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Fix assignment operand handling to preserve ARGV

After initialization, ARGV is supposed to be under the control of the
application (the awk script). However, when fulfilling the assignment
specified by a "name=value" operand, the implementation modifies the
operand's ARGV entry behind the application's back, truncating it.

This commit undoes the munge before returning control to the
application.

$ echo | ./master 'BEGIN { print ARGV[1] } { print ARGV[1] }' name=value
name=value
name

$ echo | ./a.out 'BEGIN { print ARGV[1] } { print ARGV[1] }' name=value
name=value
name=value

History: setclvar had been modifying its string argument without
ill-effect from the beginning (7th Edition UNIX, 1979) [1], when,
in 1989, SVR4 switched the source of the argument from argv to ARGV
[2]. The former cannot be accessed by the application, the latter
can. Thus, a private implementation detail unintentionally became a
public defect.

[1] https://www.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/awk/lib.c
[2] https://github.com/danfuzz/one-true-awk/blob/master/versions/1989-10-11/lib.c

--- a/FIXES

+++ b/FIXES

@@ -25,6 +25,11 @@

 This file lists all bug fixes, changes, etc., made since the AWK book

 was sent to the printers in August, 1987.

+Mar 14, 2022:

+	The fulfillment of an assignment operand had been truncating its

+	entry in ARGV (since circa 1989). Thanks to Miguel Pineiro Jr.

+	<mpj@pineiro.cc>.

+

 Mar 3, 2022:

 	Fixed file management memory leak that appears to have been

 	there since the files array was first initialized with stdin,

--- a/lib.c

+++ b/lib.c

@@ -297,12 +297,13 @@

 void setclvar(char *s)	/* set var=value from s */

 {

-	char *p;

+	char *e, *p;

 	Cell *q;

 	double result;

 	for (p=s; *p != '='; p++)

 		;

+	e = p;

 	*p++ = 0;

 	p = qstring(p, '\0');

 	q = setsymtab(s, p, 0.0, STR, symtab);

@@ -312,6 +313,7 @@

 		q->tval |= NUM;

 	}

 	DPRINTF("command line set %s to |%s|\n", s, p);

+	*e = '=';

 }

--- a/main.c

+++ b/main.c

@@ -22,7 +22,7 @@

 THIS SOFTWARE.

 ****************************************************************/

-const char	*version = "version 20220303";

+const char	*version = "version 20220314";

 #define DEBUG

 #include <stdio.h>

--- a/testdir/T.argv

+++ b/testdir/T.argv

@@ -97,6 +97,12 @@

 $awk '{print L $0}' L=11 foo0 L=22 foo0 >foo2

 diff foo1 foo2 || echo 'BAD: T.argv (L=11 L=22)'

+echo >foo0

+echo 'name=value

+name=value' >foo1

+$awk 'BEGIN { print ARGV[1] } { print ARGV[1] }' name=value foo0 >foo2

+diff foo1 foo2 || echo 'BAD: T.argv assignment operand modified'

+

 echo 3.345 >foo1

 $awk 'BEGIN { print ARGV[1] + ARGV[2]}' 1 2.345 >foo2

 diff foo1 foo2 || echo 'BAD: T.argv (ARGV[1] + ARGV[2])'

-- 

⑨