1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
|
/*-------------------------------------------------------------------------
*
* session.c
* Encapsulation of user session.
*
* This is intended to contain data that needs to be shared between backends
* performing work for a client session. In particular such a session is
* shared between the leader and worker processes for parallel queries. At
* some later point it might also become useful infrastructure for separating
* backends from client connections, e.g. for the purpose of pooling.
*
* Currently this infrastructure is used to share:
* - typemod registry for ephemeral row-types, i.e. BlessTupleDesc etc.
*
* Portions Copyright (c) 2017-2021, PostgreSQL Global Development Group
*
* src/backend/access/common/session.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/session.h"
#include "storage/lwlock.h"
#include "storage/shm_toc.h"
#include "utils/memutils.h"
#include "utils/typcache.h"
/* Magic number for per-session DSM TOC. */
#define SESSION_MAGIC 0xabb0fbc9
/*
* We want to create a DSA area to store shared state that has the same
* lifetime as a session. So far, it's only used to hold the shared record
* type registry. We don't want it to have to create any DSM segments just
* yet in common cases, so we'll give it enough space to hold a very small
* SharedRecordTypmodRegistry.
*/
#define SESSION_DSA_SIZE 0x30000
/*
* Magic numbers for state sharing in the per-session DSM area.
*/
#define SESSION_KEY_DSA UINT64CONST(0xFFFFFFFFFFFF0001)
#define SESSION_KEY_RECORD_TYPMOD_REGISTRY UINT64CONST(0xFFFFFFFFFFFF0002)
/* This backend's current session. */
Session *CurrentSession = NULL;
/*
* Set up CurrentSession to point to an empty Session object.
*/
void
InitializeSession(void)
{
CurrentSession = MemoryContextAllocZero(TopMemoryContext, sizeof(Session));
}
/*
* Initialize the per-session DSM segment if it isn't already initialized, and
* return its handle so that worker processes can attach to it.
*
* Unlike the per-context DSM segment, this segment and its contents are
* reused for future parallel queries.
*
* Return DSM_HANDLE_INVALID if a segment can't be allocated due to lack of
* resources.
*/
dsm_handle
GetSessionDsmHandle(void)
{
shm_toc_estimator estimator;
shm_toc *toc;
dsm_segment *seg;
size_t typmod_registry_size;
size_t size;
void *dsa_space;
void *typmod_registry_space;
dsa_area *dsa;
MemoryContext old_context;
/*
* If we have already created a session-scope DSM segment in this backend,
* return its handle. The same segment will be used for the rest of this
* backend's lifetime.
*/
if (CurrentSession->segment != NULL)
return dsm_segment_handle(CurrentSession->segment);
/* Otherwise, prepare to set one up. */
old_context = MemoryContextSwitchTo(TopMemoryContext);
shm_toc_initialize_estimator(&estimator);
/* Estimate space for the per-session DSA area. */
shm_toc_estimate_keys(&estimator, 1);
shm_toc_estimate_chunk(&estimator, SESSION_DSA_SIZE);
/* Estimate space for the per-session record typmod registry. */
typmod_registry_size = SharedRecordTypmodRegistryEstimate();
shm_toc_estimate_keys(&estimator, 1);
shm_toc_estimate_chunk(&estimator, typmod_registry_size);
/* Set up segment and TOC. */
size = shm_toc_estimate(&estimator);
seg = dsm_create(size, DSM_CREATE_NULL_IF_MAXSEGMENTS);
if (seg == NULL)
{
MemoryContextSwitchTo(old_context);
return DSM_HANDLE_INVALID;
}
toc = shm_toc_create(SESSION_MAGIC,
dsm_segment_address(seg),
size);
/* Create per-session DSA area. */
dsa_space = shm_toc_allocate(toc, SESSION_DSA_SIZE);
dsa = dsa_create_in_place(dsa_space,
SESSION_DSA_SIZE,
LWTRANCHE_PER_SESSION_DSA,
seg);
shm_toc_insert(toc, SESSION_KEY_DSA, dsa_space);
/* Create session-scoped shared record typmod registry. */
typmod_registry_space = shm_toc_allocate(toc, typmod_registry_size);
SharedRecordTypmodRegistryInit((SharedRecordTypmodRegistry *)
typmod_registry_space, seg, dsa);
shm_toc_insert(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY,
typmod_registry_space);
/*
* If we got this far, we can pin the shared memory so it stays mapped for
* the rest of this backend's life. If we don't make it this far, cleanup
* callbacks for anything we installed above (ie currently
* SharedRecordTypmodRegistry) will run when the DSM segment is detached
* by CurrentResourceOwner so we aren't left with a broken CurrentSession.
*/
dsm_pin_mapping(seg);
dsa_pin_mapping(dsa);
/* Make segment and area available via CurrentSession. */
CurrentSession->segment = seg;
CurrentSession->area = dsa;
MemoryContextSwitchTo(old_context);
return dsm_segment_handle(seg);
}
/*
* Attach to a per-session DSM segment provided by a parallel leader.
*/
void
AttachSession(dsm_handle handle)
{
dsm_segment *seg;
shm_toc *toc;
void *dsa_space;
void *typmod_registry_space;
dsa_area *dsa;
MemoryContext old_context;
old_context = MemoryContextSwitchTo(TopMemoryContext);
/* Attach to the DSM segment. */
seg = dsm_attach(handle);
if (seg == NULL)
elog(ERROR, "could not attach to per-session DSM segment");
toc = shm_toc_attach(SESSION_MAGIC, dsm_segment_address(seg));
/* Attach to the DSA area. */
dsa_space = shm_toc_lookup(toc, SESSION_KEY_DSA, false);
dsa = dsa_attach_in_place(dsa_space, seg);
/* Make them available via the current session. */
CurrentSession->segment = seg;
CurrentSession->area = dsa;
/* Attach to the shared record typmod registry. */
typmod_registry_space =
shm_toc_lookup(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY, false);
SharedRecordTypmodRegistryAttach((SharedRecordTypmodRegistry *)
typmod_registry_space);
/* Remain attached until end of backend or DetachSession(). */
dsm_pin_mapping(seg);
dsa_pin_mapping(dsa);
MemoryContextSwitchTo(old_context);
}
/*
* Detach from the current session DSM segment. It's not strictly necessary
* to do this explicitly since we'll detach automatically at backend exit, but
* if we ever reuse parallel workers it will become important for workers to
* detach from one session before attaching to another. Note that this runs
* detach hooks.
*/
void
DetachSession(void)
{
/* Runs detach hooks. */
dsm_detach(CurrentSession->segment);
CurrentSession->segment = NULL;
dsa_detach(CurrentSession->area);
CurrentSession->area = NULL;
}
|
