include_if_exists facility for config file.

This works the same as include, except that an error is not thrown
if the file is missing. Instead the fact that it's missing is
logged.

Greg Smith, reviewed by Euler Taveira de Oliveira.

doc/src/sgml/config.sgml

@@ -89,6 +89,18 @@ include 'filename'
     Inclusions can be nested.
    </para>
 
+   <para>
+    <indexterm>
+     <primary><literal>include_if_exists</></primary>
+     <secondary>in configuration file</secondary>
+    </indexterm>
+    Use the same approach as the <literal>include</> directive, continuing
+    normally if the file does not exist.  A regular <literal>include</>
+    will stop with an error if the referenced file is missing, while
+    <literal>include_if_exists</> does not.  A warning about the missing
+    file will be logged.
+   </para>
+
    <para>
     <indexterm>
      <primary>SIGHUP</primary>

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

@@ -129,7 +129,7 @@ ProcessConfigFile(GucContext context)
 	/* Parse the file into a list of option names and values */
 	head = tail = NULL;
 
-	if (!ParseConfigFile(ConfigFileName, NULL, 0, elevel, &head, &tail))
+	if (!ParseConfigFile(ConfigFileName, NULL, true, 0, elevel, &head, &tail))
 	{
 		/* Syntax error(s) detected in the file, so bail out */
 		error = true;
@@ -363,7 +363,7 @@ ProcessConfigFile(GucContext context)
  * and absolute-ifying the path name if necessary.
  */
 bool
-ParseConfigFile(const char *config_file, const char *calling_file,
+ParseConfigFile(const char *config_file, const char *calling_file, bool strict,
 				int depth, int elevel,
 				ConfigVariable **head_p,
 				ConfigVariable **tail_p)
@@ -414,11 +414,19 @@ ParseConfigFile(const char *config_file, const char *calling_file,
 	fp = AllocateFile(config_file, "r");
 	if (!fp)
 	{
-		ereport(elevel,
-				(errcode_for_file_access(),
-				 errmsg("could not open configuration file \"%s\": %m",
+		if (strict)
+		{
+			ereport(elevel,
+					(errcode_for_file_access(),
+					 errmsg("could not open configuration file \"%s\": %m",
+							config_file)));
+			return false;
+		}
+
+		ereport(LOG, 
+				(errmsg("skipping missing configuration file \"%s\"", 
 						config_file)));
-		return false;
+		return OK;
 	}
 
 	OK = ParseConfigFp(fp, config_file, depth, elevel, head_p, tail_p);
@@ -512,7 +520,24 @@ ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
 		}
 
 		/* OK, process the option name and value */
-		if (guc_name_compare(opt_name, "include") == 0)
+		if (guc_name_compare(opt_name, "include_if_exists") == 0)
+		{
+			/*
+			 * An include_if_exists directive isn't a variable and should be
+			 * processed immediately.
+			 */
+			unsigned int save_ConfigFileLineno = ConfigFileLineno;
+
+			if (!ParseConfigFile(opt_value, config_file, false,
+								 depth + 1, elevel,
+								 head_p, tail_p))
+				OK = false;
+			yy_switch_to_buffer(lex_buffer);
+			ConfigFileLineno = save_ConfigFileLineno;
+			pfree(opt_name);
+			pfree(opt_value);
+		}
+		else if (guc_name_compare(opt_name, "include") == 0)
 		{
 			/*
 			 * An include directive isn't a variable and should be processed
@@ -520,7 +545,7 @@ ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
 			 */
 			unsigned int save_ConfigFileLineno = ConfigFileLineno;
 
-			if (!ParseConfigFile(opt_value, config_file,
+			if (!ParseConfigFile(opt_value, config_file, true,
 								 depth + 1, elevel,
 								 head_p, tail_p))
 				OK = false;

src/include/utils/guc.h

@@ -111,7 +111,7 @@ typedef struct ConfigVariable
 } ConfigVariable;
 
 extern bool ParseConfigFile(const char *config_file, const char *calling_file,
-				int depth, int elevel,
+				bool strict, int depth, int elevel,
 				ConfigVariable **head_p, ConfigVariable **tail_p);
 extern bool ParseConfigFp(FILE *fp, const char *config_file,
 			  int depth, int elevel,