Avoid passing function pointers across process boundaries.

This back-patches commit 3247082
into 9.6, primarily to make buildfarm member culicidae happy.
Unlike the HEAD patch, avoid changing the existing API of
CreateParallelContext; instead we just switch to using
CreateParallelContextForExternalFunction, even for core functions.

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

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

src/backend/access/transam/README.parallel

@@ -198,7 +198,7 @@ pattern looks like this:
 
 	EnterParallelMode();		/* prohibit unsafe state changes */
 
-	pcxt = CreateParallelContext(entrypoint, nworkers);
+	pcxt = CreateParallelContextForExternalFunction("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"
@@ -60,7 +61,7 @@
 #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
@@ -76,7 +77,7 @@ typedef struct FixedParallelState
 	pid_t		parallel_master_pid;
 	BackendId	parallel_master_backend_id;
 
-	/* Entrypoint for parallel workers. */
+	/* Entrypoint for parallel workers (deprecated)! */
 	parallel_worker_main_type entrypoint;
 
 	/* Mutex protects remaining fields. */
@@ -106,16 +107,36 @@ 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(char *libraryname, char *funcname);
 
 
 /*
  * Establish a new parallel context.  This should be done after entering
  * parallel mode, and (unless there is an error) the context should be
  * destroyed before exiting the current subtransaction.
+ *
+ * NB: specifying the entrypoint as a function address is unportable.
+ * This will go away in Postgres 10, in favor of the API provided by
+ * CreateParallelContextForExternalFunction; in the meantime use that.
  */
 ParallelContext *
 CreateParallelContext(parallel_worker_main_type entrypoint, int nworkers)
@@ -163,9 +184,9 @@ CreateParallelContext(parallel_worker_main_type entrypoint, int nworkers)
 }
 
 /*
- * 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.
+ * Establish a new parallel context that calls a function specified by name.
+ * Unlike CreateParallelContext, this is robust against possible differences
+ * in address space layout between different processes.
  */
 ParallelContext *
 CreateParallelContextForExternalFunction(char *library_name,
@@ -179,7 +200,7 @@ CreateParallelContextForExternalFunction(char *library_name,
 	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
 
 	/* Create the context. */
-	pcxt = CreateParallelContext(ParallelExtensionTrampoline, nworkers);
+	pcxt = CreateParallelContext(NULL, nworkers);
 	pcxt->library_name = pstrdup(library_name);
 	pcxt->function_name = pstrdup(function_name);
 
@@ -248,10 +269,9 @@ InitializeParallelDSM(ParallelContext *pcxt)
 										pcxt->nworkers));
 		shm_toc_estimate_keys(&pcxt->estimator, 1);
 
-		/* Estimate how much we'll need for extension entrypoint info. */
+		/* Estimate how much we'll need for 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);
@@ -367,7 +387,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
 		}
 		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, error_queue_space);
 
-		/* Serialize extension entrypoint information. */
+		/* Serialize entrypoint information. */
 		if (pcxt->library_name != NULL)
 		{
 			Size		lnamelen = strlen(pcxt->library_name);
@@ -377,7 +397,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
 										  + 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,
+			shm_toc_insert(pcxt->toc, PARALLEL_KEY_ENTRYPOINT,
 						   extensionstate);
 		}
 	}
@@ -669,6 +689,10 @@ DestroyParallelContext(ParallelContext *pcxt)
 	}
 
 	/* Free memory. */
+	if (pcxt->library_name)
+		pfree(pcxt->library_name);
+	if (pcxt->function_name)
+		pfree(pcxt->function_name);
 	pfree(pcxt);
 }
 
@@ -939,6 +963,8 @@ ParallelWorkerMain(Datum main_arg)
 	shm_mq	   *mq;
 	shm_mq_handle *mqh;
 	char	   *libraryspace;
+	char	   *entrypointstate;
+	parallel_worker_main_type entrypt;
 	char	   *gucspace;
 	char	   *combocidspace;
 	char	   *tsnapspace;
@@ -1038,6 +1064,25 @@ 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);
+	if (entrypointstate != NULL)
+	{
+		char	   *library_name;
+		char	   *function_name;
+
+		library_name = entrypointstate;
+		function_name = entrypointstate + strlen(library_name) + 1;
+
+		entrypt = LookupParallelWorkerFunction(library_name, function_name);
+	}
+	else
+		entrypt = fps->entrypoint;
+
 	/* Restore database connection. */
 	BackgroundWorkerInitializeConnectionByOid(fps->database_id,
 											  fps->authenticated_user_id);
@@ -1101,10 +1146,11 @@ 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.
+	 * If you get a crash at this line, try using
+	 * CreateParallelContextForExternalFunction instead of
+	 * CreateParallelContext.
 	 */
-	fps->entrypoint(seg, toc);
+	entrypt(seg, toc);
 
 	/* Must exit parallel mode to pop active snapshot. */
 	ExitParallelMode();
@@ -1119,33 +1165,6 @@ 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.
@@ -1161,3 +1180,47 @@ 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(char *libraryname, 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

@@ -105,8 +105,7 @@ 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);
 
 /*
@@ -355,7 +354,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 	pstmt_data = ExecSerializePlan(planstate->plan, estate);
 
 	/* Create a parallel context. */
-	pcxt = CreateParallelContext(ParallelQueryMain, nworkers);
+	pcxt = CreateParallelContextForExternalFunction("postgres", "ParallelQueryMain", nworkers);
 	pei->pcxt = pcxt;
 
 	/*
@@ -720,7 +719,7 @@ 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;