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

1 files changed, 61 insertions, 44 deletions

diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
index 9ad3e915db3..f1194891f50 100644
--- a/src/backend/postmaster/bgworker.c
+++ b/src/backend/postmaster/bgworker.c
@@ -111,25 +111,30 @@ struct BackgroundWorkerHandle
 static BackgroundWorkerArray *BackgroundWorkerData;
 
 /*
- * List of internal background workers. These are used for mapping the
- * function name to actual function when building with EXEC_BACKEND and also
- * to allow these to be loaded outside of shared_preload_libraries.
+ * List of internal background worker entry points.  We need this for
+ * reasons explained in LookupBackgroundWorkerFunction(), below.
  */
-typedef struct InternalBGWorkerMain
+static const struct
 {
-	char			   *bgw_function_name;
-	bgworker_main_type	bgw_main;
-} InternalBGWorkerMain;
-
-static const InternalBGWorkerMain InternalBGWorkers[] = {
-	{"ParallelWorkerMain", ParallelWorkerMain},
-	{"ApplyLauncherMain", ApplyLauncherMain},
-	{"ApplyWorkerMain", ApplyWorkerMain},
-	/* Dummy entry marking end of the array. */
-	{NULL, NULL}
+	const char *fn_name;
+	bgworker_main_type fn_addr;
+}	InternalBGWorkers[] =
+
+{
+	{
+		"ParallelWorkerMain", ParallelWorkerMain
+	},
+	{
+		"ApplyLauncherMain", ApplyLauncherMain
+	},
+	{
+		"ApplyWorkerMain", ApplyWorkerMain
+	}
 };
 
-static bgworker_main_type GetInternalBgWorkerMain(BackgroundWorker *worker);
+/* Private functions. */
+static bgworker_main_type LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname);
+
 
 /*
  * Calculate shared memory needed.
@@ -812,18 +817,10 @@ StartBackgroundWorker(void)
 	}
 
 	/*
-	 * For internal workers set the entry point to known function address.
-	 * Otherwise use the entry point specified by library name (which will
-	 * be loaded, if necessary) and a function name (which will be looked up
-	 * in the named library).
+	 * Look up the entry point function, loading its library if necessary.
 	 */
-	entrypt = GetInternalBgWorkerMain(worker);
-
-	if (entrypt == NULL)
-		entrypt = (bgworker_main_type)
-			load_external_function(worker->bgw_library_name,
-								   worker->bgw_function_name,
-								   true, NULL);
+	entrypt = LookupBackgroundWorkerFunction(worker->bgw_library_name,
+											 worker->bgw_function_name);
 
 	/*
 	 * Note that in normal processes, we would call InitPostgres here.  For a
@@ -842,10 +839,11 @@ StartBackgroundWorker(void)
 }
 
 /*
- * Register a new background worker while processing shared_preload_libraries.
+ * Register a new static background worker.
  *
- * This can only be called in the _PG_init function of a module library
- * that's loaded by shared_preload_libraries; otherwise it has no effect.
+ * This can only be called directly from postmaster or in the _PG_init
+ * function of a module library that's loaded by shared_preload_libraries;
+ * otherwise it will have no effect.
  */
 void
 RegisterBackgroundWorker(BackgroundWorker *worker)
@@ -858,7 +856,7 @@ RegisterBackgroundWorker(BackgroundWorker *worker)
 		 (errmsg("registering background worker \"%s\"", worker->bgw_name)));
 
 	if (!process_shared_preload_libraries_in_progress &&
-		GetInternalBgWorkerMain(worker) == NULL)
+		strcmp(worker->bgw_library_name, "postgres") != 0)
 	{
 		if (!IsUnderPostmaster)
 			ereport(LOG,
@@ -1193,26 +1191,45 @@ TerminateBackgroundWorker(BackgroundWorkerHandle *handle)
 }
 
 /*
- * Search the known internal worker array and return its main function
- * pointer if found.
+ * Look up (and possibly load) a bgworker entry point function.
+ *
+ * For functions contained in the core code, we use library name "postgres"
+ * and consult the InternalBGWorkers array.  External functions are
+ * looked up, and loaded if necessary, using load_external_function().
  *
- * Returns NULL if not known internal worker.
+ * 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 InternalBGWorkers[]
+ * in favor of applying load_external_function() for core functions too;
+ * but that raises portability issues that are not worth addressing now.
  */
 static bgworker_main_type
-GetInternalBgWorkerMain(BackgroundWorker *worker)
+LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname)
 {
-	int i;
+	/*
+	 * If the function is to be loaded from postgres itself, search the
+	 * InternalBGWorkers array.
+	 */
+	if (strcmp(libraryname, "postgres") == 0)
+	{
+		int			i;
 
-	/* Internal workers always have to use postgres as library name. */
-	if (strncmp(worker->bgw_library_name, "postgres", BGW_MAXLEN) != 0)
-		return NULL;
+		for (i = 0; i < lengthof(InternalBGWorkers); i++)
+		{
+			if (strcmp(InternalBGWorkers[i].fn_name, funcname) == 0)
+				return InternalBGWorkers[i].fn_addr;
+		}
 
-	for (i = 0; InternalBGWorkers[i].bgw_function_name; i++)
-	{
-		if (strncmp(InternalBGWorkers[i].bgw_function_name,
-					worker->bgw_function_name, BGW_MAXLEN) == 0)
-			return InternalBGWorkers[i].bgw_main;
+		/* We can only reach this by programming error. */
+		elog(ERROR, "internal function \"%s\" not found", funcname);
 	}
 
-	return NULL;
+	/* Otherwise load from external library. */
+	return (bgworker_main_type)
+		load_external_function(libraryname, funcname, true, NULL);
 }