. + * + * If the state file is empty or the contents are garbled, it probably means + * that the operating system rebooted before the data written by the previous + * postmaster made it to disk. In that case, we can just ignore it; any shared + * memory from before the reboot should be gone anyway. + */ +static bool +dsm_read_state_file(dsm_handle *h) +{ + int statefd; + char statebuf[PG_DYNSHMEM_STATE_BUFSIZ]; + int nbytes = 0; + char *endptr, + *s; + dsm_handle handle; + + /* Read the state file to get the ID of the old control segment. */ + statefd = open(PG_DYNSHMEM_STATE_FILE, O_RDONLY | PG_BINARY, 0); + if (statefd < 0) + { + if (errno == ENOENT) + return false; + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not open file \"%s\": %m", + PG_DYNSHMEM_STATE_FILE))); + } + nbytes = read(statefd, statebuf, PG_DYNSHMEM_STATE_BUFSIZ - 1); + if (nbytes < 0) + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not read file \"%s\": %m", + PG_DYNSHMEM_STATE_FILE))); + /* make sure buffer is NUL terminated */ + statebuf[nbytes] = '\0'; + close(statefd); + + /* + * We expect to find the handle of the old control segment here, + * on a line by itself. + */ + handle = strtoul(statebuf, &endptr, 10); + for (s = endptr; *s == ' ' || *s == '\t'; ++s) + ; + if (*s != '\n' && *s != '\0') + return false; + + /* Looks good. */ + *h = handle; + return true; +} + +/* + * Write our control segment handle to the state file, so that if the + * postmaster is killed without running it's on_shmem_exit hooks, the + * next postmaster can clean things up after restart. + */ +static void +dsm_write_state_file(dsm_handle h) +{ + int statefd; + char statebuf[PG_DYNSHMEM_STATE_BUFSIZ]; + int nbytes; + + /* Create or truncate the file. */ + statefd = open(PG_DYNSHMEM_NEW_STATE_FILE, + O_RDWR | O_CREAT | O_TRUNC | PG_BINARY, 0600); + if (statefd < 0) + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not create file \"%s\": %m", + PG_DYNSHMEM_NEW_STATE_FILE))); + + /* Write contents. */ + snprintf(statebuf, PG_DYNSHMEM_STATE_BUFSIZ, "%u\n", dsm_control_handle); + nbytes = strlen(statebuf); + if (write(statefd, statebuf, nbytes) != nbytes) + { + if (errno == 0) + errno = ENOSPC; /* if no error signalled, assume no space */ + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not write file \"%s\": %m", + PG_DYNSHMEM_NEW_STATE_FILE))); + } + + /* Close file. */ + close(statefd); + + /* + * Atomically rename file into place, so that no one ever sees a partially + * written state file. + */ + if (rename(PG_DYNSHMEM_NEW_STATE_FILE, PG_DYNSHMEM_STATE_FILE) < 0) + ereport(ERROR, + (errcode_for_file_access(), + errmsg("could not rename file \"%s\": %m", + PG_DYNSHMEM_NEW_STATE_FILE))); +} + +/* + * At shutdown time, we iterate over the control segment and remove all + * remaining dynamic shared memory segments. We avoid throwing errors here; + * the postmaster is shutting down either way, and this is just non-critical + * resource cleanup. + */ +static void +dsm_postmaster_shutdown(int code, Datum arg) +{ + uint32 nitems; + uint32 i; + void *dsm_control_address; + void *junk_mapped_address = NULL; + void *junk_impl_private = NULL; + uint64 junk_mapped_size = 0; + + /* + * If some other backend exited uncleanly, it might have corrupted the + * control segment while it was dying. In that case, we warn and ignore + * the contents of the control segment. This may end up leaving behind + * stray shared memory segments, but there's not much we can do about + * that if the metadata is gone. + */ + nitems = dsm_control->nitems; + if (!dsm_control_segment_sane(dsm_control, dsm_control_mapped_size)) + { + ereport(LOG, + (errmsg("dynamic shared memory control segment is corrupt"))); + return; + } + + /* Remove any remaining segments. */ + for (i = 0; i < nitems; ++i) + { + dsm_handle handle; + + /* If the reference count is 0, the slot is actually unused. */ + if (dsm_control->item[i].refcnt == 0) + continue; + + /* Log debugging information. */ + handle = dsm_control->item[i].handle; + elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u", + handle); + + /* Destroy the segment. */ + dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private, + &junk_mapped_address, &junk_mapped_size, LOG); + } + + /* Remove the control segment itself. */ + elog(DEBUG2, + "cleaning up dynamic shared memory control segment with ID %u", + dsm_control_handle); + dsm_control_address = dsm_control; + dsm_impl_op(DSM_OP_DESTROY, dsm_control_handle, 0, + &dsm_control_impl_private, &dsm_control_address, + &dsm_control_mapped_size, LOG); + dsm_control = dsm_control_address; + + /* And, finally, remove the state file. */ + if (unlink(PG_DYNSHMEM_STATE_FILE) < 0) + ereport(LOG, + (errcode_for_file_access(), + errmsg("could not unlink file \"%s\": %m", + PG_DYNSHMEM_STATE_FILE))); +} + +/* + * Prepare this backend for dynamic shared memory usage. Under EXEC_BACKEND, + * we must reread the state file and map the control segment; in other cases, + * we'll have inherited the postmaster's mapping and global variables. + */ +static void +dsm_backend_startup(void) +{ + /* If dynamic shared memory is disabled, reject this. */ + if (dynamic_shared_memory_type == DSM_IMPL_NONE) + ereport(ERROR, + (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE), + errmsg("dynamic shared memory is disabled"), + errhint("Set dynamic_shared_memory_type to a value other than \"none\"."))); + +#ifdef EXEC_BACKEND + { + dsm_handle control_handle; + void *control_address = NULL; + + /* Read the control segment information from the state file. */ + if (!dsm_read_state_file(&control_handle)) + ereport(ERROR, + (errcode(ERRCODE_INTERNAL_ERROR), + errmsg("could not parse dynamic shared memory state file"))); + + /* Attach control segment. */ + dsm_impl_op(DSM_OP_ATTACH, control_handle, 0, + &dsm_control_impl_private, &control_address, + &dsm_control_mapped_size, ERROR); + dsm_control_handle = control_handle; + dsm_control = control_address; + /* If control segment doesn't look sane, something is badly wrong. */ + if (!dsm_control_segment_sane(dsm_control, dsm_control_mapped_size)) + { + dsm_impl_op(DSM_OP_DETACH, control_handle, 0, + &dsm_control_impl_private, &control_address, + &dsm_control_mapped_size, WARNING); + ereport(FATAL, + (errcode(ERRCODE_INTERNAL_ERROR), + errmsg("dynamic shared memory control segment is not valid"))); + } + } +#endif + + /* Arrange to detach segments on exit. */ + on_shmem_exit(dsm_backend_shutdown, 0); + + dsm_init_done = true; +} + +/* + * Create a new dynamic shared memory segment. + */ +dsm_segment * +dsm_create(uint64 size) +{ + dsm_segment *seg = dsm_create_descriptor(); + uint32 i; + uint32 nitems; + + /* Unsafe in postmaster (and pointless in a stand-alone backend). */ + Assert(IsUnderPostmaster); + + if (!dsm_init_done) + dsm_backend_startup(); + + /* Loop until we find an unused segment identifier. */ + for (;;) + { + Assert(seg->mapped_address == NULL && seg->mapped_size == 0); + seg->handle = random(); + if (dsm_impl_op(DSM_OP_CREATE, seg->handle, size, &seg->impl_private, + &seg->mapped_address, &seg->mapped_size, ERROR)) + break; + } + + /* Lock the control segment so we can register the new segment. */ + LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE); + + /* Search the control segment for an unused slot. */ + nitems = dsm_control->nitems; + for (i = 0; i < nitems; ++i) + { + if (dsm_control->item[i].refcnt == 0) + { + dsm_control->item[i].handle = seg->handle; + /* refcnt of 1 triggers destruction, so start at 2 */ + dsm_control->item[i].refcnt = 2; + seg->control_slot = i; + LWLockRelease(DynamicSharedMemoryControlLock); + return seg; + } + } + + /* Verify that we can support an additional mapping. */ + if (nitems >= dsm_control->maxitems) + ereport(ERROR, + (errcode(ERRCODE_INSUFFICIENT_RESOURCES), + errmsg("too many dynamic shared memory segments"))); + + /* Enter the handle into a new array slot. */ + dsm_control->item[nitems].handle = seg->handle; + /* refcnt of 1 triggers destruction, so start at 2 */ + dsm_control->item[nitems].refcnt = 2; + seg->control_slot = nitems; + dsm_control->nitems++; + LWLockRelease(DynamicSharedMemoryControlLock); + + return seg; +} + +/* + * Attach a dynamic shared memory segment. + * + * See comments for dsm_segment_handle() for an explanation of how this + * is intended to be used. + * + * This function will return NULL if the segment isn't known to the system. + * This can happen if we're asked to attach the segment, but then everyone + * else detaches it (causing it to be destroyed) before we get around to + * attaching it. + */ +dsm_segment * +dsm_attach(dsm_handle h) +{ + dsm_segment *seg; + dlist_iter iter; + uint32 i; + uint32 nitems; + + /* Unsafe in postmaster (and pointless in a stand-alone backend). */ + Assert(IsUnderPostmaster); + + if (!dsm_init_done) + dsm_backend_startup(); + + /* + * Since this is just a debugging cross-check, we could leave it out + * altogether, or include it only in assert-enabled builds. But since + * the list of attached segments should normally be very short, let's + * include it always for right now. + * + * If you're hitting this error, you probably want to attempt to + * find an existing mapping via dsm_find_mapping() before calling + * dsm_attach() to create a new one. + */ + dlist_foreach(iter, &dsm_segment_list) + { + seg = dlist_container(dsm_segment, node, iter.cur); + if (seg->handle == h) + elog(ERROR, "can't attach the same segment more than once"); + } + + /* Create a new segment descriptor. */ + seg = dsm_create_descriptor(); + seg->handle = h; + + /* Bump reference count for this segment in shared memory. */ + LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE); + nitems = dsm_control->nitems; + for (i = 0; i < nitems; ++i) + { + /* If the reference count is 0, the slot is actually unused. */ + if (dsm_control->item[i].refcnt == 0) + continue; + + /* + * If the reference count is 1, the slot is still in use, but the + * segment is in the process of going away. Treat that as if we + * didn't find a match. + */ + if (dsm_control->item[i].refcnt == 1) + break; + + /* Otherwise, if the descriptor matches, we've found a match. */ + if (dsm_control->item[i].handle == seg->handle) + { + dsm_control->item[i].refcnt++; + seg->control_slot = i; + break; + } + } + LWLockRelease(DynamicSharedMemoryControlLock); + + /* + * If we didn't find the handle we're looking for in the control + * segment, it probably means that everyone else who had it mapped, + * including the original creator, died before we got to this point. + * It's up to the caller to decide what to do about that. + */ + if (seg->control_slot == INVALID_CONTROL_SLOT) + { + dsm_detach(seg); + return NULL; + } + + /* Here's where we actually try to map the segment. */ + dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private, + &seg->mapped_address, &seg->mapped_size, ERROR); + + return seg; +} + +/* + * At backend shutdown time, detach any segments that are still attached. + */ +static void +dsm_backend_shutdown(int code, Datum arg) +{ + while (!dlist_is_empty(&dsm_segment_list)) + { + dsm_segment *seg; + + seg = dlist_head_element(dsm_segment, node, &dsm_segment_list); + dsm_detach(seg); + } +} + +/* + * Resize an existing shared memory segment. + * + * This may cause the shared memory segment to be remapped at a different + * address. For the caller's convenience, we return the mapped address. + */ +void * +dsm_resize(dsm_segment *seg, uint64 size) +{ + Assert(seg->control_slot != INVALID_CONTROL_SLOT); + dsm_impl_op(DSM_OP_RESIZE, seg->handle, size, &seg->impl_private, + &seg->mapped_address, &seg->mapped_size, ERROR); + return seg->mapped_address; +} + +/* + * Remap an existing shared memory segment. + * + * This is intended to be used when some other process has extended the + * mapping using dsm_resize(), but we've still only got the initial + * portion mapped. Since this might change the address at which the + * segment is mapped, we return the new mapped address. + */ +void * +dsm_remap(dsm_segment *seg) +{ + dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private, + &seg->mapped_address, &seg->mapped_size, ERROR); + + return seg->mapped_address; +} + +/* + * Detach from a shared memory segment, destroying the segment if we + * remove the last reference. + * + * This function should never fail. It will often be invoked when aborting + * a transaction, and a further error won't serve any purpose. It's not a + * complete disaster if we fail to unmap or destroy the segment; it means a + * resource leak, but that doesn't necessarily preclude further operations. + */ +void +dsm_detach(dsm_segment *seg) +{ + /* + * Try to remove the mapping, if one exists. Normally, there will be, + * but maybe not, if we failed partway through a create or attach + * operation. We remove the mapping before decrementing the reference + * count so that the process that sees a zero reference count can be + * certain that no remaining mappings exist. Even if this fails, we + * pretend that it works, because retrying is likely to fail in the + * same way. + */ + if (seg->mapped_address != NULL) + { + dsm_impl_op(DSM_OP_DETACH, seg->handle, 0, &seg->impl_private, + &seg->mapped_address, &seg->mapped_size, WARNING); + seg->impl_private = NULL; + seg->mapped_address = NULL; + seg->mapped_size = 0; + } + + /* Reduce reference count, if we previously increased it. */ + if (seg->control_slot != INVALID_CONTROL_SLOT) + { + uint32 refcnt; + uint32 control_slot = seg->control_slot; + + LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE); + Assert(dsm_control->item[control_slot].handle == seg->handle); + Assert(dsm_control->item[control_slot].refcnt > 1); + refcnt = --dsm_control->item[control_slot].refcnt; + seg->control_slot = INVALID_CONTROL_SLOT; + LWLockRelease(DynamicSharedMemoryControlLock); + + /* If new reference count is 1, try to destroy the segment. */ + if (refcnt == 1) + { + /* + * If we fail to destroy the segment here, or are killed before + * we finish doing so, the reference count will remain at 1, which + * will mean that nobody else can attach to the segment. At + * postmaster shutdown time, or when a new postmaster is started + * after a hard kill, another attempt will be made to remove the + * segment. + * + * The main case we're worried about here is being killed by + * a signal before we can finish removing the segment. In that + * case, it's important to be sure that the segment still gets + * removed. If we actually fail to remove the segment for some + * other reason, the postmaster may not have any better luck than + * we did. There's not much we can do about that, though. + */ + if (dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private, + &seg->mapped_address, &seg->mapped_size, WARNING)) + { + LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE); + Assert(dsm_control->item[control_slot].handle == seg->handle); + Assert(dsm_control->item[control_slot].refcnt == 1); + dsm_control->item[control_slot].refcnt = 0; + LWLockRelease(DynamicSharedMemoryControlLock); + } + } + } + + /* Clean up our remaining backend-private data structures. */ + if (seg->resowner != NULL) + ResourceOwnerForgetDSM(seg->resowner, seg); + dlist_delete(&seg->node); + pfree(seg); +} + +/* + * Keep a dynamic shared memory mapping until end of session. + * + * By default, mappings are owned by the current resource owner, which + * typically means they stick around for the duration of the current query + * only. + */ +void +dsm_keep_mapping(dsm_segment *seg) +{ + if (seg->resowner != NULL) + { + ResourceOwnerForgetDSM(seg->resowner, seg); + seg->resowner = NULL; + } +} + +/* + * Find an existing mapping for a shared memory segment, if there is one. + */ +dsm_segment * +dsm_find_mapping(dsm_handle h) +{ + dlist_iter iter; + dsm_segment *seg; + + dlist_foreach(iter, &dsm_segment_list) + { + seg = dlist_container(dsm_segment, node, iter.cur); + if (seg->handle == h) + return seg; + } + + return NULL; +} + +/* + * Get the address at which a dynamic shared memory segment is mapped. + */ +void * +dsm_segment_address(dsm_segment *seg) +{ + Assert(seg->mapped_address != NULL); + return seg->mapped_address; +} + +/* + * Get the size of a mapping. + */ +uint64 +dsm_segment_map_length(dsm_segment *seg) +{ + Assert(seg->mapped_address != NULL); + return seg->mapped_size; +} + +/* + * Get a handle for a mapping. + * + * To establish communication via dynamic shared memory between two backends, + * one of them should first call dsm_create() to establish a new shared + * memory mapping. That process should then call dsm_segment_handle() to + * obtain a handle for the mapping, and pass that handle to the + * coordinating backend via some means (e.g. bgw_main_arg, or via the + * main shared memory segment). The recipient, once in position of the + * handle, should call dsm_attach(). + */ +dsm_handle +dsm_segment_handle(dsm_segment *seg) +{ + return seg->handle; +} + +/* + * Create a segment descriptor. + */ +static dsm_segment * +dsm_create_descriptor(void) +{ + dsm_segment *seg; + + ResourceOwnerEnlargeDSMs(CurrentResourceOwner); + + seg = MemoryContextAlloc(TopMemoryContext, sizeof(dsm_segment)); + dlist_push_head(&dsm_segment_list, &seg->node); + + /* seg->handle must be initialized by the caller */ + seg->control_slot = INVALID_CONTROL_SLOT; + seg->impl_private = NULL; + seg->mapped_address = NULL; + seg->mapped_size = 0; + + seg->resowner = CurrentResourceOwner; + ResourceOwnerRememberDSM(CurrentResourceOwner, seg); + + return seg; +} + +/* + * Sanity check a control segment. + * + * The goal here isn't to detect everything that could possibly be wrong with + * the control segment; there's not enough information for that. Rather, the + * goal is to make sure that someone can iterate over the items in the segment + * without overrunning the end of the mapping and crashing. We also check + * the magic number since, if that's messed up, this may not even be one of + * our segments at all. + */ +static bool +dsm_control_segment_sane(dsm_control_header *control, uint64 mapped_size) +{ + if (mapped_size < offsetof(dsm_control_header, item)) + return false; /* Mapped size too short to read header. */ + if (control->magic != PG_DYNSHMEM_CONTROL_MAGIC) + return false; /* Magic number doesn't match. */ + if (dsm_control_bytes_needed(control->maxitems) > mapped_size) + return false; /* Max item count won't fit in map. */ + if (control->nitems > control->maxitems) + return false; /* Overfull. */ + return true; +} + +/* + * Compute the number of control-segment bytes needed to store a given + * number of items. + */ +static uint64 +dsm_control_bytes_needed(uint32 nitems) +{ + return offsetof(dsm_control_header, item) + + sizeof(dsm_control_item) * (uint64) nitems; +} |