-o posix: always recognise octals in "let" builtin

Though the "let" builtin is not itself a POSIX standard command, it
processes standard shell arithmetic, so it should recognise octals
by leading zeros as POSIX requires if the 'posix' option is on.
This overrides the setting of the 'letoctal' option.

Note that none of this applies to the ((...)) arithmetic command,
which has always recognised leading-octal zeros and does not listen
to 'letoctal'. So setting the posix mode makes this consistent.

src/cmd/ksh93/sh/arith.c:
- When running the 'let' builtin, test that both SH_LETOCTAL and
  SH_POSIX are off before stripping leading zeros to disable octal
  number recognition.
- Cosmetic: fix spurious newline.

src/cmd/ksh93/sh.1:
- Document the change.

src/cmd/ksh93/tests/shtests:
- Make sure to disable posix mode by default for regression tests.

NEWS

@@ -14,6 +14,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   turned on if ksh is invoked under the name 'sh'.
   For now, it:
   * disables the &> redirection shorthand
+  * causes the 'let' arithmetic command to recognise octal numbers by
+    leading zeros regardless of the setting of the 'letoctal' option
 
 2020-08-19:
 

src/cmd/ksh93/sh.1

@@ -6321,10 +6321,12 @@ to be evaluated.
 .B let
 only recognizes octal constants starting with
 .B 0
-when the
+if one of the
 .B set
-option
+options
 .B letoctal
+or
+.B posix
 is on.
 See
 .I "Arithmetic Evaluation"
@@ -6987,6 +6989,8 @@ The
 .B let
 command allows octal constants starting with
 .BR 0 .
+If the \fBposix\fR shell option is active,
+octals are recognized regardless of this option.
 .TP 8
 .B markdirs
 All directory names resulting from file name generation have a trailing
@@ -7034,6 +7038,7 @@ to fail or zero if no command has failed.
 .B posix
 Enable POSIX standard compatibility mode. This option
 is on by default if ksh is invoked as \fBsh\fR. It
+enables octal numbers in \fBlet\fR shell arithmetic (see \fBletoctal\fR), and
 disables the \fB&>\fR redirection shorthand.
 .TP 8
 .B privileged

src/cmd/ksh93/sh/arith.c

@@ -390,8 +390,11 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
 			char	lastbase=0, *val = xp, oerrno = errno;
 			lvalue->eflag = 0;
 			errno = 0;
-			if(shp->bltindata.bnode==SYSLET && !sh_isoption(SH_LETOCTAL))
-			{
+			if(shp->bltindata.bnode==SYSLET && !sh_isoption(SH_LETOCTAL) && !sh_isoption(SH_POSIX))
+			{	/*
+				 * Since we're running the "let" builtin, disable octal number processing by
+				 * skipping all initial zeros, unless the 'letoctal' or 'posix' option is on.
+				 */
 				while(*val=='0' && isdigit(val[1]))
 					val++;
 			}
@@ -415,8 +418,7 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
 				c='e';
 			else
 				c = *str;
-			if(c==GETDECIMAL(0) || c=='e' || c == 'E' || lastbase ==
- 16 && (c == 'p' || c == 'P'))
+			if(c==GETDECIMAL(0) || c=='e' || c == 'E' || lastbase == 16 && (c == 'p' || c == 'P'))
 			{
 				lvalue->isfloat=1;
 				r = strtold(val,&str);

src/cmd/ksh93/tests/arith.sh

@@ -680,9 +680,8 @@ exp='typeset -C -a x=((typeset -C -a y=( [0]=(typeset -a -l -i z=([2]=3);));))'
 unset x
 let x=010
 [[ $x == 10 ]] || err_exit 'let treating 010 as octal'
-set -o letoctal
-let x=010
-[[ $x == 8 ]] || err_exit 'let not treating 010 as octal with letoctal on'
+(set -o letoctal; let x=010; [[ $x == 8 ]]) || err_exit 'let not treating 010 as octal with letoctal on'
+(set -o posix 2>/dev/null; let x=010; [[ $x == 8 ]]) || err_exit 'let not treating 010 as octal with posix on'
 
 float z=0
 integer aa=2 a=1

src/cmd/ksh93/tests/shtests

@@ -166,7 +166,8 @@ function valxml
 	return $errors
 }
 
-unset DISPLAY FIGNORE HISTFILE
+command set +o posix 2>/dev/null
+unset DISPLAY FIGNORE HISTFILE POSIXLY_CORRECT _AST_FEATURES
 export ENV=/./dev/null
 trap + PIPE # unadvertized -- set SIGPIPE to SIG_DFL #