Fix truncating of files with <>;file combined with <#pattern

The issue with truncating files was caused by out-of-sync streams.
Details and discussion: https://github.com/att/ast/issues/61

src/cmd/ksh93/sh/io.c: sh_iorestore():
- To be safe, sync all streams before restoring file descriptors.

src/cmd/ksh93/tests/io.sh:
- Add two regression tests for truncating files with this
  combination of redirections.
- The second test, which invokes a -c script, is disabled for now
  as this triggers another corner case bug involving the SH_NOFORK
  optimisaton for -c scripts. That fix is for another commit.

(cherry picked from commit 18fb64840365c2ff4608188e5487bd79d08f67d1)

NEWS

@@ -4,6 +4,12 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-05-21:
+
+- Fix truncating of files with the combined redirections '<>;file' and
+  '<#pattern'. The bug was caused by out-of-sync streams.
+  Details and discussion: https://github.com/att/ast/issues/61
+
 2020-05-20:
 
 - Fix BUG_ISSETLOOP. Expansions like ${var+set} remained static when used

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-05-20"
+#define SH_RELEASE	"93u+m 2020-05-21"

src/cmd/ksh93/sh/io.c

@@ -1749,6 +1749,11 @@ void	sh_iorestore(Shell_t *shp, int last, int jmpval)
 	register int 	origfd, savefd, fd;
 	int flag = (last&IOSUBSHELL);
 	last &= ~IOSUBSHELL;
+	/*
+	 * There was an issue with truncating files (see 'ftruncate' below) that was caused by
+	 * out-of-sync streams. So, to be safe, sync all streams before restoring file descriptors.
+	 */
+	sfsync(NULL);
 	for (fd = shp->topfd - 1; fd >= last; fd--)
 	{
 		if(!flag && filemap[fd].subshell)

src/cmd/ksh93/tests/io.sh

@@ -502,4 +502,24 @@ done	{n}< /dev/null
 n=$( exec {n}< /dev/null; print -r -- $n)
 [[ -r /dev/fd/$n ]] && err_exit "file descriptor n=$n left open after subshell"
 
+# ======
+# Truncating a file using <> and >#num
+# https://github.com/att/ast/issues/61
+
+for ((i=1; i<=10; i++)) do print "$i"; done | tee "$tmp/nums2" > "$tmp/nums1"
+expect=$'1\n2\n3\n4'
+
+(1<>;"$tmp/nums1" >#5)
+actual=$(cat "$tmp/nums1")
+[[ "$actual" = "$expect" ]] || err_exit "Failed to truncate file in subshell \
+(expected $(printf %q "$expect"), got $(printf %q "$actual"))"
+
+: <<\INACTIVE	# TODO: the >#5 is optimised away by a '-c' optimisation corner case bug
+"$SHELL" -c '1<>;"$1/nums2" >#5' x "$tmp"
+actual=$(cat "$tmp/nums2")
+[[ "$actual" = "$expect" ]] || err_exit "Failed to truncate file in -c script \
+(expected $(printf %q "$expect"), got $(printf %q "$actual"))"
+INACTIVE
+
+# ======
 exit $((Errors<125?Errors:125))