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session.c
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1 /*-------------------------------------------------------------------------
2  *
3  * session.c
4  * Encapsulation of user session.
5  *
6  * This is intended to contain data that needs to be shared between backends
7  * performing work for a client session. In particular such a session is
8  * shared between the leader and worker processes for parallel queries. At
9  * some later point it might also become useful infrastructure for separating
10  * backends from client connections, e.g. for the purpose of pooling.
11  *
12  * Currently this infrastructure is used to share:
13  * - typemod registry for ephemeral row-types, i.e. BlessTupleDesc etc.
14  *
15  * Portions Copyright (c) 2017-2019, PostgreSQL Global Development Group
16  *
17  * src/backend/access/common/session.c
18  *
19  *-------------------------------------------------------------------------
20  */
21 #include "postgres.h"
22 
23 #include "access/session.h"
24 #include "storage/lwlock.h"
25 #include "storage/shm_toc.h"
26 #include "utils/memutils.h"
27 #include "utils/typcache.h"
28 
29 /* Magic number for per-session DSM TOC. */
30 #define SESSION_MAGIC 0xabb0fbc9
31 
32 /*
33  * We want to create a DSA area to store shared state that has the same
34  * lifetime as a session. So far, it's only used to hold the shared record
35  * type registry. We don't want it to have to create any DSM segments just
36  * yet in common cases, so we'll give it enough space to hold a very small
37  * SharedRecordTypmodRegistry.
38  */
39 #define SESSION_DSA_SIZE 0x30000
40 
41 /*
42  * Magic numbers for state sharing in the per-session DSM area.
43  */
44 #define SESSION_KEY_DSA UINT64CONST(0xFFFFFFFFFFFF0001)
45 #define SESSION_KEY_RECORD_TYPMOD_REGISTRY UINT64CONST(0xFFFFFFFFFFFF0002)
46 
47 /* This backend's current session. */
49 
50 /*
51  * Set up CurrentSession to point to an empty Session object.
52  */
53 void
55 {
56  CurrentSession = MemoryContextAllocZero(TopMemoryContext, sizeof(Session));
57 }
58 
59 /*
60  * Initialize the per-session DSM segment if it isn't already initialized, and
61  * return its handle so that worker processes can attach to it.
62  *
63  * Unlike the per-context DSM segment, this segment and its contents are
64  * reused for future parallel queries.
65  *
66  * Return DSM_HANDLE_INVALID if a segment can't be allocated due to lack of
67  * resources.
68  */
71 {
72  shm_toc_estimator estimator;
73  shm_toc *toc;
74  dsm_segment *seg;
75  size_t typmod_registry_size;
76  size_t size;
77  void *dsa_space;
78  void *typmod_registry_space;
79  dsa_area *dsa;
80  MemoryContext old_context;
81 
82  /*
83  * If we have already created a session-scope DSM segment in this backend,
84  * return its handle. The same segment will be used for the rest of this
85  * backend's lifetime.
86  */
87  if (CurrentSession->segment != NULL)
88  return dsm_segment_handle(CurrentSession->segment);
89 
90  /* Otherwise, prepare to set one up. */
92  shm_toc_initialize_estimator(&estimator);
93 
94  /* Estimate space for the per-session DSA area. */
95  shm_toc_estimate_keys(&estimator, 1);
97 
98  /* Estimate space for the per-session record typmod registry. */
99  typmod_registry_size = SharedRecordTypmodRegistryEstimate();
100  shm_toc_estimate_keys(&estimator, 1);
101  shm_toc_estimate_chunk(&estimator, typmod_registry_size);
102 
103  /* Set up segment and TOC. */
104  size = shm_toc_estimate(&estimator);
106  if (seg == NULL)
107  {
108  MemoryContextSwitchTo(old_context);
109 
110  return DSM_HANDLE_INVALID;
111  }
113  dsm_segment_address(seg),
114  size);
115 
116  /* Create per-session DSA area. */
117  dsa_space = shm_toc_allocate(toc, SESSION_DSA_SIZE);
118  dsa = dsa_create_in_place(dsa_space,
121  seg);
122  shm_toc_insert(toc, SESSION_KEY_DSA, dsa_space);
123 
124 
125  /* Create session-scoped shared record typmod registry. */
126  typmod_registry_space = shm_toc_allocate(toc, typmod_registry_size);
128  typmod_registry_space, seg, dsa);
130  typmod_registry_space);
131 
132  /*
133  * If we got this far, we can pin the shared memory so it stays mapped for
134  * the rest of this backend's life. If we don't make it this far, cleanup
135  * callbacks for anything we installed above (ie currently
136  * SharedRecordTypmodRegistry) will run when the DSM segment is detached
137  * by CurrentResourceOwner so we aren't left with a broken CurrentSession.
138  */
139  dsm_pin_mapping(seg);
140  dsa_pin_mapping(dsa);
141 
142  /* Make segment and area available via CurrentSession. */
143  CurrentSession->segment = seg;
144  CurrentSession->area = dsa;
145 
146  MemoryContextSwitchTo(old_context);
147 
148  return dsm_segment_handle(seg);
149 }
150 
151 /*
152  * Attach to a per-session DSM segment provided by a parallel leader.
153  */
154 void
156 {
157  dsm_segment *seg;
158  shm_toc *toc;
159  void *dsa_space;
160  void *typmod_registry_space;
161  dsa_area *dsa;
162  MemoryContext old_context;
163 
165 
166  /* Attach to the DSM segment. */
167  seg = dsm_attach(handle);
168  if (seg == NULL)
169  elog(ERROR, "could not attach to per-session DSM segment");
171 
172  /* Attach to the DSA area. */
173  dsa_space = shm_toc_lookup(toc, SESSION_KEY_DSA, false);
174  dsa = dsa_attach_in_place(dsa_space, seg);
175 
176  /* Make them available via the current session. */
177  CurrentSession->segment = seg;
178  CurrentSession->area = dsa;
179 
180  /* Attach to the shared record typmod registry. */
181  typmod_registry_space =
184  typmod_registry_space);
185 
186  /* Remain attached until end of backend or DetachSession(). */
187  dsm_pin_mapping(seg);
188  dsa_pin_mapping(dsa);
189 
190  MemoryContextSwitchTo(old_context);
191 }
192 
193 /*
194  * Detach from the current session DSM segment. It's not strictly necessary
195  * to do this explicitly since we'll detach automatically at backend exit, but
196  * if we ever reuse parallel workers it will become important for workers to
197  * detach from one session before attaching to another. Note that this runs
198  * detach hooks.
199  */
200 void
202 {
203  /* Runs detach hooks. */
204  dsm_detach(CurrentSession->segment);
205  CurrentSession->segment = NULL;
206  dsa_detach(CurrentSession->area);
207  CurrentSession->area = NULL;
208 }
#define SESSION_KEY_RECORD_TYPMOD_REGISTRY
Definition: session.c:45
shm_toc * shm_toc_create(uint64 magic, void *address, Size nbytes)
Definition: shm_toc.c:40
uint32 dsm_handle
Definition: dsm_impl.h:54
#define SESSION_DSA_SIZE
Definition: session.c:39
Session * CurrentSession
Definition: session.c:48
void InitializeSession(void)
Definition: session.c:54
dsm_segment * dsm_attach(dsm_handle h)
Definition: dsm.c:533
dsa_area * dsa_attach_in_place(void *place, dsm_segment *segment)
Definition: dsa.c:541
static MemoryContext MemoryContextSwitchTo(MemoryContext context)
Definition: palloc.h:109
dsm_handle dsm_segment_handle(dsm_segment *seg)
Definition: dsm.c:966
void DetachSession(void)
Definition: session.c:201
Size shm_toc_estimate(shm_toc_estimator *e)
Definition: shm_toc.c:263
#define shm_toc_estimate_chunk(e, sz)
Definition: shm_toc.h:51
void SharedRecordTypmodRegistryAttach(SharedRecordTypmodRegistry *registry)
Definition: typcache.c:2009
void SharedRecordTypmodRegistryInit(SharedRecordTypmodRegistry *registry, dsm_segment *segment, dsa_area *area)
Definition: typcache.c:1910
#define DSM_HANDLE_INVALID
Definition: dsm.h:23
#define ERROR
Definition: elog.h:43
void dsm_pin_mapping(dsm_segment *seg)
Definition: dsm.c:765
dsa_area * area
Definition: session.h:28
#define DSM_CREATE_NULL_IF_MAXSEGMENTS
Definition: dsm.h:20
#define SESSION_KEY_DSA
Definition: session.c:44
void dsa_detach(dsa_area *area)
Definition: dsa.c:1913
void AttachSession(dsm_handle handle)
Definition: session.c:155
#define shm_toc_initialize_estimator(e)
Definition: shm_toc.h:49
void dsa_pin_mapping(dsa_area *area)
Definition: dsa.c:631
dsa_area * dsa_create_in_place(void *place, size_t size, int tranche_id, dsm_segment *segment)
Definition: dsa.c:469
MemoryContext TopMemoryContext
Definition: mcxt.c:44
dsm_segment * dsm_create(Size size, int flags)
Definition: dsm.c:437
shm_toc * shm_toc_attach(uint64 magic, void *address)
Definition: shm_toc.c:64
void * MemoryContextAllocZero(MemoryContext context, Size size)
Definition: mcxt.c:839
size_t SharedRecordTypmodRegistryEstimate(void)
Definition: typcache.c:1888
#define SESSION_MAGIC
Definition: session.c:30
void * dsm_segment_address(dsm_segment *seg)
Definition: dsm.c:938
#define shm_toc_estimate_keys(e, cnt)
Definition: shm_toc.h:53
void * shm_toc_allocate(shm_toc *toc, Size nbytes)
Definition: shm_toc.c:88
Definition: dsa.c:354
void dsm_detach(dsm_segment *seg)
Definition: dsm.c:664
void shm_toc_insert(shm_toc *toc, uint64 key, void *address)
Definition: shm_toc.c:171
#define elog(elevel,...)
Definition: elog.h:228
dsm_handle GetSessionDsmHandle(void)
Definition: session.c:70
void * shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
Definition: shm_toc.c:232
dsm_segment * segment
Definition: session.h:27