Replace va_list fmt() overload with vfmt()

Using an overload that takes a va_list argument potentially causes
accidental misuse on platforms (e.g. 32-bit) where va_list is
implemented as a type that may collide with commonly-used argument
types.

For example:

    char* c = copy_string("hi");
    fmt("%s", (const char*)c);
    fmt("%s", c);

The first fmt() call correctly goes through fmt(const char*, ...) first,
but the second mistakenly goes through fmt(const char*, va_list) first
because variadic function overloads have lower priority during overload
resolution and va_list on a 32-bit system happens to be defined as a
pointer type that can match with "char*" but not "const char*".

NEWS

@@ -15,6 +15,12 @@ Changed Functionality
 Removed Functionality
 ---------------------
 
+- The fmt() function which takes a va_list argument is replaced, use
+  the new vfmt() function for equivalent functionality.  The former is
+  deprecated because overloading it with the variadic fmt() function
+  can cause the unintended overload to be chosen depending on how the
+  platform implements va_list.
+
 Deprecated Functionality
 ------------------------
 

src/broker/Manager.cc

@@ -625,7 +625,7 @@ void Manager::Error(const char* format, ...)
 	{
 	va_list args;
 	va_start(args, format);
-	auto msg = fmt(format, args);
+	auto msg = vfmt(format, args);
 	va_end(args);
 
 	if ( script_scope )

src/supervisor/Supervisor.cc

@@ -724,7 +724,7 @@ void Stem::ReportStatus(const Supervisor::Node& node) const
 
 void Stem::Log(std::string_view type, const char* format, va_list args) const
 	{
-	auto raw_msg = fmt(format, args);
+	auto raw_msg = vfmt(format, args);
 
 	if ( getenv("ZEEK_DEBUG_STEM_STDERR") )
 		{

src/util.cc

@@ -817,7 +817,7 @@ const char* fmt_bytes(const char* data, int len)
 	return buf;
 	}
 
-const char* fmt(const char* format, va_list al)
+const char* vfmt(const char* format, va_list al)
 	{
 	static char* buf = 0;
 	static unsigned int buf_len = 1024;
@@ -848,7 +848,7 @@ const char* fmt(const char* format, ...)
 	{
 	va_list al;
 	va_start(al, format);
-	auto rval = fmt(format, al);
+	auto rval = vfmt(format, al);
 	va_end(al);
 	return rval;
 	}

src/util.h

@@ -188,7 +188,7 @@ extern std::string strtolower(const std::string& s);
 extern const char* fmt_bytes(const char* data, int len);
 
 // Note: returns a pointer into a shared buffer.
-extern const char* fmt(const char* format, va_list args);
+extern const char* vfmt(const char* format, va_list args);
 // Note: returns a pointer into a shared buffer.
 extern const char* fmt(const char* format, ...)
 	__attribute__((format (printf, 1, 2)));