Move routine building restore_command to src/common/

restore_command has only been used until now by the backend, but there
is a pending patch for pg_rewind to make use of that in the frontend.

Author: Alexey Kondratov
Reviewed-by: Andrey Borodin, Andres Freund, Alvaro Herrera, Alexander
Korotkov, Michael Paquier
Discussion: https://postgr.es/m/a3acff50-5a0d-9a2c-b3b2-ee36168955c1@postgrespro.ru

src/common/archive.c

@@ -0,0 +1,121 @@
+/*-------------------------------------------------------------------------
+ *
+ * archive.c
+ *	  Common WAL archive routines
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/common/archive.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef FRONTEND
+#include "postgres.h"
+#else
+#include "postgres_fe.h"
+#endif
+
+#include "common/archive.h"
+#include "lib/stringinfo.h"
+
+/*
+ * BuildRestoreCommand
+ *
+ * Builds a restore command to retrieve a file from WAL archives, replacing
+ * the supported aliases with values supplied by the caller as defined by
+ * the GUC parameter restore_command: xlogpath for %p, xlogfname for %f and
+ * lastRestartPointFname for %r.
+ *
+ * The result is a palloc'd string for the restore command built.  The
+ * caller is responsible for freeing it.  If any of the required arguments
+ * is NULL and that the corresponding alias is found in the command given
+ * by the caller, then NULL is returned.
+ */
+char *
+BuildRestoreCommand(const char *restoreCommand,
+					const char *xlogpath,
+					const char *xlogfname,
+					const char *lastRestartPointFname)
+{
+	StringInfoData result;
+	const char *sp;
+
+	/*
+	 * Build the command to be executed.
+	 */
+	initStringInfo(&result);
+
+	for (sp = restoreCommand; *sp; sp++)
+	{
+		if (*sp == '%')
+		{
+			switch (sp[1])
+			{
+				case 'p':
+					{
+						char	   *nativePath;
+
+						/* %p: relative path of target file */
+						if (xlogpath == NULL)
+						{
+							pfree(result.data);
+							return NULL;
+						}
+						sp++;
+
+						/*
+						 * This needs to use a placeholder to not modify the
+						 * input with the conversion done via
+						 * make_native_path().
+						 */
+						nativePath = pstrdup(xlogpath);
+						make_native_path(nativePath);
+						appendStringInfoString(&result,
+											   nativePath);
+						pfree(nativePath);
+						break;
+					}
+				case 'f':
+					/* %f: filename of desired file */
+					if (xlogfname == NULL)
+					{
+						pfree(result.data);
+						return NULL;
+					}
+					sp++;
+					appendStringInfoString(&result, xlogfname);
+					break;
+				case 'r':
+					/* %r: filename of last restartpoint */
+					if (lastRestartPointFname == NULL)
+					{
+						pfree(result.data);
+						return NULL;
+					}
+					sp++;
+					appendStringInfoString(&result,
+										   lastRestartPointFname);
+					break;
+				case '%':
+					/* convert %% to a single % */
+					sp++;
+					appendStringInfoChar(&result, *sp);
+					break;
+				default:
+					/* otherwise treat the % as not special */
+					appendStringInfoChar(&result, *sp);
+					break;
+			}
+		}
+		else
+		{
+			appendStringInfoChar(&result, *sp);
+		}
+	}
+
+	return result.data;
+}