Date: Tue, 25 Nov 2025 11:45:34 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: pgsql: Teach DSM registry to ERROR if attaching to an
 uninitialized ent
Message-ID: <aSXrPqV3Bcm2HLV4@nathan>
References: <aR-fqzfZYSd3TVKa@nathan>
 <CA+TgmoZsxbVhoxNGTWSd6=vHvyoqOSDNXJZabBZx+St8uPduLQ@mail.gmail.com>
 <aSIR4Z4U887f_KcQ@nathan>
 <aSSlqWwLVuXMmN-o@nathan>
 <aSSr6j3JzeLcqOWo@nathan>
 <CA+TgmoYq7U0hHgOEOqQgf=6cFXz_7Ab8QTRMFqFpLNfav0G4Ug@mail.gmail.com>
 <aSTNXyXnr9HYsLo6@nathan>
 <CA+Tgmob6knHwEbQv11n_Yy0GyFzPgjhC33LroWso0prt7hCksg@mail.gmail.com>
 <aSXQOAZuA4rX9ooU@nathan>
 <CA+TgmobGYDczmVa-UDGPgcyN=63PJzzyQ9ZCofou4VYH8TVLSQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="c08aH/hgV4/mNMsI"
Content-Disposition: inline
In-Reply-To: 
 <CA+TgmobGYDczmVa-UDGPgcyN=63PJzzyQ9ZCofou4VYH8TVLSQ@mail.gmail.com>
Archived-At: <https://www.postgresql.org/message-id/aSXrPqV3Bcm2HLV4%40nathan>
Precedence: bulk


--c08aH/hgV4/mNMsI
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

On Tue, Nov 25, 2025 at 11:25:09AM -0500, Robert Haas wrote:
> The downside is that then we have to rely on PG_CATCH() to make things
> whole. I am not sure that there's any problem with that, but I'm also
> not sure that there isn't. The timing of PG_CATCH() block execution
> often creates bugs, because it runs before (sub)transaction abort.
> That means that there's a real risk that you try to acquire an LWLock
> you already hold, for example. It's a lot easier to be confident that
> cleanup actions will reliably succeed when they run inside the
> transaction abort path that knows the order in which various resources
> should be released. Generally, I think it's PG_CATCH() is fine if
> you're releasing a resource that is decoupled from everything else,
> like using a third-party library's free function to free memory
> allocated by that library. But if you're releasing PostgreSQL
> resources that are layered on top of other PostgreSQL resources, like
> a DSA that depends on DSM and LWLock, I think it's a lot more
> difficult to be certain that you aren't going to end up trying to
> release the same stuff multiple times or in the wrong order. I'm not
> saying you can't make it work, but I've banged my head on this
> particular doorframe enough times that my reflex is to duck.

I gave your idea a try, and I like the result a lot.  IMHO it makes the
code much easier to reason about.

-- 
nathan

--c08aH/hgV4/mNMsI
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment;
	filename=v3-0001-Revert-Teach-DSM-registry-to-ERROR-if-attaching-t.patch

From 3fddb959db906ff6560d73c08ba7695812020275 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 24 Nov 2025 11:03:52 -0600
Subject: [PATCH v3 1/2] Revert "Teach DSM registry to ERROR if attaching to an
 uninitialized entry."

This reverts commit 1165a933aab1355757a43cfd9193b6cce06f573b.
---
 src/backend/storage/ipc/dsm_registry.c | 34 +++-----------------------
 1 file changed, 4 insertions(+), 30 deletions(-)

diff --git a/src/backend/storage/ipc/dsm_registry.c b/src/backend/storage/ipc/dsm_registry.c
index 7eba8a8cffb..a926b9c3f32 100644
--- a/src/backend/storage/ipc/dsm_registry.c
+++ b/src/backend/storage/ipc/dsm_registry.c
@@ -93,7 +93,6 @@ typedef struct DSMRegistryEntry
 {
 	char		name[NAMEDATALEN];
 	DSMREntryType type;
-	bool		initialized;
 	union
 	{
 		NamedDSMState dsm;
@@ -217,7 +216,6 @@ GetNamedDSMSegment(const char *name, size_t size,
 		dsm_segment *seg;
 
 		entry->type = DSMR_ENTRY_TYPE_DSM;
-		entry->initialized = false;
 
 		/* Initialize the segment. */
 		seg = dsm_create(size, 0);
@@ -230,21 +228,13 @@ GetNamedDSMSegment(const char *name, size_t size,
 
 		if (init_callback)
 			(*init_callback) (ret);
-
-		entry->initialized = true;
 	}
 	else if (entry->type != DSMR_ENTRY_TYPE_DSM)
 		ereport(ERROR,
-				(errmsg("requested DSM segment \"%s\" does not match type of existing entry",
-						name)));
-	else if (!entry->initialized)
-		ereport(ERROR,
-				(errmsg("requested DSM segment \"%s\" failed initialization",
-						name)));
+				(errmsg("requested DSM segment does not match type of existing entry")));
 	else if (entry->dsm.size != size)
 		ereport(ERROR,
-				(errmsg("requested DSM segment \"%s\" does not match size of existing entry",
-						name)));
+				(errmsg("requested DSM segment size does not match size of existing segment")));
 	else
 	{
 		NamedDSMState *state = &entry->dsm;
@@ -307,7 +297,6 @@ GetNamedDSA(const char *name, bool *found)
 		NamedDSAState *state = &entry->dsa;
 
 		entry->type = DSMR_ENTRY_TYPE_DSA;
-		entry->initialized = false;
 
 		/* Initialize the LWLock tranche for the DSA. */
 		state->tranche = LWLockNewTrancheId(name);
@@ -319,17 +308,10 @@ GetNamedDSA(const char *name, bool *found)
 
 		/* Store handle for other backends to use. */
 		state->handle = dsa_get_handle(ret);
-
-		entry->initialized = true;
 	}
 	else if (entry->type != DSMR_ENTRY_TYPE_DSA)
 		ereport(ERROR,
-				(errmsg("requested DSA \"%s\" does not match type of existing entry",
-						name)));
-	else if (!entry->initialized)
-		ereport(ERROR,
-				(errmsg("requested DSA \"%s\" failed initialization",
-						name)));
+				(errmsg("requested DSA does not match type of existing entry")));
 	else
 	{
 		NamedDSAState *state = &entry->dsa;
@@ -390,7 +372,6 @@ GetNamedDSHash(const char *name, const dshash_parameters *params, bool *found)
 		dsa_area   *dsa;
 
 		entry->type = DSMR_ENTRY_TYPE_DSH;
-		entry->initialized = false;
 
 		/* Initialize the LWLock tranche for the hash table. */
 		dsh_state->tranche = LWLockNewTrancheId(name);
@@ -408,17 +389,10 @@ GetNamedDSHash(const char *name, const dshash_parameters *params, bool *found)
 		/* Store handles for other backends to use. */
 		dsh_state->dsa_handle = dsa_get_handle(dsa);
 		dsh_state->dsh_handle = dshash_get_hash_table_handle(ret);
-
-		entry->initialized = true;
 	}
 	else if (entry->type != DSMR_ENTRY_TYPE_DSH)
 		ereport(ERROR,
-				(errmsg("requested DSHash \"%s\" does not match type of existing entry",
-						name)));
-	else if (!entry->initialized)
-		ereport(ERROR,
-				(errmsg("requested DSHash \"%s\" failed initialization",
-						name)));
+				(errmsg("requested DSHash does not match type of existing entry")));
 	else
 	{
 		NamedDSHState *dsh_state = &entry->dsh;
-- 
2.39.5 (Apple Git-154)


--c08aH/hgV4/mNMsI
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment;
	filename=v3-0002-handle-ERRORs-in-DSM-registry-functions.patch

From c40884c967755b3630d0419a7d6585d3f4aae170 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 24 Nov 2025 12:28:23 -0600
Subject: [PATCH v3 2/2] handle ERRORs in DSM registry functions

---
 src/backend/storage/ipc/dsm_registry.c | 110 +++++++++++++++----------
 1 file changed, 67 insertions(+), 43 deletions(-)

diff --git a/src/backend/storage/ipc/dsm_registry.c b/src/backend/storage/ipc/dsm_registry.c
index a926b9c3f32..5290cc1c8e3 100644
--- a/src/backend/storage/ipc/dsm_registry.c
+++ b/src/backend/storage/ipc/dsm_registry.c
@@ -155,9 +155,10 @@ init_dsm_registry(void)
 	{
 		/* Initialize dynamic shared hash table for registry. */
 		dsm_registry_dsa = dsa_create(LWTRANCHE_DSM_REGISTRY_DSA);
+		dsm_registry_table = dshash_create(dsm_registry_dsa, &dsh_params, NULL);
+
 		dsa_pin(dsm_registry_dsa);
 		dsa_pin_mapping(dsm_registry_dsa);
-		dsm_registry_table = dshash_create(dsm_registry_dsa, &dsh_params, NULL);
 
 		/* Store handles in shared memory for other backends to use. */
 		DSMRegistryCtx->dsah = dsa_get_handle(dsm_registry_dsa);
@@ -188,6 +189,8 @@ GetNamedDSMSegment(const char *name, size_t size,
 	DSMRegistryEntry *entry;
 	MemoryContext oldcontext;
 	void	   *ret;
+	NamedDSMState *state;
+	dsm_segment *seg;
 
 	Assert(found);
 
@@ -210,36 +213,36 @@ GetNamedDSMSegment(const char *name, size_t size,
 	init_dsm_registry();
 
 	entry = dshash_find_or_insert(dsm_registry_table, name, found);
+	state = &entry->dsm;
 	if (!(*found))
 	{
-		NamedDSMState *state = &entry->dsm;
-		dsm_segment *seg;
-
 		entry->type = DSMR_ENTRY_TYPE_DSM;
+		state->handle = DSM_HANDLE_INVALID;
+		state->size = size;
+	}
+	else if (entry->type != DSMR_ENTRY_TYPE_DSM)
+		ereport(ERROR,
+				(errmsg("requested DSM segment does not match type of existing entry")));
+	else if (state->size != size)
+		ereport(ERROR,
+				(errmsg("requested DSM segment size does not match size of existing segment")));
+
+	if (state->handle == DSM_HANDLE_INVALID)
+	{
+		*found = false;
 
 		/* Initialize the segment. */
 		seg = dsm_create(size, 0);
 
+		if (init_callback)
+			(*init_callback) (dsm_segment_address(seg));
+
 		dsm_pin_segment(seg);
 		dsm_pin_mapping(seg);
 		state->handle = dsm_segment_handle(seg);
-		state->size = size;
-		ret = dsm_segment_address(seg);
-
-		if (init_callback)
-			(*init_callback) (ret);
 	}
-	else if (entry->type != DSMR_ENTRY_TYPE_DSM)
-		ereport(ERROR,
-				(errmsg("requested DSM segment does not match type of existing entry")));
-	else if (entry->dsm.size != size)
-		ereport(ERROR,
-				(errmsg("requested DSM segment size does not match size of existing segment")));
 	else
 	{
-		NamedDSMState *state = &entry->dsm;
-		dsm_segment *seg;
-
 		/* If the existing segment is not already attached, attach it now. */
 		seg = dsm_find_mapping(state->handle);
 		if (seg == NULL)
@@ -250,10 +253,9 @@ GetNamedDSMSegment(const char *name, size_t size,
 
 			dsm_pin_mapping(seg);
 		}
-
-		ret = dsm_segment_address(seg);
 	}
 
+	ret = dsm_segment_address(seg);
 	dshash_release_lock(dsm_registry_table, entry);
 	MemoryContextSwitchTo(oldcontext);
 
@@ -274,6 +276,7 @@ GetNamedDSA(const char *name, bool *found)
 	DSMRegistryEntry *entry;
 	MemoryContext oldcontext;
 	dsa_area   *ret;
+	NamedDSAState *state;
 
 	Assert(found);
 
@@ -292,14 +295,28 @@ GetNamedDSA(const char *name, bool *found)
 	init_dsm_registry();
 
 	entry = dshash_find_or_insert(dsm_registry_table, name, found);
+	state = &entry->dsa;
 	if (!(*found))
 	{
-		NamedDSAState *state = &entry->dsa;
-
 		entry->type = DSMR_ENTRY_TYPE_DSA;
+		state->handle = DSA_HANDLE_INVALID;
+		state->tranche = -1;
+	}
+	else if (entry->type != DSMR_ENTRY_TYPE_DSA)
+		ereport(ERROR,
+				(errmsg("requested DSA does not match type of existing entry")));
+
+	if (state->tranche == -1)
+	{
+		*found = false;
 
 		/* Initialize the LWLock tranche for the DSA. */
 		state->tranche = LWLockNewTrancheId(name);
+	}
+
+	if (state->handle == DSA_HANDLE_INVALID)
+	{
+		*found = false;
 
 		/* Initialize the DSA. */
 		ret = dsa_create(state->tranche);
@@ -309,17 +326,11 @@ GetNamedDSA(const char *name, bool *found)
 		/* Store handle for other backends to use. */
 		state->handle = dsa_get_handle(ret);
 	}
-	else if (entry->type != DSMR_ENTRY_TYPE_DSA)
+	else if (dsa_is_attached(state->handle))
 		ereport(ERROR,
-				(errmsg("requested DSA does not match type of existing entry")));
+				(errmsg("requested DSA already attached to current process")));
 	else
 	{
-		NamedDSAState *state = &entry->dsa;
-
-		if (dsa_is_attached(state->handle))
-			ereport(ERROR,
-					(errmsg("requested DSA already attached to current process")));
-
 		/* Attach to existing DSA. */
 		ret = dsa_attach(state->handle);
 		dsa_pin_mapping(ret);
@@ -346,6 +357,7 @@ GetNamedDSHash(const char *name, const dshash_parameters *params, bool *found)
 	DSMRegistryEntry *entry;
 	MemoryContext oldcontext;
 	dshash_table *ret;
+	NamedDSHState *dsh_state;
 
 	Assert(params);
 	Assert(found);
@@ -365,45 +377,57 @@ GetNamedDSHash(const char *name, const dshash_parameters *params, bool *found)
 	init_dsm_registry();
 
 	entry = dshash_find_or_insert(dsm_registry_table, name, found);
+	dsh_state = &entry->dsh;
 	if (!(*found))
 	{
-		NamedDSHState *dsh_state = &entry->dsh;
-		dshash_parameters params_copy;
-		dsa_area   *dsa;
-
 		entry->type = DSMR_ENTRY_TYPE_DSH;
+		dsh_state->dsa_handle = DSA_HANDLE_INVALID;
+		dsh_state->dsh_handle = DSHASH_HANDLE_INVALID;
+		dsh_state->tranche = -1;
+	}
+	else if (entry->type != DSMR_ENTRY_TYPE_DSH)
+		ereport(ERROR,
+				(errmsg("requested DSHash does not match type of existing entry")));
+
+	if (dsh_state->tranche == -1)
+	{
+		*found = false;
 
 		/* Initialize the LWLock tranche for the hash table. */
 		dsh_state->tranche = LWLockNewTrancheId(name);
+	}
+
+	if (dsh_state->dsa_handle == DSA_HANDLE_INVALID)
+	{
+		dshash_parameters params_copy;
+		dsa_area   *dsa;
+
+		*found = false;
 
 		/* Initialize the DSA for the hash table. */
 		dsa = dsa_create(dsh_state->tranche);
-		dsa_pin(dsa);
-		dsa_pin_mapping(dsa);
 
 		/* Initialize the dshash table. */
 		memcpy(&params_copy, params, sizeof(dshash_parameters));
 		params_copy.tranche_id = dsh_state->tranche;
 		ret = dshash_create(dsa, &params_copy, NULL);
 
+		dsa_pin(dsa);
+		dsa_pin_mapping(dsa);
+
 		/* Store handles for other backends to use. */
 		dsh_state->dsa_handle = dsa_get_handle(dsa);
 		dsh_state->dsh_handle = dshash_get_hash_table_handle(ret);
 	}
-	else if (entry->type != DSMR_ENTRY_TYPE_DSH)
+	else if (dsa_is_attached(dsh_state->dsa_handle))
 		ereport(ERROR,
-				(errmsg("requested DSHash does not match type of existing entry")));
+				(errmsg("requested DSHash already attached to current process")));
 	else
 	{
-		NamedDSHState *dsh_state = &entry->dsh;
 		dsa_area   *dsa;
 
 		/* XXX: Should we verify params matches what table was created with? */
 
-		if (dsa_is_attached(dsh_state->dsa_handle))
-			ereport(ERROR,
-					(errmsg("requested DSHash already attached to current process")));
-
 		/* Attach to existing DSA for the hash table. */
 		dsa = dsa_attach(dsh_state->dsa_handle);
 		dsa_pin_mapping(dsa);
-- 
2.39.5 (Apple Git-154)


--c08aH/hgV4/mNMsI--