Clean up assorted issues in ALTER SYSTEM coding.

Fix unsafe use of a non-volatile variable in PG_TRY/PG_CATCH in
AlterSystemSetConfigFile().  While at it, clean up a bundle of other
infelicities and outright bugs, including corner-case-incorrect linked list
manipulation, a poorly designed and worse documented parse-and-validate
function (which even included some randomly chosen hard-wired substitutes
for the specified elevel in one code path ... wtf?), direct use of open()
instead of fd.c's facilities, inadequate checking of write()'s return
value, and generally poorly written commentary.

src/backend/utils/misc/guc-file.l

@@ -118,11 +118,11 @@ ProcessConfigFile(GucContext context)
 	bool		error = false;
 	bool		apply = false;
 	int			elevel;
+	const char *ConfFileWithError;
 	ConfigVariable *item,
-				   *head,
-				   *tail;
+			   *head,
+			   *tail;
 	int			i;
-	char		*ErrorConfFile = ConfigFileName;
 
 	/*
 	 * Config files are processed on startup (by the postmaster only)
@@ -138,6 +138,7 @@ ProcessConfigFile(GucContext context)
 	elevel = IsUnderPostmaster ? DEBUG2 : LOG;
 
 	/* Parse the main config file into a list of option names and values */
+	ConfFileWithError = ConfigFileName;
 	head = tail = NULL;
 
 	if (!ParseConfigFile(ConfigFileName, NULL, true, 0, elevel, &head, &tail))
@@ -160,25 +161,23 @@ ProcessConfigFile(GucContext context)
 		{
 			/* Syntax error(s) detected in the file, so bail out */
 			error = true;
-			ErrorConfFile = PG_AUTOCONF_FILENAME;
+			ConfFileWithError = PG_AUTOCONF_FILENAME;
 			goto cleanup_list;
 		}
 	}
 	else
 	{
-		ConfigVariable *prev = NULL;
-
 		/*
-		 * Pick up only the data_directory if DataDir is not set, which
-		 * means that the configuration file is read for the first time and
-		 * PG_AUTOCONF_FILENAME file cannot be read yet. In this case,
-		 * we shouldn't pick any settings except the data_directory
-		 * from postgresql.conf because they might be overwritten
-		 * with the settings in PG_AUTOCONF_FILENAME file which will be
-		 * read later. OTOH, since it's ensured that data_directory doesn't
-		 * exist in PG_AUTOCONF_FILENAME file, it will never be overwritten
-		 * later.
+		 * If DataDir is not set, the PG_AUTOCONF_FILENAME file cannot be
+		 * read.  In this case, we don't want to accept any settings but
+		 * data_directory from postgresql.conf, because they might be
+		 * overwritten with settings in the PG_AUTOCONF_FILENAME file which
+		 * will be read later. OTOH, since data_directory isn't allowed in the
+		 * PG_AUTOCONF_FILENAME file, it will never be overwritten later.
 		 */
+		ConfigVariable *prev = NULL;
+
+		/* Prune all items except "data_directory" from the list */
 		for (item = head; item;)
 		{
 			ConfigVariable *ptr = item;
@@ -189,15 +188,9 @@ ProcessConfigFile(GucContext context)
 				if (prev == NULL)
 					head = ptr->next;
 				else
-				{
 					prev->next = ptr->next;
-					/*
-					 * On removing last item in list, we need to update tail
-					 * to ensure that list will be maintianed.
-					 */
-					if (prev->next == NULL)
-						tail = prev;
-				}
+				if (ptr->next == NULL)
+					tail = prev;
 				FreeConfigVariable(ptr);
 			}
 			else
@@ -205,10 +198,10 @@ ProcessConfigFile(GucContext context)
 		}
 
 		/*
-		 * Quick exit if data_directory is not present in list.
+		 * Quick exit if data_directory is not present in file.
 		 *
-		 * Don't remember when we last successfully loaded the config file in
-		 * this case because that time will be set soon by subsequent load of
+		 * We need not do any further processing, in particular we don't set
+		 * PgReloadTime; that will be set soon by subsequent full loading of
 		 * the config file.
 		 */
 		if (head == NULL)
@@ -263,7 +256,7 @@ ProcessConfigFile(GucContext context)
 							item->name,
 							item->filename, item->sourceline)));
 			error = true;
-			ErrorConfFile = item->filename;
+			ConfFileWithError = item->filename;
 		}
 	}
 
@@ -392,7 +385,7 @@ ProcessConfigFile(GucContext context)
 		else if (scres == 0)
 		{
 			error = true;
-			ErrorConfFile = item->filename;
+			ConfFileWithError = item->filename;
 		}
 		/* else no error but variable's active value was not changed */
 
@@ -421,23 +414,23 @@ ProcessConfigFile(GucContext context)
 			ereport(ERROR,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 					 errmsg("configuration file \"%s\" contains errors",
-							ErrorConfFile)));
+							ConfFileWithError)));
 		else if (apply)
 			ereport(elevel,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 					 errmsg("configuration file \"%s\" contains errors; unaffected changes were applied",
-							ErrorConfFile)));
+							ConfFileWithError)));
 		else
 			ereport(elevel,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 					 errmsg("configuration file \"%s\" contains errors; no changes were applied",
-							ErrorConfFile)));
+							ConfFileWithError)));
 	}
 
 	/*
 	 * Calling FreeConfigVariables() any earlier than this can cause problems,
-	 * because ErrorConfFile could be pointing to a string that will be freed
-	 * here.
+	 * because ConfFileWithError could be pointing to a string that will be
+	 * freed here.
 	 */
 	FreeConfigVariables(head);
 }
@@ -477,8 +470,11 @@ AbsoluteConfigLocation(const char *location, const char *calling_file)
  * Read and parse a single configuration file.  This function recurses
  * to handle "include" directives.
  *
- * See ParseConfigFp for details.  This one merely adds opening the
- * file rather than working from a caller-supplied file descriptor,
+ * If "strict" is true, treat failure to open the config file as an error,
+ * otherwise just skip the file.
+ *
+ * See ParseConfigFp for further details.  This one merely adds opening the
+ * config file rather than working from a caller-supplied file descriptor,
  * and absolute-ifying the path name if necessary.
  */
 bool
@@ -516,12 +512,13 @@ ParseConfigFile(const char *config_file, const char *calling_file, bool strict,
 					 errmsg("could not open configuration file \"%s\": %m",
 							abs_path)));
 			OK = false;
-			goto cleanup;
 		}
-
-		ereport(LOG,
-				(errmsg("skipping missing configuration file \"%s\"",
-						abs_path)));
+		else
+		{
+			ereport(LOG,
+					(errmsg("skipping missing configuration file \"%s\"",
+							abs_path)));
+		}
 		goto cleanup;
 	}
 
@@ -616,9 +613,7 @@ ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
 	{
 		char	   *opt_name = NULL;
 		char	   *opt_value = NULL;
-		ConfigVariable *item,
-					   *cur_item = NULL,
-					   *prev_item = NULL;
+		ConfigVariable *item;
 
 		if (token == GUC_EOL)	/* empty or comment line */
 			continue;
@@ -701,41 +696,13 @@ ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
 		}
 		else
 		{
-			/*
-			 * ordinary variable, append to list.  For multiple items of
-			 * same parameter, retain only which comes later.
-			 */
+			/* ordinary variable, append to list */
 			item = palloc(sizeof *item);
 			item->name = opt_name;
 			item->value = opt_value;
 			item->filename = pstrdup(config_file);
 			item->sourceline = ConfigFileLineno-1;
 			item->next = NULL;
-
-			/* Remove the existing item of same parameter from the list */
-			for (cur_item = *head_p; cur_item; prev_item = cur_item,
-					 cur_item = cur_item->next)
-			{
-				if (strcmp(item->name, cur_item->name) == 0)
-				{
-					if (prev_item == NULL)
-						*head_p = cur_item->next;
-					else
-					{
-						prev_item->next = cur_item->next;
-						/*
-						 * On removing last item in list, we need to update tail
-						 * to ensure that list will be maintianed.
-						 */
-						if (prev_item->next == NULL)
-							*tail_p = prev_item;
-					}
-
-					FreeConfigVariable(cur_item);
-					break;
-				}
-			}
-
 			if (*head_p == NULL)
 				*head_p = item;
 			else
@@ -911,21 +878,6 @@ cleanup:
 	return status;
 }
 
-/*
- * Free a ConfigVariable
- */
-static void
-FreeConfigVariable(ConfigVariable *item)
-{
-	if (item != NULL)
-	{
-		pfree(item->name);
-		pfree(item->value);
-		pfree(item->filename);
-		pfree(item);
-	}
-}
-
 /*
  * Free a list of ConfigVariables, including the names and the values
  */
@@ -944,6 +896,18 @@ FreeConfigVariables(ConfigVariable *list)
 	}
 }
 
+/*
+ * Free a single ConfigVariable
+ */
+static void
+FreeConfigVariable(ConfigVariable *item)
+{
+	pfree(item->name);
+	pfree(item->value);
+	pfree(item->filename);
+	pfree(item);
+}
+
 
 /*
  *		scanstr