Fix command history corruption on syntax error (re: e999f6b1)

Analysis: When a syntax error occurs, the shell performs a
longjmp(3) back to exfile() in main.c on line 417:
415|	if(jmpval)
416|	{
417|		Sfio_t *top;
418|		sh_iorestore((void*)shp,0,jmpval);
419|		hist_flush(shp->gd->hist_ptr);
420|		sfsync(shp->outpool);
The first thing it does is restore the file descriptor state
(sh_iorestore), then it flushes the history file (hist_flush), then
it synchronises sfio's logical stream state with the physical
stream state using (sfsync).

However, the fix applied in e999f6b1 caused sh_iorestore() to sync
all sfio streams unconditionally. So this was done before
hist_flush(), which caused unpredictable behaviour, including
temporary and/or permanent history corruption, as this also synched
shp->outpool before hist_flush() had a chance to do its thing.

The fix is to only call sfsync() in sh_iorestore() if we're
actually about to call ftruncate(2), and not otherwise.

Moral of the story: bug fixes should be as specific as possible to
minimise the risk of side effects.

src/cmd/ksh93/sh/io.c: sh_iorestore():
- Only call sfsync() if we're about to truncate a file.

src/cmd/ksh93/tests/pty.sh:
- Add test.

Thanks to Marc Wilson for reporting the bug and to Johnothan King
for finding the commit that introduced it.

Resolves: https://github.com/ksh93/ksh/issues/209
Relevant: https://github.com/att/ast/issues/61

NEWS

@@ -12,6 +12,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed a bug introduced on 2020-08-19: Ctrl+D would break after an
   interactive shell received SIGWINCH.
 
+- Fixed a bug introduced on 2020-05-21: on an interactive shell, command lines
+  containing a syntax error were not added to the command history file and
+  sometimes corrupted the command history.
+
 2021-03-05:
 
 - Unbalanced quotes and backticks now correctly produce a syntax error

src/cmd/ksh93/sh/io.c

@@ -1712,11 +1712,6 @@ 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)
@@ -1740,7 +1735,10 @@ void	sh_iorestore(Shell_t *shp, int last, int jmpval)
 			return;
 		}
 		if(filemap[fd].tname == Empty && shp->exitval==0)
+		{
+			sfsync(NIL(Sfio_t*));
 			ftruncate(origfd,lseek(origfd,0,SEEK_CUR));
+		}
 		else if(filemap[fd].tname)
 			io_usename(filemap[fd].tname,(int*)0,origfd,shp->exitval?2:1);
 		sh_close(origfd);

src/cmd/ksh93/tests/pty.sh

@@ -107,7 +107,6 @@ if	! pty $bintrue < /dev/null
 then	err_exit pty command hangs on $bintrue -- tests skipped
 	exit 0
 fi
-
 # err_exit #
 tst $LINENO <<"!"
 L POSIX sh 026(C)
@@ -712,5 +711,21 @@ r ^:test-3: ls vi_completion_A_file\r\n$
 r ^vi_completion_A_file\r\n$
 !
 
+# err_exit #
+tst $LINENO <<"!"
+L syntax error added to history file
+
+# https://github.com/ksh93/ksh/issues/209
+
+d 10
+p :test-1:
+w do something
+r ^:test-1: do something\r\n$
+r : syntax error: `do' unexpected\r\n$
+w fc -lN1
+r ^:test-2: fc -lN1\r\n$
+r \tdo something\r\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))