Line data Source code
1 : /*--------------------------------------------------------------------------
2 : *
3 : * worker.c
4 : * Code for sample worker making use of shared memory message queues.
5 : * Our test worker simply reads messages from one message queue and
6 : * writes them back out to another message queue. In a real
7 : * application, you'd presumably want the worker to do some more
8 : * complex calculation rather than simply returning the input,
9 : * but it should be possible to use much of the control logic just
10 : * as presented here.
11 : *
12 : * Copyright (c) 2013-2026, PostgreSQL Global Development Group
13 : *
14 : * IDENTIFICATION
15 : * src/test/modules/test_shm_mq/worker.c
16 : *
17 : * -------------------------------------------------------------------------
18 : */
19 :
20 : #include "postgres.h"
21 :
22 : #include "miscadmin.h"
23 : #include "storage/ipc.h"
24 : #include "storage/procarray.h"
25 : #include "storage/shm_mq.h"
26 : #include "storage/shm_toc.h"
27 : #include "tcop/tcopprot.h"
28 :
29 : #include "test_shm_mq.h"
30 :
31 : static void attach_to_queues(dsm_segment *seg, shm_toc *toc,
32 : int myworkernumber, shm_mq_handle **inqhp,
33 : shm_mq_handle **outqhp);
34 : static void copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh);
35 :
36 : /*
37 : * Background worker entrypoint.
38 : *
39 : * This is intended to demonstrate how a background worker can be used to
40 : * facilitate a parallel computation. Most of the logic here is fairly
41 : * boilerplate stuff, designed to attach to the shared memory segment,
42 : * notify the user backend that we're alive, and so on. The
43 : * application-specific bits of logic that you'd replace for your own worker
44 : * are attach_to_queues() and copy_messages().
45 : */
46 : void
47 0 : test_shm_mq_main(Datum main_arg)
48 : {
49 0 : dsm_segment *seg;
50 0 : shm_toc *toc;
51 0 : shm_mq_handle *inqh;
52 0 : shm_mq_handle *outqh;
53 0 : volatile test_shm_mq_header *hdr;
54 0 : int myworkernumber;
55 0 : PGPROC *registrant;
56 :
57 : /*
58 : * Establish signal handlers.
59 : *
60 : * We want CHECK_FOR_INTERRUPTS() to kill off this worker process just as
61 : * it would a normal user backend. To make that happen, we use die().
62 : */
63 0 : pqsignal(SIGTERM, die);
64 0 : BackgroundWorkerUnblockSignals();
65 :
66 : /*
67 : * Connect to the dynamic shared memory segment.
68 : *
69 : * The backend that registered this worker passed us the ID of a shared
70 : * memory segment to which we must attach for further instructions. Once
71 : * we've mapped the segment in our address space, attach to the table of
72 : * contents so we can locate the various data structures we'll need to
73 : * find within the segment.
74 : *
75 : * Note: at this point, we have not created any ResourceOwner in this
76 : * process. This will result in our DSM mapping surviving until process
77 : * exit, which is fine. If there were a ResourceOwner, it would acquire
78 : * ownership of the mapping, but we have no need for that.
79 : */
80 0 : seg = dsm_attach(DatumGetUInt32(main_arg));
81 0 : if (seg == NULL)
82 0 : ereport(ERROR,
83 : (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
84 : errmsg("unable to map dynamic shared memory segment")));
85 0 : toc = shm_toc_attach(PG_TEST_SHM_MQ_MAGIC, dsm_segment_address(seg));
86 0 : if (toc == NULL)
87 0 : ereport(ERROR,
88 : (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
89 : errmsg("bad magic number in dynamic shared memory segment")));
90 :
91 : /*
92 : * Acquire a worker number.
93 : *
94 : * By convention, the process registering this background worker should
95 : * have stored the control structure at key 0. We look up that key to
96 : * find it. Our worker number gives our identity: there may be just one
97 : * worker involved in this parallel operation, or there may be many.
98 : */
99 0 : hdr = shm_toc_lookup(toc, 0, false);
100 0 : SpinLockAcquire(&hdr->mutex);
101 0 : myworkernumber = ++hdr->workers_attached;
102 0 : SpinLockRelease(&hdr->mutex);
103 0 : if (myworkernumber > hdr->workers_total)
104 0 : ereport(ERROR,
105 : (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
106 : errmsg("too many message queue testing workers already")));
107 :
108 : /*
109 : * Attach to the appropriate message queues.
110 : */
111 0 : attach_to_queues(seg, toc, myworkernumber, &inqh, &outqh);
112 :
113 : /*
114 : * Indicate that we're fully initialized and ready to begin the main part
115 : * of the parallel operation.
116 : *
117 : * Once we signal that we're ready, the user backend is entitled to assume
118 : * that our on_dsm_detach callbacks will fire before we disconnect from
119 : * the shared memory segment and exit. Generally, that means we must have
120 : * attached to all relevant dynamic shared memory data structures by now.
121 : */
122 0 : SpinLockAcquire(&hdr->mutex);
123 0 : ++hdr->workers_ready;
124 0 : SpinLockRelease(&hdr->mutex);
125 0 : registrant = BackendPidGetProc(MyBgworkerEntry->bgw_notify_pid);
126 0 : if (registrant == NULL)
127 : {
128 0 : elog(DEBUG1, "registrant backend has exited prematurely");
129 0 : proc_exit(1);
130 : }
131 0 : SetLatch(®istrant->procLatch);
132 :
133 : /* Do the work. */
134 0 : copy_messages(inqh, outqh);
135 :
136 : /*
137 : * We're done. For cleanliness, explicitly detach from the shared memory
138 : * segment (that would happen anyway during process exit, though).
139 : */
140 0 : dsm_detach(seg);
141 0 : proc_exit(1);
142 : }
143 :
144 : /*
145 : * Attach to shared memory message queues.
146 : *
147 : * We use our worker number to determine to which queue we should attach.
148 : * The queues are registered at keys 1..<number-of-workers>. The user backend
149 : * writes to queue #1 and reads from queue #<number-of-workers>; each worker
150 : * reads from the queue whose number is equal to its worker number and writes
151 : * to the next higher-numbered queue.
152 : */
153 : static void
154 0 : attach_to_queues(dsm_segment *seg, shm_toc *toc, int myworkernumber,
155 : shm_mq_handle **inqhp, shm_mq_handle **outqhp)
156 : {
157 0 : shm_mq *inq;
158 0 : shm_mq *outq;
159 :
160 0 : inq = shm_toc_lookup(toc, myworkernumber, false);
161 0 : shm_mq_set_receiver(inq, MyProc);
162 0 : *inqhp = shm_mq_attach(inq, seg, NULL);
163 0 : outq = shm_toc_lookup(toc, myworkernumber + 1, false);
164 0 : shm_mq_set_sender(outq, MyProc);
165 0 : *outqhp = shm_mq_attach(outq, seg, NULL);
166 0 : }
167 :
168 : /*
169 : * Loop, receiving and sending messages, until the connection is broken.
170 : *
171 : * This is the "real work" performed by this worker process. Everything that
172 : * happens before this is initialization of one form or another, and everything
173 : * after this point is cleanup.
174 : */
175 : static void
176 0 : copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh)
177 : {
178 0 : Size len;
179 0 : void *data;
180 0 : shm_mq_result res;
181 :
182 0 : for (;;)
183 : {
184 : /* Notice any interrupts that have occurred. */
185 0 : CHECK_FOR_INTERRUPTS();
186 :
187 : /* Receive a message. */
188 0 : res = shm_mq_receive(inqh, &len, &data, false);
189 0 : if (res != SHM_MQ_SUCCESS)
190 0 : break;
191 :
192 : /* Send it back out. */
193 0 : res = shm_mq_send(outqh, len, data, false, true);
194 0 : if (res != SHM_MQ_SUCCESS)
195 0 : break;
196 : }
197 0 : }
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