Warn about uninitialized variables; meaningful use of string variables in boolean context

* src/mailfromd.h (variable) <initialized>: New member.
(variable_check_initialized): New proto.
* src/symbols.c (init_variable): Set initialized to 0.
* src/gram.y: Convert the use of string variable in boolean context
to s != ''.
(create_asgn_node): Set var->initialized  to 1.
(create_node_variable): Check if the variable is initialized.
(variable_check_initialized): New function.
* src/lex.l (variable_or_const): Check if the variable is
initialized.

* mflib/dns.mf4 (dns_getname, dns_getaddr)
(getns,getmx): Initialize result.

* tests/arginit.at: New test.
* tests/Makefile.am: Add new test.
* tests/testsuite.at: Include new test.

* doc/mailfromd.texi: Elaborate on default values of variables.

* NEWS: Version 8.9.90
* configure.ac: Likewise.

11 files changed, 165 insertions, 21 deletions

diff --git a/NEWS b/NEWS
index 84d3d8ff..e1d6a555 100644
--- a/NEWS
+++ b/NEWS
@@ -1,8 +1,31 @@
-Mailfromd NEWS -- history of user-visible changes. 2020-12-29
+Mailfromd NEWS -- history of user-visible changes. 2021-02-15
 See the end of file for copying conditions.
 
 Please send Mailfromd bug reports to <bug-mailfromd@gnu.org.ua>
 
+Version 8.9.90 (git)
+
+* Use of uninitialized automatic variables
+
+The MFL compiler issues a warning if it encounters the use of a
+previously uninitialized automatic variable.  In future versions
+the warning will change to error.
+
+* Use of string variables in boolean context
+
+Strings can meaningfully be used in boolean context.  For example
+
+  func f(string s)
+  do
+    if s
+       echo "non-empty
+    fi
+  done
+
+The use of "s" in conditional is equivalent to
+
+  if s != ""
+
 Version 8.9, 2020-12-29
 
 * The sed function.
diff --git a/configure.ac b/configure.ac
index 7d75d27f..287f09d0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,6 +17,7 @@
 AC_PREREQ(2.63)
 m4_define([MF_VERSION_MAJOR], 8)
 m4_define([MF_VERSION_MINOR], 9)
+m4_define([MF_VERSION_PATCH], 90)
 AC_INIT([mailfromd],
         MF_VERSION_MAJOR.MF_VERSION_MINOR[]m4_ifdef([MF_VERSION_PATCH],.MF_VERSION_PATCH),
         [bug-mailfromd@gnu.org.ua],
diff --git a/doc/mailfromd.texi b/doc/mailfromd.texi
index b88ca5e3..8bfbda57 100644
--- a/doc/mailfromd.texi
+++ b/doc/mailfromd.texi
@@ -5184,6 +5184,11 @@ variables and @samp{number} for numeric ones.
 string var
 @end example
 
+  If a variable declaration occurs within a function
+(@pxref{Functions,User-defined}) or handler (@pxref{Handlers}), it
+declares an automatic variable, local to this function or handler.
+Otherwise, it declares a global variable.
+
 @cindex qualifiers, variable declaration
 @kwindex public
 @kwindex static
@@ -5215,23 +5220,26 @@ or
 precious static string rcpt_list
 @end example
 
-  The declaration can be followed by any valid @acronym{MFL}
-expression, which supplies the @dfn{initial value} for the
+  Declaration can be followed by any valid @acronym{MFL}
+expression, which supplies the initial value or @dfn{initializer} for the
 variable, for example:
 
 @example
 string var "test"
 @end example
 
-  If a variable declaration occurs within a function
-(@pxref{Functions,User-defined}) or handler (@pxref{Handlers}), it
-declares an automatic variable, local to this function or handler.
-Otherwise, it declares a global variable.
+  A global variable declared without initializer is implicitly
+initialized to a null value: numeric variables assume initial value 0,
+string variable are initialized to empty string.
+
+  The value of an automatic variable declared without initializer is
+unspecified.  It is an error to use such variable prior to assigning
+it a value.
 
 @cindex variable, assigning a value
 @cindex variable assignment
 @kwindex set
-  A variable is assigned a value using @code{set} statement:
+  A variable is assigned a value using the @code{set} statement:
 
 @example
 set @var{name} @var{expr}
@@ -5256,9 +5264,11 @@ topmost lexical level.  This is called @dfn{implicit variable
 declaration}.
 
 @cindex variables, referencing
-  Variables are referenced using the notation @samp{%@var{name}}.  The
-variable being referenced must have been declared earlier (either
-explicitly or implicitly).  
+  In the @acronym{MFL} program, variables are referenced by their
+name.  When appearing inside a double-quoted string, variables are
+referenced using the notation @samp{%@var{name}}.  Any variable being
+referenced must have been declared earlier (either explicitly or
+implicitly).
 
 @menu
 * Predefined variables::
diff --git a/mflib/dns.mf4 b/mflib/dns.mf4
index 9c7e7d85..3d52f086 100644
--- a/mflib/dns.mf4
+++ b/mflib/dns.mf4
@@ -85,7 +85,7 @@ done
 func dns_getname (string ipstr)
   returns string
 do
-  string result
+  string result ''
   set n dns_query(DNS_TYPE_PTR, ipstr, 1)
   loop for set i 0,
        while i < dns_reply_count(n),
@@ -103,7 +103,7 @@ done
 func dns_getaddr (string domain)
   returns string
 do
-  string result
+  string result ''
   set n dns_query(DNS_TYPE_A, domain, 1)
   loop for set i 0,
        while i < dns_reply_count(n),
@@ -121,7 +121,7 @@ done
 func getns (string domain; number resolve_names, number sort_names)
   returns string
 do
-  string result
+  string result ''
   set n dns_query(DNS_TYPE_NS, domain, sort_names, resolve_names)
   loop for set i 0,
        while i < dns_reply_count(n),
@@ -139,7 +139,7 @@ done
 func getmx (string domain; number resolve_names)
   returns string
 do
-  string result
+  string result ''
   set n dns_query(DNS_TYPE_MX, domain, 0, resolve_names)
   loop for set i 0,
        while i < dns_reply_count(n),
diff --git a/src/gram.y b/src/gram.y
index faa3884f..8384a115 100644
--- a/src/gram.y
+++ b/src/gram.y
@@ -449,7 +449,7 @@ _create_alias(void *item, void *data)
 
 %type <node> decl stmt condition action sendmail_action header_action
              if_cond else_cond on_cond atom argref paren_argref
-             funcall expr maybe_expr maybe_xcode_expr
+             funcall expr bool_expr maybe_expr maybe_xcode_expr
              simp_expr atom_expr
              asgn catch simple_catch try_block throw return
              case_cond autodcl constdecl
@@ -1392,7 +1392,7 @@ condition  : if_cond
            | on_cond
            ;
 
-if_cond    : T_IF expr stmtlist else_cond T_FI
+if_cond    : T_IF bool_expr stmtlist else_cond T_FI
              {
                      $$ = alloc_node(node_type_if, &@1);
 		     $$->v.cond.cond = $2;
@@ -1405,7 +1405,7 @@ else_cond  : /* empty */
              {
                      $$ = NULL;
              }
-           | T_ELIF expr stmtlist else_cond
+           | T_ELIF bool_expr stmtlist else_cond
              {
                      $$ = alloc_node(node_type_if, &@1);
 		     $$->v.cond.cond = $2;
@@ -2246,6 +2246,30 @@ return      : T_RETURN
 	      }
             ;
 
+bool_expr   : expr
+              {
+		      switch (node_type($1)) {
+		      case dtype_string:
+			      $$ = alloc_node(node_type_bin, &@1);
+			      $$->v.bin.opcode = bin_ne;
+			      $$->v.bin.arg[0] = $1;
+			      $$->v.bin.arg[1] = alloc_node(node_type_string, &@1);
+			      $$->v.bin.arg[1]->v.literal = literal_lookup("");
+			      break;
+			      
+		      case dtype_number:
+		      case dtype_pointer:
+			      $$ = $1;
+			      break;
+			      
+		      default:
+			      parse_error_locus(&@1,
+						_("unspecified data type in conditional expression: please, report"));
+			      YYERROR;
+		      }
+	      }
+            ;
+
 /* *************************** */
 /*         ON statement        */
 /* *************************** */
@@ -3674,6 +3698,7 @@ create_asgn_node(struct variable *var, NODE *expr,
         node->v.asgn.var = var;
         node->v.asgn.nframes = catch_nesting;
         node->v.asgn.node = cast_to(var->type, expr);
+	var->initialized = 1;
         return node;
 }
 
@@ -4487,7 +4512,10 @@ declare_function(struct function *func, struct mu_locus_range const *loc,
 NODE *
 create_node_variable(struct variable *var, struct mu_locus_range const *locus)
 {
-        NODE *node = alloc_node(node_type_variable, locus);
+        NODE *node;
+
+	variable_check_initialized(var, locus);
+	node = alloc_node(node_type_variable, locus);
         node->v.var_ref.variable = var;
         node->v.var_ref.nframes = catch_nesting;
         return node;
@@ -4548,3 +4576,21 @@ create_node_backref(long num, struct mu_locus_range const *locus)
         node->v.number = num;
         return node;
 }
+
+static inline int
+variable_is_initialized(struct variable *var)
+{
+	return var->storage_class != storage_auto || var->initialized;
+}
+
+void
+variable_check_initialized(struct variable *var,
+			   struct mu_locus_range const *loc)
+{
+	if (!variable_is_initialized(var)) {
+		parse_warning_locus(loc,
+				    _("use of uninitialized variable '%s'"),
+				    var->sym.name);
+		var->initialized = 1;
+	}
+}
diff --git a/src/lex.l b/src/lex.l
index 7e24408d..7385d4b7 100644
--- a/src/lex.l
+++ b/src/lex.l
@@ -261,7 +261,7 @@ lex_new_source(const char *name, int flag)
 /* Return constant or variable token corresponding to the current
    value of yylval.literal->text. */
 static int
-variable_or_const()
+variable_or_const(void)
 {
 	struct variable *vptr;
 	const struct constant *cptr;
@@ -289,6 +289,7 @@ variable_or_const()
 			    yylval.literal->text);
 		return T_BOGUS;
 	}
+	variable_check_initialized(vptr, &yylloc);
 	add_xref(vptr, &yylloc);
 	yylval.var = vptr;
 	return T_VARIABLE;
diff --git a/src/mailfromd.h b/src/mailfromd.h
index d6582e05..e0610a2f 100644
--- a/src/mailfromd.h
+++ b/src/mailfromd.h
@@ -502,6 +502,8 @@ struct variable {
 	size_t *addrptr;           /* Address pointer (for built-in vars) */ 
 	struct variable *shadowed; /* Points to the variable shadowed by
 				      this one */
+	int initialized;           /* Is the variable initialized (for
+				      automatic variables) */
 	mu_list_t xref;            /* List of struct mu_locus_range */
 	struct variable *next;     /* Next variable in this class */
 };
@@ -765,6 +767,9 @@ void register_macro(enum smtp_state tag, const char *macro);
 char *get_stage_macro_string(enum gacopyz_stage i);
 struct exmask *exmask_create(void);
 
+void variable_check_initialized(struct variable *var,
+				struct mu_locus_range const *loc);
+
 
 /* Data types and declarations for handling compiled configuration code */
 
diff --git a/src/symbols.c b/src/symbols.c
index ad0b6994..bed911b5 100644
--- a/src/symbols.c
+++ b/src/symbols.c
@@ -736,6 +736,7 @@ init_variable(struct variable *var)
 	var->off = 0;
 	var->addrptr = NULL;
 	var->shadowed = NULL;
+	var->initialized = 0;
 	var->xref = NULL;
 	var->next = NULL;
 }	
@@ -901,7 +902,6 @@ literal_lookup(const char *text)
 	}
 	return lit;
 }
-
 
 struct constant *
 define_constant(const char *name, struct value *value, unsigned flags,
diff --git a/tests/Makefile.am b/tests/Makefile.am
index dcea96dc..e6655304 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -44,6 +44,7 @@ TESTSUITE_AT = \
   ack.at\
   alias.at\
   arg.at\
+  arginit.at\
   ashadow.at\
   bctx00.at\
   bctx01.at\
diff --git a/tests/arginit.at b/tests/arginit.at
new file mode 100644
index 00000000..212ce62c
--- /dev/null
+++ b/tests/arginit.at
@@ -0,0 +1,56 @@
+# This file is part of Mailfromd testsuite. -*- Autotest -*-
+# Copyright (C) 2021 Sergey Poznyakoff
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3, or (at your option)
+# any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+AT_SETUP([Use of uninitialized variables])
+AT_KEYWORDS([arginit variable variables])
+MF_RUN_TEXT([
+func x()
+do
+  string result
+  set y result
+done
+
+func main(...) returns number
+do
+  x()
+  x()
+done
+],
+[0],
+[],
+[],
+[mailfromd: prog:5.9-14: warning: use of uninitialized variable 'result'
+])
+
+MF_RUN_TEXT([
+func x()
+do
+  string result
+  set y "result is %result"
+done
+
+func main(...) returns number
+do
+  x()
+  x()
+done
+],
+[0],
+[],
+[],
+[mailfromd: prog:5.20-26: warning: use of uninitialized variable 'result'
+])
+
+AT_CLEANUP
diff --git a/tests/testsuite.at b/tests/testsuite.at
index 19532d08..3400738a 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -189,6 +189,7 @@ m4_include([ml.at])
 m4_include([ml01.at])
 
 m4_include([declvar.at])
+m4_include([arginit.at])
 
 AT_BANNER([Macros])
 m4_include([macros.at])