Avoid passing function pointers across process boundaries.

We'd already recognized that we can't pass function pointers across process
boundaries for functions in loadable modules, since a shared library could
get loaded at different addresses in different processes.  But actually the
practice doesn't work for functions in the core backend either, if we're
using EXEC_BACKEND.  This is the cause of recent failures on buildfarm
member culicidae.  Switch to passing a string function name in all cases.

Something like this needs to be back-patched into 9.6, but let's see
if the buildfarm likes it first.

Petr Jelinek, with a bunch of basically-cosmetic adjustments by me

Discussion: https://postgr.es/m/548f9c1d-eafa-e3fa-9da8-f0cc2f654e60@2ndquadrant.com

Author: tglsfdc

src/backend/access/transam/README.parallel

@@ -198,7 +198,7 @@ pattern looks like this:
 
 	EnterParallelMode();		/* prohibit unsafe state changes */
 
-	pcxt = CreateParallelContext(entrypoint, nworkers);
+	pcxt = CreateParallelContext("library_name", "function_name", nworkers);
 
 	/* Allow space for application-specific data here. */
 	shm_toc_estimate_chunk(&pcxt->estimator, size);

src/backend/access/transam/parallel.c

@@ -19,6 +19,7 @@
 #include "access/xlog.h"
 #include "catalog/namespace.h"
 #include "commands/async.h"
+#include "executor/execParallel.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "libpq/pqmq.h"
@@ -61,7 +62,7 @@
 #define PARALLEL_KEY_TRANSACTION_SNAPSHOT	UINT64CONST(0xFFFFFFFFFFFF0006)
 #define PARALLEL_KEY_ACTIVE_SNAPSHOT		UINT64CONST(0xFFFFFFFFFFFF0007)
 #define PARALLEL_KEY_TRANSACTION_STATE		UINT64CONST(0xFFFFFFFFFFFF0008)
-#define PARALLEL_KEY_EXTENSION_TRAMPOLINE	UINT64CONST(0xFFFFFFFFFFFF0009)
+#define PARALLEL_KEY_ENTRYPOINT				UINT64CONST(0xFFFFFFFFFFFF0009)
 
 /* Fixed-size parallel state. */
 typedef struct FixedParallelState
@@ -77,9 +78,6 @@ typedef struct FixedParallelState
 	pid_t		parallel_master_pid;
 	BackendId	parallel_master_backend_id;
 
-	/* Entrypoint for parallel workers. */
-	parallel_worker_main_type entrypoint;
-
 	/* Mutex protects remaining fields. */
 	slock_t		mutex;
 
@@ -107,10 +105,26 @@ static FixedParallelState *MyFixedParallelState;
 /* List of active parallel contexts. */
 static dlist_head pcxt_list = DLIST_STATIC_INIT(pcxt_list);
 
+/*
+ * List of internal parallel worker entry points.  We need this for
+ * reasons explained in LookupParallelWorkerFunction(), below.
+ */
+static const struct
+{
+	const char *fn_name;
+	parallel_worker_main_type fn_addr;
+}	InternalParallelWorkers[] =
+
+{
+	{
+		"ParallelQueryMain", ParallelQueryMain
+	}
+};
+
 /* Private functions. */
 static void HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg);
-static void ParallelExtensionTrampoline(dsm_segment *seg, shm_toc *toc);
 static void WaitForParallelWorkersToExit(ParallelContext *pcxt);
+static parallel_worker_main_type LookupParallelWorkerFunction(const char *libraryname, const char *funcname);
 
 
 /*
@@ -119,7 +133,8 @@ static void WaitForParallelWorkersToExit(ParallelContext *pcxt);
  * destroyed before exiting the current subtransaction.
  */
 ParallelContext *
-CreateParallelContext(parallel_worker_main_type entrypoint, int nworkers)
+CreateParallelContext(const char *library_name, const char *function_name,
+					  int nworkers)
 {
 	MemoryContext oldcontext;
 	ParallelContext *pcxt;
@@ -152,7 +167,8 @@ CreateParallelContext(parallel_worker_main_type entrypoint, int nworkers)
 	pcxt = palloc0(sizeof(ParallelContext));
 	pcxt->subid = GetCurrentSubTransactionId();
 	pcxt->nworkers = nworkers;
-	pcxt->entrypoint = entrypoint;
+	pcxt->library_name = pstrdup(library_name);
+	pcxt->function_name = pstrdup(function_name);
 	pcxt->error_context_stack = error_context_stack;
 	shm_toc_initialize_estimator(&pcxt->estimator);
 	dlist_push_head(&pcxt_list, &pcxt->node);
@@ -163,33 +179,6 @@ CreateParallelContext(parallel_worker_main_type entrypoint, int nworkers)
 	return pcxt;
 }
 
-/*
- * Establish a new parallel context that calls a function provided by an
- * extension.  This works around the fact that the library might get mapped
- * at a different address in each backend.
- */
-ParallelContext *
-CreateParallelContextForExternalFunction(char *library_name,
-										 char *function_name,
-										 int nworkers)
-{
-	MemoryContext oldcontext;
-	ParallelContext *pcxt;
-
-	/* We might be running in a very short-lived memory context. */
-	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
-
-	/* Create the context. */
-	pcxt = CreateParallelContext(ParallelExtensionTrampoline, nworkers);
-	pcxt->library_name = pstrdup(library_name);
-	pcxt->function_name = pstrdup(function_name);
-
-	/* Restore previous memory context. */
-	MemoryContextSwitchTo(oldcontext);
-
-	return pcxt;
-}
-
 /*
  * Establish the dynamic shared memory segment for a parallel context and
  * copy state and other bookkeeping information that will be needed by
@@ -249,15 +238,10 @@ InitializeParallelDSM(ParallelContext *pcxt)
 										pcxt->nworkers));
 		shm_toc_estimate_keys(&pcxt->estimator, 1);
 
-		/* Estimate how much we'll need for extension entrypoint info. */
-		if (pcxt->library_name != NULL)
-		{
-			Assert(pcxt->entrypoint == ParallelExtensionTrampoline);
-			Assert(pcxt->function_name != NULL);
-			shm_toc_estimate_chunk(&pcxt->estimator, strlen(pcxt->library_name)
-								   + strlen(pcxt->function_name) + 2);
-			shm_toc_estimate_keys(&pcxt->estimator, 1);
-		}
+		/* Estimate how much we'll need for the entrypoint info. */
+		shm_toc_estimate_chunk(&pcxt->estimator, strlen(pcxt->library_name) +
+							   strlen(pcxt->function_name) + 2);
+		shm_toc_estimate_keys(&pcxt->estimator, 1);
 	}
 
 	/*
@@ -297,7 +281,6 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	fps->parallel_master_pgproc = MyProc;
 	fps->parallel_master_pid = MyProcPid;
 	fps->parallel_master_backend_id = MyBackendId;
-	fps->entrypoint = pcxt->entrypoint;
 	SpinLockInit(&fps->mutex);
 	fps->last_xlog_end = 0;
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);
@@ -312,6 +295,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
 		char	   *asnapspace;
 		char	   *tstatespace;
 		char	   *error_queue_space;
+		char	   *entrypointstate;
+		Size		lnamelen;
 
 		/* Serialize shared libraries we have loaded. */
 		libraryspace = shm_toc_allocate(pcxt->toc, library_len);
@@ -368,19 +353,19 @@ InitializeParallelDSM(ParallelContext *pcxt)
 		}
 		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, error_queue_space);
 
-		/* Serialize extension entrypoint information. */
-		if (pcxt->library_name != NULL)
-		{
-			Size		lnamelen = strlen(pcxt->library_name);
-			char	   *extensionstate;
-
-			extensionstate = shm_toc_allocate(pcxt->toc, lnamelen
-										  + strlen(pcxt->function_name) + 2);
-			strcpy(extensionstate, pcxt->library_name);
-			strcpy(extensionstate + lnamelen + 1, pcxt->function_name);
-			shm_toc_insert(pcxt->toc, PARALLEL_KEY_EXTENSION_TRAMPOLINE,
-						   extensionstate);
-		}
+		/*
+		 * Serialize entrypoint information.  It's unsafe to pass function
+		 * pointers across processes, as the function pointer may be different
+		 * in each process in EXEC_BACKEND builds, so we always pass library
+		 * and function name.  (We use library name "postgres" for functions
+		 * in the core backend.)
+		 */
+		lnamelen = strlen(pcxt->library_name);
+		entrypointstate = shm_toc_allocate(pcxt->toc, lnamelen +
+										   strlen(pcxt->function_name) + 2);
+		strcpy(entrypointstate, pcxt->library_name);
+		strcpy(entrypointstate + lnamelen + 1, pcxt->function_name);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ENTRYPOINT, entrypointstate);
 	}
 
 	/* Restore previous memory context. */
@@ -671,6 +656,8 @@ DestroyParallelContext(ParallelContext *pcxt)
 	}
 
 	/* Free memory. */
+	pfree(pcxt->library_name);
+	pfree(pcxt->function_name);
 	pfree(pcxt);
 }
 
@@ -941,6 +928,10 @@ ParallelWorkerMain(Datum main_arg)
 	shm_mq	   *mq;
 	shm_mq_handle *mqh;
 	char	   *libraryspace;
+	char	   *entrypointstate;
+	char	   *library_name;
+	char	   *function_name;
+	parallel_worker_main_type entrypt;
 	char	   *gucspace;
 	char	   *combocidspace;
 	char	   *tsnapspace;
@@ -1040,6 +1031,18 @@ ParallelWorkerMain(Datum main_arg)
 	Assert(libraryspace != NULL);
 	RestoreLibraryState(libraryspace);
 
+	/*
+	 * Identify the entry point to be called.  In theory this could result in
+	 * loading an additional library, though most likely the entry point is in
+	 * the core backend or in a library we just loaded.
+	 */
+	entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT);
+	Assert(entrypointstate != NULL);
+	library_name = entrypointstate;
+	function_name = entrypointstate + strlen(library_name) + 1;
+
+	entrypt = LookupParallelWorkerFunction(library_name, function_name);
+
 	/* Restore database connection. */
 	BackgroundWorkerInitializeConnectionByOid(fps->database_id,
 											  fps->authenticated_user_id);
@@ -1102,11 +1105,8 @@ ParallelWorkerMain(Datum main_arg)
 
 	/*
 	 * Time to do the real work: invoke the caller-supplied code.
-	 *
-	 * If you get a crash at this line, see the comments for
-	 * ParallelExtensionTrampoline.
 	 */
-	fps->entrypoint(seg, toc);
+	entrypt(seg, toc);
 
 	/* Must exit parallel mode to pop active snapshot. */
 	ExitParallelMode();
@@ -1121,33 +1121,6 @@ ParallelWorkerMain(Datum main_arg)
 	pq_putmessage('X', NULL, 0);
 }
 
-/*
- * It's unsafe for the entrypoint invoked by ParallelWorkerMain to be a
- * function living in a dynamically loaded module, because the module might
- * not be loaded in every process, or might be loaded but not at the same
- * address.  To work around that problem, CreateParallelContextForExtension()
- * arranges to call this function rather than calling the extension-provided
- * function directly; and this function then looks up the real entrypoint and
- * calls it.
- */
-static void
-ParallelExtensionTrampoline(dsm_segment *seg, shm_toc *toc)
-{
-	char	   *extensionstate;
-	char	   *library_name;
-	char	   *function_name;
-	parallel_worker_main_type entrypt;
-
-	extensionstate = shm_toc_lookup(toc, PARALLEL_KEY_EXTENSION_TRAMPOLINE);
-	Assert(extensionstate != NULL);
-	library_name = extensionstate;
-	function_name = extensionstate + strlen(library_name) + 1;
-
-	entrypt = (parallel_worker_main_type)
-		load_external_function(library_name, function_name, true, NULL);
-	entrypt(seg, toc);
-}
-
 /*
  * Update shared memory with the ending location of the last WAL record we
  * wrote, if it's greater than the value already stored there.
@@ -1163,3 +1136,47 @@ ParallelWorkerReportLastRecEnd(XLogRecPtr last_xlog_end)
 		fps->last_xlog_end = last_xlog_end;
 	SpinLockRelease(&fps->mutex);
 }
+
+/*
+ * Look up (and possibly load) a parallel worker entry point function.
+ *
+ * For functions contained in the core code, we use library name "postgres"
+ * and consult the InternalParallelWorkers array.  External functions are
+ * looked up, and loaded if necessary, using load_external_function().
+ *
+ * The point of this is to pass function names as strings across process
+ * boundaries.  We can't pass actual function addresses because of the
+ * possibility that the function has been loaded at a different address
+ * in a different process.  This is obviously a hazard for functions in
+ * loadable libraries, but it can happen even for functions in the core code
+ * on platforms using EXEC_BACKEND (e.g., Windows).
+ *
+ * At some point it might be worthwhile to get rid of InternalParallelWorkers[]
+ * in favor of applying load_external_function() for core functions too;
+ * but that raises portability issues that are not worth addressing now.
+ */
+static parallel_worker_main_type
+LookupParallelWorkerFunction(const char *libraryname, const char *funcname)
+{
+	/*
+	 * If the function is to be loaded from postgres itself, search the
+	 * InternalParallelWorkers array.
+	 */
+	if (strcmp(libraryname, "postgres") == 0)
+	{
+		int			i;
+
+		for (i = 0; i < lengthof(InternalParallelWorkers); i++)
+		{
+			if (strcmp(InternalParallelWorkers[i].fn_name, funcname) == 0)
+				return InternalParallelWorkers[i].fn_addr;
+		}
+
+		/* We can only reach this by programming error. */
+		elog(ERROR, "internal function \"%s\" not found", funcname);
+	}
+
+	/* Otherwise load from external library. */
+	return (parallel_worker_main_type)
+		load_external_function(libraryname, funcname, true, NULL);
+}

src/backend/executor/execParallel.c

@@ -112,8 +112,7 @@ static shm_mq_handle **ExecParallelSetupTupleQueues(ParallelContext *pcxt,
 static bool ExecParallelRetrieveInstrumentation(PlanState *planstate,
 							 SharedExecutorInstrumentation *instrumentation);
 
-/* Helper functions that run in the parallel worker. */
-static void ParallelQueryMain(dsm_segment *seg, shm_toc *toc);
+/* Helper function that runs in the parallel worker. */
 static DestReceiver *ExecParallelGetReceiver(dsm_segment *seg, shm_toc *toc);
 
 /*
@@ -410,7 +409,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 	pstmt_data = ExecSerializePlan(planstate->plan, estate);
 
 	/* Create a parallel context. */
-	pcxt = CreateParallelContext(ParallelQueryMain, nworkers);
+	pcxt = CreateParallelContext("postgres", "ParallelQueryMain", nworkers);
 	pei->pcxt = pcxt;
 
 	/*
@@ -831,7 +830,7 @@ ExecParallelInitializeWorker(PlanState *planstate, shm_toc *toc)
  * to do this are also stored in the dsm_segment and can be accessed through
  * the shm_toc.
  */
-static void
+void
 ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 {
 	BufferUsage *buffer_usage;