trace: update the documentation

=head1 NAME

OSSL_trace_enabled, OSSL_trace_begin, OSSL_trace_end

OSSL_trace_enabled, OSSL_trace_begin, OSSL_trace_end,

OSSL_TRACE_BEGIN, OSSL_TRACE_END, OSSL_TRACE1, OSSL_TRACE2, OSSL_TRACE9

- OpenSSL Tracing API

=head1 SYNOPSIS

 BIO *OSSL_trace_begin(int category);

 void OSSL_trace_end(int category, BIO *channel);

 /* trace group macros */

 OSSL_TRACE_BEGIN(category) {

    ...

 } OSSL_TRACE_END(category);

 /* one-shot trace macros */

 OSSL_TRACE1(category, format, arg1)

 OSSL_TRACE2(category, format, arg1, arg2)

 ...

 OSSL_TRACE9(category, format, arg1, ..., arg9)

=head1 DESCRIPTION

The functions described here are mainly interesting for those who provide

The fallback type C<OSSL_TRACE_CATEGORY_ANY> should I<not> be used

with the functions described here.

Tracing for a specific category is enabled if a so called

I<trace channel> is attached to it. A trace channel is simply a

BIO object to which the application can write its trace output.

The application has two different ways of registering a trace channel,

either by directly providing a BIO object using OSSL_trace_set_channel(),

or by providing a callback routine using OSSL_trace_set_callback().

The latter is wrapped internally by a dedicated BIO object, so for the

tracing code both channel types are effectively indistinguishable.

We call them a I<simple trace channel> and a I<callback trace channel>,

respectively.

To produce trace output, it is necessary to obtain a pointer to the

trace channel (i.e., the BIO object) using OSSL_trace_begin(), write

to it using arbitrary BIO output routines, and finally releases the

channel using OSSL_trace_end(). The OSSL_trace_begin()/OSSL_trace_end()

calls surrounding the trace output create a group, which acts as a

critical section (guarded by a mutex) to ensure that the trace output

of different threads does not get mixed up.

The tracing code normally does not call OSSL_trace_{begin,end}() directly,

but rather uses a set of convenience macros, see the L</Macros> section below.

=head2 Functions

OSSL_trace_enabled() can be used to check if tracing for the given

The result of trying to produce tracing output outside of such

sections is undefined.

=head2 Convenience Macros

=head2 Macros

There are a number of convenience macros defined, to make tracing

easy and consistent.

 } OSSL_TRACE_END(TLS);

This will normally expands to:

This will normally expand to:

 do {

     BIO *trc_out = OSSL_trace_begin(OSSL_TRACE_CATEGORY_TLS);

     OSSL_trace_end(OSSL_TRACE_CATEGORY_TLS, trc_out);

 } while (0);

C<OSSL_TRACE1()>, ... C<OSSL_TRACE9()> are one-shot macros which essentially wrap

a single BIO_printf() into a tracing group.

The call OSSL_TRACEn(category, format, arg1, ..., argN) expands to:

  OSSL_TRACE_BEGIN(category) {

    BIO_printf(trc_out, format, arg1, ..., argN)

  } OSSL_TRACE_END(category)

=head1 NOTES

It is advisable to always check that a trace type is enabled with

=item *

the convenience macros are defined to produce dead code.

For example, take this example from L</Convenience Macros> above:

For example, take this example from L</Macros> section above:

 OSSL_TRACE_BEGIN(TLS) {

The trace output is divided into categories which can be

enabled individually.

They are enabled by giving them a channel in form of a BIO, or a

tracer callback, which is responsible for performing the actual

output.

Every category can be enabled individually by attaching a so called

I<trace channel> to it, which in the simplest case is just a BIO object

to which the application can write the tracing output for this category.

Alternatively, the application can provide a tracer callback in order to

get more finegrained trace information. This callback will be wrapped

internally by a dedicated BIO object.

For the tracing code, both trace channel types are indistinguishable.

These are called a I<simple trace channel> and a I<callback trace channel>,

respectively.

=head2 Functions

OSSL_trace_set_channel() is used to enable the given trace C<category>

by giving it the B<BIO> C<bio>.

by attaching the B<BIO> C<bio> object as (simple) trace channel.

OSSL_trace_set_prefix() and OSSL_trace_set_suffix() can be used to add

an extra line for each channel, to be output before and after group of

OSSL_trace_set_callback() is used to enable the given trace

C<category> by giving it the tracer callback C<cb> with the associated

data C<data>, which will simply be passed through to C<cb> whenever

it's called.

it's called. The callback function is internally wrapped by a

dedicated BIO object, the so called I<callback trace channel>.

This should be used when it's desirable to do form the trace output to

something suitable for application needs where a prefix and suffix

line aren't enough.

prefix that should be output at the beginning of each line, or

something other.

=item C<OSSL_TRACE_CTRL_DURING>

=item C<OSSL_TRACE_CTRL_WRITE>

The callback is called from any regular BIO output routine.

This callback is called whenever data is written to the BIO by some

regular BIO output routine.

An arbitrary number of C<OSSL_TRACE_CTRL_WRITE> callbacks can occur

inside a group marked by a pair of C<OSSL_TRACE_CTRL_BEGIN> and

C<OSSL_TRACE_CTRL_END> calls, but never outside such a group.

=item C<OSSL_TRACE_CTRL_END>

=head1 EXAMPLES

In all examples below, we assume that the trace producing code is

this:

In all examples below, the trace producing code is assumed to be

the following:

 int foo = 42;

 const char bar[] = { 0,  1,  2,  3,  4,  5,  6,  7,

ASYNC_STATUS_OK                         define

ASYNC_STATUS_ERR                        define

ASYNC_STATUS_UNSUPPORTED                define

OSSL_TRACE_BEGIN                        define

OSSL_TRACE_END                          define

OSSL_TRACE_CANCEL                       define

OSSL_TRACE1                             define

OSSL_TRACE2                             define

OSSL_TRACE9                             define