8000 Invent a memory context reset/delete callback mechanism. · postgres/postgres@f65e827 · GitHub
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Invent a memory context reset/delete callback mechanism.
This allows cleanup actions to be registered to be called just before a particular memory context's contents are flushed (either by deletion or MemoryContextReset). The patch in itself has no use-cases for this, but several likely reasons for wanting this exist. In passing, per discussion, rearrange some boolean fields in struct MemoryContextData so as to avoid wasted padding space. For safety, this requires making allowInCritSection's existence unconditional; but I think that's a better approach than what was there anyway.
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3 files changed

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‎src/backend/utils/mmgr/mcxt.c

Lines changed: 63 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -54,6 +54,7 @@ MemoryContext CurTransactionContext = NULL;
5454
/* This is a transient link to the active portal's memory context: */
5555
MemoryContext PortalContext = NULL;
5656

57+
static void MemoryContextCallResetCallbacks(MemoryContext context);
5758
static void MemoryContextStatsInternal(MemoryContext context, int level);
5859

5960
/*
@@ -115,9 +116,8 @@ MemoryContextInit(void)
115116
* where retained memory in a context is *essential* --- we want to be
116117
* sure ErrorContext still has some memory even if we've run out
117118
* elsewhere! Also, allow allocations in ErrorContext within a critical
118-
* section. Otherwise a PANIC will cause an assertion failure in the
119-
* error reporting code, before printing out the real cause of the
120-
* failure.
119+
* section. Otherwise a PANIC will cause an assertion failure in the error
120+
* reporting code, before printing out the real cause of the failure.
121121
*
122122
* This should be the last step in this function, as elog.c assumes memory
123123
* management works once ErrorContext is non-null.
@@ -150,6 +150,7 @@ MemoryContextReset(MemoryContext context)
150150
/* Nothing to do if no pallocs since startup or last reset */
151151
if (!context->isReset)
152152
{
153+
MemoryContextCallResetCallbacks(context);
153154
(*context->methods->reset) (context);
154155
context->isReset = true;
155156
VALGRIND_DESTROY_MEMPOOL(context);
@@ -195,6 +196,14 @@ MemoryContextDelete(MemoryContext context)
195196

196197
MemoryContextDeleteChildren(context);
197198

199+
/*
200+
* It's not entirely clear whether 'tis better to do this before or after
201+
* delinking the context; but an error in a callback will likely result in
202+
* leaking the whole context (if it's not a root context) if we do it
203+
* after, so let's do it before.
204+
*/
205+
MemoryContextCallResetCallbacks(context);
206+
198207
/*
199208
* We delink the context from its parent before deleting it, so that if
200209
* there's an error we won't have deleted/busted contexts still attached
@@ -242,6 +251,56 @@ MemoryContextResetAndDeleteChildren(MemoryContext context)
242251
MemoryContextReset(context);
243252
}
244253

254+
/*
255+
* MemoryContextRegisterResetCallback
256+
* Register a function to be called before next context reset/delete.
257+
* Such callbacks will be called in reverse order of registration.
258+
*
259+
* The caller is responsible for allocating a MemoryContextCallback struct
260+
* to hold the info about this callback request, and for filling in the
261+
* "func" and "arg" fields in the struct to show what function to call with
262+
* what argument. Typically the callback struct should be allocated within
263+
* the specified context, since that means it will automatically be freed
264+
* when no longer needed.
265+
*
266+
* There is no API for deregistering a callback once registered. If you
267+
* want it to not do anything anymore, adjust the state pointed to by its
268+
* "arg" to indicate that.
269+
*/
270+
void
271+
MemoryContextRegisterResetCallback(MemoryContext context,
272+
MemoryContextCallback *cb)
273+
{
274+
AssertArg(MemoryContextIsValid(context));
275+
276+
/* Push onto head so this will be called before older registrants. */
277+
cb->next = context->reset_cbs;
278+
context->reset_cbs = cb;
279+
/* Mark the context as non-reset (it probably is already). */
280+
context->isReset = false;
281+
}
282+
283+
/*
284+
* MemoryContextCallResetCallbacks
285+
* Internal function to call all registered callbacks for context.
286+
*/
287+
static void
288+
MemoryContextCallResetCallbacks(MemoryContext context)
289+
{
290+
MemoryContextCallback *cb;
291+
292+
/*
293+
* We pop each callback from the list before calling. That way, if an
294+
* error occurs inside the callback, we won't try to call it a second time
295+
* in the likely event that we reset or delete the context later.
296+
*/
297+
while ((cb = context->reset_cbs) != NULL)
298+
{
299+
context->reset_cbs = cb->next;
300+
(*cb->func) (cb->arg);
301+
}
302+
}
303+
245304
/*
246305
* MemoryContextSetParent
247306
* Change a context to belong to a new parent (or no parent).
@@ -318,9 +377,8 @@ void
318377
MemoryContextAllowInCriticalSection(MemoryContext context, bool allow)
319378
{
320379
AssertArg(MemoryContextIsValid(context));
321-
#ifdef USE_ASSERT_CHECKING
380+
322381
context->allowInCritSection = allow;
323-
#endif
324382
}
325383

326384
/*
@@ -589,11 +647,8 @@ MemoryContextCreate(NodeTag tag, Size size,
589647
node->parent = parent;
590648
node->nextchild = parent->firstchild;
591649
parent->firstchild = node;
592-
593-
#ifdef USE_ASSERT_CHECKING
594650
/* inherit allowInCritSection flag from parent */
595651
node->allowInCritSection = parent->allowInCritSection;
596-
#endif
597652
}
598653

599654
VALGRIND_CREATE_MEMPOOL(node, 0, false);

‎src/include/nodes/memnodes.h

Lines changed: 20 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -16,6 +16,22 @@
1616

1717
#include "nodes/nodes.h"
1818

19+
/*
20+
* A memory context can have callback functions registered on it. Any such
21+
* function will be called once just before the context is next reset or
22+
* deleted. The MemoryContextCallback struct describing such a callback
23+
* typically would be allocated within the context itself, thereby avoiding
24+
* any need to manage it explicitly (the reset/delete action will free it).
25+
*/
26+
typedef void (*MemoryContextCallbackFunction) (void *arg);
27+
28+
typedef struct MemoryContextCallback
29+
{
30+
MemoryContextCallbackFunction func; /* function to call */
31+
void *arg; /* argument to pass it */
32+
struct MemoryContextCallback *next; /* next in list of callbacks */
33+
} MemoryContextCallback;
34+
1935
/*
2036
* MemoryContext
2137
* A logical context in which memory allocations occur.
@@ -54,15 +70,15 @@ typedef struct MemoryContextMethods
5470
typedef struct MemoryContextData
5571
{
5672
NodeTag type; /* identifies exact kind of context */
73+
/* these two fields are placed here to minimize alignment wastage: */
74+
bool isReset; /* T = no space alloced since last reset */
75+
bool allowInCritSection; /* allow palloc in critical section */
5776
MemoryContextMethods *methods; /* virtual function table */
5877
MemoryContext parent; /* NULL if no parent (toplevel context) */
5978
MemoryContext firstchild; /* head of linked list of children */
6079
MemoryContext nextchild; /* next child of same parent */
6180
char *name; /* context name (just for debugging) */
62-
bool isReset; /* T = no space alloced since last reset */
63-
#ifdef USE_ASSERT_CHECKING
64-
bool allowInCritSection; /* allow palloc in critical section */
65-
#endif
81+
MemoryContextCallback *reset_cbs; /* list of reset/delete callbacks */
6682
} MemoryContextData;
6783

6884
/* utils/palloc.h contains typedef struct MemoryContextData *MemoryContext */

‎src/include/utils/memutils.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -94,6 +94,8 @@ extern void MemoryContextDelete(MemoryContext context);
9494
extern void MemoryContextResetChildren(MemoryContext context);
9595
extern void MemoryContextDeleteChildren(MemoryContext context);
9696
extern void MemoryContextResetAndDeleteChildren(MemoryContext context);
97+
extern void MemoryContextRegisterResetCallback(MemoryContext context,
98+
MemoryContextCallback *cb);
9799
extern void MemoryContextSetParent(MemoryContext context,
98100
MemoryContext new_parent);
99101
extern Size GetMemoryChunkSpace(void *pointer);

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