Only replace file when newly downloaded file is different

Previously, when a file was downloaded, for example when no ETags were
available or used and/or Last-Modified header isn't available, we always
replaced the output file with the newly downloaded file making the
--keep=n functionality useless because the same file could replace all
stored versions after n attempts to update the file.

Now, we compare file size and if file size is equal we try to compare
sha256sum or md5sum to determine if the newly downloaded file is different
from the current file.

ibfupdater

@@ -35,7 +35,7 @@
 # Read-only variables:
 declare -r      PROGRAM_NAME="ibfupdater" # The name of the application
 declare -r PROGRAM_SHORTNAME="ibfu"       # The short name of the application
-declare -r   PROGRAM_VERSION="0.9.2"      # The application's version number
+declare -r   PROGRAM_VERSION="0.10.0"     # The application's version number
 declare -r              OIFS="$IFS"       # Save the IFS if we need to restore
 
 # From <sysexits.h>
@@ -194,6 +194,63 @@ do_check_config() {
 	IS_CONFIGURED=1
 }
 
+##
+# Check if downloaded file is really newer then existing file
+# in case server doesn't provide any useful headers
+do_check_if_downloaded_file_is_newer() {
+	if [[ ! -e "${config[output_file]}" ]] ; then
+		print_verbose "Cannot check if downloaded file is newer than output file because output file does not exist! Maybe first run?"
+		return 0
+	fi
+
+	local _file_size_old=$(stat --format %s "${config[output_file]}" 2>/dev/null)
+	local _file_size_new=$(stat --format %s "${config[output_file_temp]}" 2>/dev/null)
+
+	if [[ -z "${_file_size_old}" || -z "${_file_size_new}" ]] ; then
+		print_verbose "Cannot check if downloaded file is newer because one required file size is not available!"
+		return 0
+	fi
+
+	if [[ "${_file_size_old}" == "${_file_size_new}" ]] ; then
+		if ! hash awk 2>/dev/null ; then
+			print_verbose "awk is not available -- cannot compare downloaded file"
+			return 0
+		fi
+
+		local _command_checksum=
+
+		if hash sha256sum 2>/dev/null ; then
+			# Use sha256sum when available
+			print_verbose "Using ‘sha256sum’ to determine if downloaded file has changed ..."
+			_command_checksum="sha256sum"
+
+		elif hash md5sum 2>/dev/null ; then
+			print_verbose "Using ‘md5sum’ to determine if downloaded file has changed ..."
+			_command_checksum="md5sum"
+		else
+			print_verbose "Cannot compare downloaded file: Neither ‘sha256sum’ nor ‘md5sum’ is available!"
+			return 0
+		fi
+
+		local _file_checksum_old=$(${_command_checksum} "${config[output_file]}" 2>/dev/null | awk '{ print $1 }')
+		local _file_checksum_new=$(${_command_checksum} "${config[output_file_temp]}" 2>/dev/null | awk '{ print $1 }')
+
+		if [[ "${_file_checksum_old}" != "${_file_checksum_new}" ]] ; then
+			print_verbose "Checksums do not match -- newly downloaded file is probably different!"
+			return 0
+		else
+			print_verbose "Checksums match -- newly downloaded file is not different!"
+			return 1
+		fi
+	else
+		print_verbose "File size is different -- newly downloaded file is probably different!"
+		return 0
+	fi
+
+	print_verbose "Was unable to determine if newly downloaded file is newer!"
+	return 0
+}
+
 do_handle_curl_errors() {
 	[[ $1 -eq 0 ]] && return 0
 
@@ -237,7 +294,10 @@ do_handle_http_status() {
 				die $EX_ERROR "$error_message"
 			fi
 
-			print_verbose "New file version downloaded"
+			if ! do_check_if_downloaded_file_is_newer ; then
+				return 0
+			fi
+
 			do_rotate_file
 			do_update_file
 			;;