Fix BUG_PUTIOERR: Check for and report I/O error in output builtins

This allows scripts to check for a nonzero exit status on the
'print', 'printf' and 'echo' builtins and prevent possible infinite
loops if SIGPIPE is ignored.

sfsync() was already returning a negative value on I/O error, so
all we need to do is add a check. The stream buffer will need to be
filled before an I/O error can be detected, but this is the same on
other shells. See manual page: src/lib/libast/man/sfio.3

Ref.: https://github.com/att/ast/issues/1093
      https://github.com/att/ast/pull/1363

src/cmd/ksh93/bltins/print.c: b_print():
- Make sure an error result from sfsync() is reflected in the
  output builtin's exit status (exitval).
- Write an I/O error message (e_io) if the exit status is nonzero.

src/cmd/ksh93/data/msg.c, src/cmd/ksh93/include/io.h:
- Add the e_io[] error message.

src/cmd/ksh93/tests/builtins.sh:
- Add I/O error regression test, checking for the correct error
  message and exit status. All three output builtins use the same
  b_print() function so we only need to test one.

src/cmd/ksh93/tests/basic.sh,
src/cmd/ksh93/tests/coprocess.sh:
- Redirect stderr on a few 'print' commands to /dev/null; these
  now issue an expected I/O error. This does not cause failures.

NEWS, TODO:
- Update.

(cherry picked from commit 9011fa933552e483dab460f7dd1593d64e059d94)

NEWS

@@ -12,6 +12,11 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   Ref.:	https://github.com/att/ast/issues/425
 	https://github.com/att/ast/pull/442
 
+- Fix BUG_PUTIOERR: Output builtins now correctly detect and report
+  input/output errors. This allows scripts to check for a nonzero exit
+  status on the 'print', 'printf' and 'echo' builtins and prevent possible
+  infinite loops if SIGPIPE is ignored.
+
 2020-05-13:
 
 - Fix BUG_CASELIT: an undocumented 'case' pattern matching misbehaviour that

TODO

@@ -92,10 +92,5 @@ https://github.com/modernish/modernish/tree/0.16/lib/modernish/cap/
   example, "$*" joins positional parameters on the first byte of IFS instead
   of the first character.
 
-- BUG_PUTIOERR: Shell builtins that output strings (echo, printf, ksh/zsh
-  print), and thus also modernish put and putln, do not check for I/O errors
-  on output. This means a script cannot check for them, and a script process
-  in a pipe can get stuck in an infinite loop if SIGPIPE is ignored.
-
 - BUG_TESTERR1A: test/[ exits with a non-error false status (1) if an
   invalid argument is given to an operator.

src/cmd/ksh93/bltins/print.c

@@ -336,9 +336,11 @@ skip2:
 		 * https://github.com/att/ast/issues/425
 		 */
 		if(!sflag && sffileno(outfile)!=sffileno(sfstderr))
-			sfsync(outfile);
+			if (sfsync(outfile) < 0)
+				exitval = 1;
 		sfpool(sfstderr,pool,SF_WRITE);
-		exitval = pdata.err;
+		if (pdata.err)
+			exitval = 1;
 	}
 	else if(vflag)
 	{
@@ -353,7 +355,10 @@ skip2:
 	{
 		/* echo style print */
 		if(nflag && !argv[0])
-			sfsync((Sfio_t*)0);
+		{
+			if (sfsync((Sfio_t*)0) < 0)
+				exitval = 1;
+		}
 		else if(sh_echolist(shp,outfile,rflag,argv) && !nflag)
 			sfputc(outfile,'\n');
 	}
@@ -365,8 +370,11 @@ skip2:
 	else if(n&SF_SHARE)
 	{
 		sfset(outfile,SF_SHARE|SF_PUBLIC,1);
-		sfsync(outfile);
+		if (sfsync(outfile) < 0)
+			exitval = 1;
 	}
+	if (exitval)
+		errormsg(SH_DICT, 2, e_io);
 	return(exitval);
 }
 

src/cmd/ksh93/data/msg.c

@@ -89,6 +89,7 @@ const char e_access[]		= "permission denied";
 const char e_direct[]		= "bad directory";
 const char e_file[]		= "%s: bad file unit number";
 const char e_redirect[]		= "redirection failed";
+const char e_io[]		= "I/O error";
 const char e_trap[]		= "%s: bad trap";
 const char e_readonly[]		= "%s: is read only";
 const char e_badfield[]		= "%d: negative field size";

src/cmd/ksh93/include/io.h

@@ -97,6 +97,7 @@ extern const char	e_tmpcreate[];
 extern const char	e_exists[];
 extern const char	e_file[];
 extern const char	e_redirect[];
+extern const char	e_io[];
 extern const char	e_formspec[];
 extern const char	e_badregexp[];
 extern const char	e_open[];

src/cmd/ksh93/tests/basic.sh

@@ -431,7 +431,7 @@ done | while read sec; do ( $sleep $sec; $sleep $sec) done
 s=SECONDS
 set -o pipefail
 for ((i=0; i < 30; i++))
-do	print hello
+do	print hello 2>/dev/null
 	sleep .1
 done |  $sleep 1
 (( (SECONDS-s) < 2 )) || err_exit 'early termination not causing broken pipe'
@@ -470,7 +470,7 @@ bintrue=$(whence -p true)
 set -o pipefail
 float start=$SECONDS end 
 for ((i=0; i < 2; i++))
-do	print foo
+do	print foo 2>/dev/null
 	sleep 1.5
 done | { read; $bintrue; end=$SECONDS ;}
 (( (SECONDS-start) < 1 )) && err_exit "pipefail not waiting for pipe to finish"

src/cmd/ksh93/tests/builtins.sh

@@ -639,4 +639,26 @@ read baz <<< 'foo\\\\bar'
 : ~root
 [[ $(builtin) == *.sh.tilde* ]] &&  err_exit 'builtin contains .sh.tilde'
 
+# ======
+# Check that I/O errors are detected <https://github.com/att/ast/issues/1093>
+actual=$(
+    {
+        (
+            trap "" PIPE
+            for ((i = SECONDS + 1; SECONDS < i; )); do
+                print hi || {
+                    print $? >&2
+                    exit
+                }
+            done
+        ) | true
+    } 2>&1
+)
+expect=$': print: I/O error\n1'
+if [[ $actual != *"$expect" ]]
+then
+    err_exit "I/O error not detected (expected '$expect', got '$actual')"
+fi
+
+# ======
 exit $((Errors<125?Errors:125))

src/cmd/ksh93/tests/coprocess.sh

@@ -266,7 +266,7 @@ _COSHELL_msgfd=5
 function cop
 {
 	read
-	print ok
+	print ok 2>/dev/null
 }
 
 exp=ok
@@ -342,7 +342,7 @@ wait
 function cop
 {
         read
-        print ok
+        print ok 2>/dev/null
 }
 exp=ok
 cop |&