mk: fetch to temporary file

Download to a temporary file and move the file to the final location
when the download is finished - so the file is there or not, but not a
partially downloaded copy.

The script already supported resuming downloads - the support is still
there but basically will never happen again, because it only looks at
a partial file of the correct name (which can still happen if the
download finishes but provides an incomplete file, and the file cannot
be verified by during download e.g. because the distinfo file is not
available - a theoretically supported use case)

As proposed on tech-pkg.

Author: 0-wiz-0

mk/fetch/fetch

@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# $NetBSD: fetch,v 1.22 2026/02/07 15:15:02 wiz Exp $
+# $NetBSD: fetch,v 1.23 2026/02/15 09:46:33 wiz Exp $
 #
 # Copyright (c) 2006, 2015 The NetBSD Foundation, Inc.
 # All rights reserved.
@@ -45,6 +45,9 @@
 #	If the file cannot be fetched successfully, then we try the next
 #	listed site.
 #
+#	The file will be downloaded under a temporary name and
+#	renamed to the proper name when the download is done.
+#
 #	If the file already exists on the disk and is verified, then
 #	no fetch action is taken.
 #
@@ -189,7 +192,7 @@ if ${TEST} -n "$distinfo"; then
 	done < $distinfo
 fi
 
-# verify_file [-v] $file
+# verify_file [-v] $file $suffix
 #	If we can checksum the file, then see if it matches the listed
 #	checksums in the distinfo file.  If we can check the size, then
 #	check that instead.  We strip off ".pkgsrc.resume" from the
@@ -199,12 +202,13 @@ fi
 verify_file() {
 	_if_verbose=:; if [ "x$1" = "x-v" ]; then shift; _if_verbose=; fi
 	_file="${1#./}"
+	_suffix="$2"
 	${TEST} -f $_file || {
 		$_if_verbose ${ECHO} 1>&2 "$self: File $_file does not exist."
 		return 1
 	}
 	if ${TEST} -n "$checksum"; then
-		${CHECKSUM} -s ".pkgsrc.resume" $distinfo ${_file} || {
+		${CHECKSUM} -s "${_suffix}" $distinfo ${_file} || {
 			$_if_verbose ${ECHO} 1>&2 "$self: Checksum of the file $_file doesn't match."
 			return 1
 		}
@@ -223,7 +227,7 @@ verify_file() {
 # If the file already exists and it verifies, then we don't need to fetch
 # it again.
 #
-if verify_file $path; then
+if verify_file $path ""; then
 	exit 0
 fi
 
@@ -232,32 +236,23 @@ ${TEST} -d $fetchdir || ${MKDIR} -p $fetchdir 2>/dev/null
 ${TEST} -w $fetchdir || ${ECHO} 1>&2 "$self: WARNING: DISTDIR `cd $fetchdir && pwd` looks non-writable."
 
 # Set the name of the output file.  In the "resume" case, we initialize
-# the fetch loop by ensuring that the temporary output file already
-# exists.
+# the fetch loop by providing the existing, possibly incomplete, file.
 #
-outputfile="$file"
+tmp_suffix=".$$.part"
+outputfile="$file$tmp_suffix"
 outputpath="$fetchdir/$outputfile"
+if ${TEST} -f $outputpath; then
+    ${ECHO} "Temporary download file ${outputpath} already existed, deleting"
+    ${RM} -f $outputpath
+fi
 if ${TEST} -n "$resume"; then
-	outputfile="${file}.pkgsrc.resume"
-	outputpath="$fetchdir/$outputfile"
-	if ${TEST} ! -f $outputpath; then
-		if ${TEST} -f $path; then
-			${CP} -f $path $outputpath
-		else
-			${RM} -f $outputpath
-			${TOUCH} $outputpath
-		fi
-	fi
-	#
-	# If the temporary file verifies, then we don't need to resume
-	# fetching it.
-	#
-	if verify_file $outputpath; then
-		${MV} -f $outputpath $path
-		exit 0
+	if ${TEST} -f $path; then
+		${CP} -f $path $outputpath
+	else
+		${TOUCH} $outputpath
 	fi
 	size=`${WC} -c < $outputpath`
-	${ECHO} "=> Downloaded size: $size bytes"
+	${ECHO} "=> Downloaded size (before resume): $size bytes"
 fi
 ${TEST} -z "$distsize" || ${ECHO} "=> Total size: $distsize $distunits"
 
@@ -287,8 +282,8 @@ while ${TEST} $# -gt 0; do
 		${ECHO} 1>&2 "$self: Unable to fetch expected file $file"
 		continue
 	fi
-	if verify_file -v $outputpath; then
-		${TEST} -z "$resume" || ${MV} -f $outputpath $path
+	if verify_file -v $outputpath $tmp_suffix; then
+		${MV} -f $outputpath $path
 		break
 	fi
 	if ${TEST} -n "$resume"; then