Fix shmem allocation size.

rjuju · rjuju · commit 277a4e5abcd9 · 2022-03-27T21:03:23.000+08:00
MaxBackends is still 0 when _PG_init() is called, which means that we
don't request enough memory in RequestAddinShmemSpace(), while the rest of
the code sees (and allocate) a correct value.

It's technically usually not a problem as postgres adds an extra 100kB of
memory for small unaccounted memory usage, but it's better to avoid relying on
it too much.

Note that the value is still not guaranteed to be exact as other modules
_PG_init() could later change the underlying GUCs, but there is not available
API to handle that case accurately.
diff --git a/pg_wait_sampling.c b/pg_wait_sampling.c
@@ -17,6 +17,10 @@
 #include "miscadmin.h"
 #include "optimizer/planner.h"
 #include "pgstat.h"
+#include "postmaster/autovacuum.h"
+#if PG_VERSION_NUM >= 120000
+#include "replication/walsender.h"
+#endif
 #include "storage/ipc.h"
 #include "storage/pg_shmem.h"
 #include "storage/procarray.h"
@@ -66,20 +70,46 @@ static void pgws_ExecutorEnd(QueryDesc *queryDesc);
 
 /*
  * Calculate max processes count.
- * Look at InitProcGlobal (proc.c) and TotalProcs variable in it
- * if something wrong here.
+ *
+ * The value has to be in sync with ProcGlobal->allProcCount, initialized in
+ * InitProcGlobal() (proc.c).
+ *
+ * We calculate the value here as it won't initialized when we need need
+ * it during _PG_init().
+ *
+ * Note that the value returned during _PG_init() might be different from the
+ * value returned later if some third-party modules change one of the
+ * underlying GUC.  This isn't ideal but can't lead to a crash, as the value
+ * returned during _PG_init() is only used to ask for additional shmem with
+ * RequestAddinShmemSpace(), and postgres has an extra 100kB of shmem to
+ * compensate some small unaccounted usage.  So if the value later changes, we
+ * will allocate and initialize the new (and correct) memory size, which
+ * will either work thanks for the extra 100kB of shmem, of fail (and prevent
+ * postgres startup) due to an out of shared memory error.
  */
 static int
 get_max_procs_count(void)
 {
 	int count = 0;
 
-	/* MyProcs, including autovacuum workers and launcher */
-	count += MaxBackends;
+	/*
+	 * MaxBackends: bgworkers, autovacuum workers and launcher.
+	 * This has to be in sync with the value computed in
+	 * InitializeMaxBackends() (postinit.c)
+	 */
+	count += MaxConnections + autovacuum_max_workers + 1
+			+ max_worker_processes;
+
+	/*
+	 * Starting with pg12, wal senders aren't part of MaxConnections anymore
+	 * and have to be accounted for.
+	 */
+#if PG_VERSION_NUM >= 120000
+	count += max_wal_senders;
+#endif
+
 	/* AuxiliaryProcs */
 	count += NUM_AUXILIARY_PROCS;
-	/* Prepared xacts */
-	count += max_prepared_xacts;
 
 	return count;
 }
