Move frontend-side archive APIs from src/common/ to src/fe_utils/

fe_archive.c was compiled only for the frontend in src/common/, but as
it will never share anything with the backend, it makes most sense to
move this file to src/fe_utils/.

Reported-by: Peter Eisentraut
Discussion: https://postgr.es/m/e9766d71-8655-ac86-bdf6-77e0e7169977@2ndquadrant.com
Backpatch-through: 13

src/fe_utils/Makefile

@@ -20,6 +20,7 @@ include $(top_builddir)/src/Makefile.global
 override CPPFLAGS := -DFRONTEND -I$(libpq_srcdir) $(CPPFLAGS)
 
 OBJS = \
+	archive.o \
 	cancel.o \
 	conditional.o \
 	mbprint.o \

src/fe_utils/archive.c

@@ -0,0 +1,124 @@
+/*-------------------------------------------------------------------------
+ *
+ * archive.c
+ *	  Routines to access WAL archives from frontend
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/fe_utils/archive.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres_fe.h"
+
+#include <unistd.h>
+#include <sys/stat.h>
+
+#include "access/xlog_internal.h"
+#include "common/archive.h"
+#include "common/logging.h"
+#include "fe_utils/archive.h"
+
+
+/*
+ * RestoreArchivedFile
+ *
+ * Attempt to retrieve the specified file from off-line archival storage.
+ * If successful, return a file descriptor of the restored file, else
+ * return -1.
+ *
+ * For fixed-size files, the caller may pass the expected size as an
+ * additional crosscheck on successful recovery.  If the file size is not
+ * known, set expectedSize = 0.
+ */
+int
+RestoreArchivedFile(const char *path, const char *xlogfname,
+					off_t expectedSize, const char *restoreCommand)
+{
+	char		xlogpath[MAXPGPATH];
+	char	   *xlogRestoreCmd;
+	int			rc;
+	struct stat stat_buf;
+
+	snprintf(xlogpath, MAXPGPATH, "%s/" XLOGDIR "/%s", path, xlogfname);
+
+	xlogRestoreCmd = BuildRestoreCommand(restoreCommand, xlogpath,
+										 xlogfname, NULL);
+	if (xlogRestoreCmd == NULL)
+	{
+		pg_log_fatal("could not use restore_command with %%r alias");
+		exit(1);
+	}
+
+	/*
+	 * Execute restore_command, which should copy the missing file from
+	 * archival storage.
+	 */
+	rc = system(xlogRestoreCmd);
+	pfree(xlogRestoreCmd);
+
+	if (rc == 0)
+	{
+		/*
+		 * Command apparently succeeded, but let's make sure the file is
+		 * really there now and has the correct size.
+		 */
+		if (stat(xlogpath, &stat_buf) == 0)
+		{
+			if (expectedSize > 0 && stat_buf.st_size != expectedSize)
+			{
+				pg_log_fatal("unexpected file size for \"%s\": %lu instead of %lu",
+							 xlogfname, (unsigned long) stat_buf.st_size,
+							 (unsigned long) expectedSize);
+				exit(1);
+			}
+			else
+			{
+				int			xlogfd = open(xlogpath, O_RDONLY | PG_BINARY, 0);
+
+				if (xlogfd < 0)
+				{
+					pg_log_fatal("could not open file \"%s\" restored from archive: %m",
+								 xlogpath);
+					exit(1);
+				}
+				else
+					return xlogfd;
+			}
+		}
+		else
+		{
+			if (errno != ENOENT)
+			{
+				pg_log_fatal("could not stat file \"%s\": %m",
+							 xlogpath);
+				exit(1);
+			}
+		}
+	}
+
+	/*
+	 * If the failure was due to a signal, then it would be misleading to
+	 * return with a failure at restoring the file.  So just bail out and
+	 * exit.  Hard shell errors such as "command not found" are treated as
+	 * fatal too.
+	 */
+	if (wait_result_is_any_signal(rc, true))
+	{
+		pg_log_fatal("restore_command failed due to the signal: %s",
+					 wait_result_to_str(rc));
+		exit(1);
+	}
+
+	/*
+	 * The file is not available, so just let the caller decide what to do
+	 * next.
+	 */
+	pg_log_error("could not restore file \"%s\" from archive",
+				 xlogfname);
+	return -1;
+}