Move code related to configuration files in directories to new file

The code in charge of listing and classifying a set of configuration
files in a directory was located in guc-file.l, being used currently for
GUCs under "include_dir".  This code is planned to be used for an
upcoming feature able to include configuration files for ident and HBA
files from a directory, similarly to GUCs.  In both cases, the file
names, suffixed by ".conf", have to be ordered alphabetically.  This
logic is moved to a new file, called conffiles.c, so as it is easier to
share this facility between GUCs and the HBA/ident parsing logic.

Author: Julien Rouhaud, Michael Paquier
Discussion: https://postgr.es/m/Y2IgaH5YzIq2b+iR@paquier.xyz

src/backend/utils/misc/conffiles.c

@@ -0,0 +1,164 @@
+/*--------------------------------------------------------------------
+ * conffiles.c
+ *
+ * Utilities related to the handling of configuration files.
+ *
+ * This file contains some generic tools to work on configuration files
+ * used by PostgreSQL, be they related to GUCs or authentication.
+ *
+ *
+ * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/misc/conffiles.c
+ *
+ *--------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include <dirent.h>
+
+#include "common/file_utils.h"
+#include "miscadmin.h"
+#include "storage/fd.h"
+#include "utils/conffiles.h"
+
+/*
+ * AbsoluteConfigLocation
+ *
+ * Given a configuration file or directory location that may be a relative
+ * path, return an absolute one.  We consider the location to be relative to
+ * the directory holding the calling file, or to DataDir if no calling file.
+ */
+char *
+AbsoluteConfigLocation(const char *location, const char *calling_file)
+{
+	char		abs_path[MAXPGPATH];
+
+	if (is_absolute_path(location))
+		return pstrdup(location);
+	else
+	{
+		if (calling_file != NULL)
+		{
+			strlcpy(abs_path, calling_file, sizeof(abs_path));
+			get_parent_directory(abs_path);
+			join_path_components(abs_path, abs_path, location);
+			canonicalize_path(abs_path);
+		}
+		else
+		{
+			Assert(DataDir);
+			join_path_components(abs_path, DataDir, location);
+			canonicalize_path(abs_path);
+		}
+		return pstrdup(abs_path);
+	}
+}
+
+
+/*
+ * GetConfFilesInDir
+ *
+ * Returns the list of config files located in a directory, in alphabetical
+ * order.  On error, returns NULL with details about the error stored in
+ * "err_msg".
+ */
+char	  **
+GetConfFilesInDir(const char *includedir, const char *calling_file,
+				  int elevel, int *num_filenames, char **err_msg)
+{
+	char	   *directory;
+	DIR		   *d;
+	struct dirent *de;
+	char	  **filenames = NULL;
+	int			size_filenames;
+
+	/*
+	 * Reject directory name that is all-blank (including empty), as that
+	 * leads to confusion --- we'd read the containing directory, typically
+	 * resulting in recursive inclusion of the same file(s).
+	 */
+	if (strspn(includedir, " \t\r\n") == strlen(includedir))
+	{
+		ereport(elevel,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("empty configuration directory name: \"%s\"",
+						includedir)));
+		*err_msg = "empty configuration directory name";
+		return NULL;
+	}
+
+	directory = AbsoluteConfigLocation(includedir, calling_file);
+	d = AllocateDir(directory);
+	if (d == NULL)
+	{
+		ereport(elevel,
+				(errcode_for_file_access(),
+				 errmsg("could not open configuration directory \"%s\": %m",
+						directory)));
+		*err_msg = psprintf("could not open directory \"%s\"", directory);
+		goto cleanup;
+	}
+
+	/*
+	 * Read the directory and put the filenames in an array, so we can sort
+	 * them prior to caller processing the contents.
+	 */
+	size_filenames = 32;
+	filenames = (char **) palloc(size_filenames * sizeof(char *));
+	*num_filenames = 0;
+
+	while ((de = ReadDir(d, directory)) != NULL)
+	{
+		PGFileType	de_type;
+		char		filename[MAXPGPATH];
+
+		/*
+		 * Only parse files with names ending in ".conf".  Explicitly reject
+		 * files starting with ".".  This excludes things like "." and "..",
+		 * as well as typical hidden files, backup files, and editor debris.
+		 */
+		if (strlen(de->d_name) < 6)
+			continue;
+		if (de->d_name[0] == '.')
+			continue;
+		if (strcmp(de->d_name + strlen(de->d_name) - 5, ".conf") != 0)
+			continue;
+
+		join_path_components(filename, directory, de->d_name);
+		canonicalize_path(filename);
+		de_type = get_dirent_type(filename, de, true, elevel);
+		if (de_type == PGFILETYPE_ERROR)
+		{
+			*err_msg = psprintf("could not stat file \"%s\"", filename);
+			pfree(filenames);
+			filenames = NULL;
+			goto cleanup;
+		}
+		else if (de_type != PGFILETYPE_DIR)
+		{
+			/* Add file to array, increasing its size in blocks of 32 */
+			if (*num_filenames >= size_filenames)
+			{
+				size_filenames += 32;
+				filenames = (char **) repalloc(filenames,
+											   size_filenames * sizeof(char *));
+			}
+			filenames[*num_filenames] = pstrdup(filename);
+			(*num_filenames)++;
+		}
+	}
+
+	/* Sort the files by name before leaving */
+	if (*num_filenames > 0)
+		qsort(filenames, *num_filenames, sizeof(char *), pg_qsort_strcmp);
+
+cleanup:
+	if (d)
+		FreeDir(d);
+	pfree(directory);
+	return filenames;
+}