From 07befa0c2640069db239ed55cfbf068c462e5a26 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 29 May 2025 10:43:10 -0400 Subject: [PATCH v6 1/4] Fix memory leakage in postgres_fdw's DirectModify code path. postgres_fdw tries to use PG_TRY blocks to ensure that it will eventually free the PGresult created by the remote modify command. However, it's fundamentally impossible for this scheme to work reliably when there's RETURNING data, because the query could fail in between invocations of postgres_fdw's DirectModify methods. There is at least one instance of exactly this situation in the regression tests, and the ensuing session-lifespan leak is visible under Valgrind. We can improve matters by using a memory context reset callback attached to the ExecutorState context. That ensures that the PGresult will be freed when the ExecutorState context is torn down, even if control never reaches postgresEndDirectModify. This patch adds infrastructure that wraps each PGresult in a "libpqsrv_PGresult" that provides the reset callback. Code using this abstraction is inherently memory-safe (so long as it attaches the reset callbacks to an appropriate memory context). Furthermore, we add some macros that automatically redirect calls of the libpq functions concerned with PGresults to use this infrastructure, so that almost no source-code changes are needed to wheel this infrastructure into place in all the backend code that uses libpq. This patch just creates the infrastructure and makes relevant code use it, which is enough to fix the DirectModify leak (and any others that may exist). A good deal of follow-on simplification is possible now that we don't have to be so cautious about freeing PGresults, but I'll put that in a separate patch. Author: Tom Lane Discussion: https://postgr.es/m/2976982.1748049023@sss.pgh.pa.us --- contrib/postgres_fdw/postgres_fdw.c | 7 + contrib/postgres_fdw/postgres_fdw.h | 2 +- src/backend/utils/mmgr/mcxt.c | 39 +++- src/include/libpq/libpq-be-fe-helpers.h | 12 +- src/include/libpq/libpq-be-fe.h | 259 ++++++++++++++++++++++++ src/include/utils/palloc.h | 2 + src/tools/pgindent/typedefs.list | 1 + 7 files changed, 307 insertions(+), 15 deletions(-) create mode 100644 src/include/libpq/libpq-be-fe.h diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c index 331f3fc088d..12bdecb32f4 100644 --- a/contrib/postgres_fdw/postgres_fdw.c +++ b/contrib/postgres_fdw/postgres_fdw.c @@ -4600,6 +4600,13 @@ execute_dml_stmt(ForeignScanState *node) pgfdw_report_error(ERROR, dmstate->result, dmstate->conn, true, dmstate->query); + /* + * The result potentially needs to survive across multiple executor row + * cycles, so move it to the context where the dmstate is. + */ + dmstate->result = libpqsrv_PGresultSetParent(dmstate->result, + GetMemoryChunkContext(dmstate)); + /* Get the number of rows affected. */ if (dmstate->has_returning) dmstate->num_tuples = PQntuples(dmstate->result); diff --git a/contrib/postgres_fdw/postgres_fdw.h b/contrib/postgres_fdw/postgres_fdw.h index 81358f3bde7..9cb4ee84139 100644 --- a/contrib/postgres_fdw/postgres_fdw.h +++ b/contrib/postgres_fdw/postgres_fdw.h @@ -15,7 +15,7 @@ #include "foreign/foreign.h" #include "lib/stringinfo.h" -#include "libpq-fe.h" +#include "libpq/libpq-be-fe.h" #include "nodes/execnodes.h" #include "nodes/pathnodes.h" #include "utils/relcache.h" diff --git a/src/backend/utils/mmgr/mcxt.c b/src/backend/utils/mmgr/mcxt.c index 15fa4d0a55e..ce01dce9861 100644 --- a/src/backend/utils/mmgr/mcxt.c +++ b/src/backend/utils/mmgr/mcxt.c @@ -560,9 +560,7 @@ MemoryContextDeleteChildren(MemoryContext context) * the specified context, since that means it will automatically be freed * when no longer needed. * - * There is no API for deregistering a callback once registered. If you - * want it to not do anything anymore, adjust the state pointed to by its - * "arg" to indicate that. + * Note that callers can assume this cannot fail. */ void MemoryContextRegisterResetCallback(MemoryContext context, @@ -577,6 +575,41 @@ MemoryContextRegisterResetCallback(MemoryContext context, context->isReset = false; } +/* + * MemoryContextUnregisterResetCallback + * Undo the effects of MemoryContextRegisterResetCallback. + * + * This can be used if a callback's effects are no longer required + * at some point before the context has been reset/deleted. It is the + * caller's responsibility to pfree the callback struct (if needed). + * + * An assertion failure occurs if the callback was not registered. + * We could alternatively define that case as a no-op, but that seems too + * likely to mask programming errors such as passing the wrong context. + */ +void +MemoryContextUnregisterResetCallback(MemoryContext context, + MemoryContextCallback *cb) +{ + MemoryContextCallback *prev, + *cur; + + Assert(MemoryContextIsValid(context)); + + for (prev = NULL, cur = context->reset_cbs; cur != NULL; + prev = cur, cur = cur->next) + { + if (cur != cb) + continue; + if (prev) + prev->next = cur->next; + else + context->reset_cbs = cur->next; + return; + } + Assert(false); +} + /* * MemoryContextCallResetCallbacks * Internal function to call all registered callbacks for context. diff --git a/src/include/libpq/libpq-be-fe-helpers.h b/src/include/libpq/libpq-be-fe-helpers.h index 16205b824fa..9a0373212da 100644 --- a/src/include/libpq/libpq-be-fe-helpers.h +++ b/src/include/libpq/libpq-be-fe-helpers.h @@ -30,17 +30,7 @@ #ifndef LIBPQ_BE_FE_HELPERS_H #define LIBPQ_BE_FE_HELPERS_H -/* - * Despite the name, BUILDING_DLL is set only when building code directly part - * of the backend. Which also is where libpq isn't allowed to be - * used. Obviously this doesn't protect against libpq-fe.h getting included - * otherwise, but perhaps still protects against a few mistakes... - */ -#ifdef BUILDING_DLL -#error "libpq may not be used code directly built into the backend" -#endif - -#include "libpq-fe.h" +#include "libpq/libpq-be-fe.h" #include "miscadmin.h" #include "storage/fd.h" #include "storage/latch.h" diff --git a/src/include/libpq/libpq-be-fe.h b/src/include/libpq/libpq-be-fe.h new file mode 100644 index 00000000000..e3f796b0230 --- /dev/null +++ b/src/include/libpq/libpq-be-fe.h @@ -0,0 +1,259 @@ +/*------------------------------------------------------------------------- + * + * libpq-be-fe.h + * Wrapper functions for using libpq in extensions + * + * Code built directly into the backend is not allowed to link to libpq + * directly. Extension code is allowed to use libpq however. One of the + * main risks in doing so is leaking the malloc-allocated structures + * returned by libpq, causing a process-lifespan memory leak. + * + * This file provides wrapper objects to help in building memory-safe code. + * A PGresult object wrapped this way acts much as if it were palloc'd: + * it will go away when the specified context is reset or deleted. + * We might later extend the concept to other objects such as PGconns. + * + * See also the libpq-be-fe-helpers.h file, which provides additional + * facilities built on top of this one. + * + * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group + * Portions Copyright (c) 1994, Regents of the University of California + * + * src/include/libpq/libpq-be-fe.h + * + *------------------------------------------------------------------------- + */ +#ifndef LIBPQ_BE_FE_H +#define LIBPQ_BE_FE_H + +/* + * Despite the name, BUILDING_DLL is set only when building code directly part + * of the backend. Which also is where libpq isn't allowed to be + * used. Obviously this doesn't protect against libpq-fe.h getting included + * otherwise, but perhaps still protects against a few mistakes... + */ +#ifdef BUILDING_DLL +#error "libpq may not be used in code directly built into the backend" +#endif + +#include "libpq-fe.h" + +/* + * Memory-context-safe wrapper object for a PGresult. + */ +typedef struct libpqsrv_PGresult +{ + PGresult *res; /* the wrapped PGresult */ + MemoryContext ctx; /* the MemoryContext it's attached to */ + MemoryContextCallback cb; /* the callback that implements freeing */ +} libpqsrv_PGresult; + + +/* + * Wrap the given PGresult in a libpqsrv_PGresult object, so that it will + * go away automatically if the current memory context is reset or deleted. + * + * To avoid potential memory leaks, backend code must always apply this + * immediately to the output of any PGresult-yielding libpq function. + */ +static inline libpqsrv_PGresult * +libpqsrv_PQwrap(PGresult *res) +{ + libpqsrv_PGresult *bres; + MemoryContext ctx = CurrentMemoryContext; + + /* We pass through a NULL result as-is, since there's nothing to free */ + if (res == NULL) + return NULL; + /* Attempt to allocate the wrapper ... this had better not throw error */ + bres = (libpqsrv_PGresult *) + MemoryContextAllocExtended(ctx, + sizeof(libpqsrv_PGresult), + MCXT_ALLOC_NO_OOM); + /* If we failed to allocate a wrapper, free the PGresult before failing */ + if (bres == NULL) + { + PQclear(res); + ereport(ERROR, + (errcode(ERRCODE_OUT_OF_MEMORY), + errmsg("out of memory"))); + } + /* Okay, set up the wrapper */ + bres->res = res; + bres->ctx = ctx; + bres->cb.func = (MemoryContextCallbackFunction) PQclear; + bres->cb.arg = res; + MemoryContextRegisterResetCallback(ctx, &bres->cb); + return bres; +} + +/* + * Free a wrapped PGresult, after detaching it from the memory context. + * Like PQclear(), allow the argument to be NULL. + */ +static inline void +libpqsrv_PQclear(libpqsrv_PGresult *bres) +{ + if (bres) + { + MemoryContextUnregisterResetCallback(bres->ctx, &bres->cb); + PQclear(bres->res); + pfree(bres); + } +} + +/* + * Move a wrapped PGresult to have a different parent context. + */ +static inline libpqsrv_PGresult * +libpqsrv_PGresultSetParent(libpqsrv_PGresult *bres, MemoryContext ctx) +{ + libpqsrv_PGresult *newres; + + /* We pass through a NULL result as-is */ + if (bres == NULL) + return NULL; + /* Make a new wrapper in the target context, raising error on OOM */ + newres = (libpqsrv_PGresult *) + MemoryContextAlloc(ctx, sizeof(libpqsrv_PGresult)); + /* Okay, set up the new wrapper */ + newres->res = bres->res; + newres->ctx = ctx; + newres->cb.func = (MemoryContextCallbackFunction) PQclear; + newres->cb.arg = bres->res; + MemoryContextRegisterResetCallback(ctx, &newres->cb); + /* Disarm and delete the old wrapper */ + MemoryContextUnregisterResetCallback(bres->ctx, &bres->cb); + pfree(bres); + return newres; +} + +/* + * Convenience wrapper for PQgetResult. + * + * We could supply wrappers for other PGresult-returning functions too, + * but at present there's no need. + */ +static inline libpqsrv_PGresult * +libpqsrv_PQgetResult(PGconn *conn) +{ + return libpqsrv_PQwrap(PQgetResult(conn)); +} + +/* + * Accessor functions for libpqsrv_PGresult. While it's not necessary to use + * these, they emulate the behavior of the underlying libpq functions when + * passed a NULL pointer. This is particularly important for PQresultStatus, + * which is often the first check on a result. + */ + +static inline ExecStatusType +libpqsrv_PQresultStatus(const libpqsrv_PGresult *res) +{ + if (!res) + return PGRES_FATAL_ERROR; + return PQresultStatus(res->res); +} + +static inline const char * +libpqsrv_PQresultErrorMessage(const libpqsrv_PGresult *res) +{ + if (!res) + return ""; + return PQresultErrorMessage(res->res); +} + +static inline char * +libpqsrv_PQresultErrorField(const libpqsrv_PGresult *res, int fieldcode) +{ + if (!res) + return NULL; + return PQresultErrorField(res->res, fieldcode); +} + +static inline char * +libpqsrv_PQcmdStatus(const libpqsrv_PGresult *res) +{ + if (!res) + return NULL; + return PQcmdStatus(res->res); +} + +static inline int +libpqsrv_PQntuples(const libpqsrv_PGresult *res) +{ + if (!res) + return 0; + return PQntuples(res->res); +} + +static inline int +libpqsrv_PQnfields(const libpqsrv_PGresult *res) +{ + if (!res) + return 0; + return PQnfields(res->res); +} + +static inline char * +libpqsrv_PQgetvalue(const libpqsrv_PGresult *res, int tup_num, int field_num) +{ + if (!res) + return NULL; + return PQgetvalue(res->res, tup_num, field_num); +} + +static inline int +libpqsrv_PQgetlength(const libpqsrv_PGresult *res, int tup_num, int field_num) +{ + if (!res) + return 0; + return PQgetlength(res->res, tup_num, field_num); +} + +static inline int +libpqsrv_PQgetisnull(const libpqsrv_PGresult *res, int tup_num, int field_num) +{ + if (!res) + return 1; /* pretend it is null */ + return PQgetisnull(res->res, tup_num, field_num); +} + +static inline char * +libpqsrv_PQfname(const libpqsrv_PGresult *res, int field_num) +{ + if (!res) + return NULL; + return PQfname(res->res, field_num); +} + +static inline const char * +libpqsrv_PQcmdTuples(const libpqsrv_PGresult *res) +{ + if (!res) + return ""; + return PQcmdTuples(res->res); +} + +/* + * Redefine these libpq entry point names concerned with PGresults so that + * they will operate on libpqsrv_PGresults instead. This avoids needing to + * convert a lot of pre-existing code, and reduces the notational differences + * between frontend and backend libpq-using code. + */ +#define PGresult libpqsrv_PGresult +#define PQclear libpqsrv_PQclear +#define PQgetResult libpqsrv_PQgetResult +#define PQresultStatus libpqsrv_PQresultStatus +#define PQresultErrorMessage libpqsrv_PQresultErrorMessage +#define PQresultErrorField libpqsrv_PQresultErrorField +#define PQcmdStatus libpqsrv_PQcmdStatus +#define PQntuples libpqsrv_PQntuples +#define PQnfields libpqsrv_PQnfields +#define PQgetvalue libpqsrv_PQgetvalue +#define PQgetlength libpqsrv_PQgetlength +#define PQgetisnull libpqsrv_PQgetisnull +#define PQfname libpqsrv_PQfname +#define PQcmdTuples libpqsrv_PQcmdTuples + +#endif /* LIBPQ_BE_FE_H */ diff --git a/src/include/utils/palloc.h b/src/include/utils/palloc.h index e1b42267b22..039b9cba61a 100644 --- a/src/include/utils/palloc.h +++ b/src/include/utils/palloc.h @@ -133,6 +133,8 @@ MemoryContextSwitchTo(MemoryContext context) /* Registration of memory context reset/delete callbacks */ extern void MemoryContextRegisterResetCallback(MemoryContext context, MemoryContextCallback *cb); +extern void MemoryContextUnregisterResetCallback(MemoryContext context, + MemoryContextCallback *cb); /* * These are like standard strdup() except the copied string is diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list index a8346cda633..0b1a8f71af6 100644 --- a/src/tools/pgindent/typedefs.list +++ b/src/tools/pgindent/typedefs.list @@ -3714,6 +3714,7 @@ lclTocEntry leafSegmentInfo leaf_item libpq_source +libpqsrv_PGresult line_t lineno_t list_sort_comparator -- 2.43.5