Closes #1837

destroy_async_data() assumes that if the flag "done" is not set yet, the
thread itself will clean up once the request is complete.  But if an
error (generally OOM) occurs before the thread even has a chance to
start, it will never get a chance to clean up and memory will be leaked.
By clearing "done" only just before starting the thread, the correct
cleanup sequence will happen in all cases.

This used to be set in some configurations to EAI_MEMORY which is not a
valid value for errno and caused Curl_strerror to fail an assertion.

There is a mode in which libcurl is compiled with versioned symbols,
depending on the active SSL backend.

When multiple SSL backends are active, it does not make sense to favor
one over the others, so let's not: introduce a new prefix for the case
where multiple SSL backends are compiled into cURL.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Previously, we used as default SSL backend whatever was first in the
`available_backends` array.

However, some users may want to override that default without patching
the source code.

Now they can: with the --with-default-ssl-backend=<backend> option of
the ./configure script.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

When only one SSL backend is configured, it is totally unnecessary to
let multissl_init() configure the backend at runtime, we can select the
correct backend at build time already.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

To discern the active one from the inactive ones, put the latter into
parentheses.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This new feature flag reports When cURL was built with multiple SSL
backends.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Previously, the code assumed that at most one of the SSL backends would
be compiled in, emulating OpenSSL's functions if the configured backend
was not OpenSSL itself.

However, now we allow building with multiple SSL backends and choosing
one at runtime. Therefore, metalink needs to be adjusted to handle this
scenario, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The newly-introduced curl_global_sslset() function deserves to be
show-cased.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Let's add a compile time safe API to select an SSL backend. This
function needs to be called *before* curl_global_init(), and can be
called only once.

Side note: we do not explicitly test that it is called before
curl_global_init(), but we do verify that it is not called multiple times
(even implicitly).

If SSL is used before the function was called, it will use whatever the
CURL_SSL_BACKEND environment variable says (or default to the first
available SSL backend), and if a subsequent call to
curl_global_sslset() disagrees with the previous choice, it will fail
with CURLSSLSET_TOO_LATE.

The function also accepts an "avail" parameter to point to a (read-only)
NULL-terminated list of available backends. This comes in real handy if
an application wants to let the user choose between whatever SSL backends
the currently available libcurl has to offer: simply call

	curl_global_sslset(-1, NULL, &avail);

which will return CURLSSLSET_UNKNOWN_BACKEND and populate the avail
variable to point to the relevant information to present to the user.

Just like with the HTTP/2 push functions, we have to add the function
declaration of curl_global_sslset() function to the header file
*multi.h* because VMS and OS/400 require a stable order of functions
declared in include/curl/*.h (where the header files are sorted
alphabetically). This looks a bit funny, but it cannot be helped.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

There is information about the compiled-in SSL backends that is really
no concern of any code other than the SSL backend itself, such as which
function (if any) implements SHA-256 summing.

And there is information that is really interesting to the user, such as
the name, or the curl_sslbackend value.

Let's factor out the latter into a publicly visible struct. This
information will be used in the upcoming API to set the SSL backend
globally.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

When building software for the masses, it is sometimes not possible to
decide for all users which SSL backend is appropriate.

Git for Windows, for example,  uses cURL to perform clones, fetches and
pushes via HTTPS, and some users strongly prefer OpenSSL, while other
users really need to use Secure Channel because it offers
enterprise-ready tools to manage credentials via Windows' Credential
Store.

The current Git for Windows versions use the ugly work-around of
building libcurl once with OpenSSL support and once with Secure Channel
support, and switching out the binaries in the installer depending on
the user's choice.

Needless to say, this is a super ugly workaround that actually only
works in some cases: Git for Windows also comes in a portable form, and
in a form intended for third-party applications requiring Git
functionality, in which cases this "swap out libcurl-4.dll" simply is
not an option.

Therefore, the Git for Windows project has a vested interest in teaching
cURL to make the SSL backend a *runtime* option.

This patch makes that possible.

By running ./configure with multiple --with-<backend> options, cURL will
be built with multiple backends.

For the moment, the backend can be configured using the environment
variable CURL_SSL_BACKEND (valid values are e.g. "openssl" and
"schannel").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

So far, all of the SSL backends' private data has been declared as
part of the ssl_connect_data struct, in one big #if .. #elif .. #endif
block.

This can only work as long as the SSL backend is a compile-time option,
something we want to change in the next commits.

Therefore, let's encapsulate the exact data needed by each SSL backend
into a private struct, and let's avoid bleeding any SSL backend-specific
information into urldata.h. This is also necessary to allow multiple SSL
backends to be compiled in at the same time, as e.g. OpenSSL's and
CyaSSL's headers cannot be included in the same .c file.

To avoid too many malloc() calls, we simply append the private structs
to the connectdata struct in allocate_conn().

This requires us to take extra care of alignment issues: struct fields
often need to be aligned on certain boundaries e.g. 32-bit values need to
be stored at addresses that divide evenly by 4 (= 32 bit / 8
bit-per-byte).

We do that by assuming that no SSL backend's private data contains any
fields that need to be aligned on boundaries larger than `long long`
(typically 64-bit) would need. Under this assumption, we simply add a
dummy field of type `long long` to the `struct connectdata` struct. This
field will never be accessed but acts as a placeholder for the four
instances of ssl_backend_data instead. the size of each ssl_backend_data
struct is stored in the SSL backend-specific metadata, to allow
allocate_conn() to know how much extra space to allocate, and how to
initialize the ssl[sockindex]->backend and proxy_ssl[sockindex]->backend
pointers.

This would appear to be a little complicated at first, but is really
necessary to encapsulate the private data of each SSL backend correctly.
And we need to encapsulate thusly if we ever want to allow selecting
CyaSSL and OpenSSL at runtime, as their headers cannot be included within
the same .c file (there are just too many conflicting definitions and
declarations for that).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

At the moment, cURL's SSL backend needs to be configured at build time.
As such, it is totally okay for them to hard-code their backend-specific
data in the ssl_connect_data struct.

In preparation for making the SSL backend a runtime option, let's make
the access of said private data a bit more abstract so that it can be
adjusted later in an easy manner.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

In 86b88948 (sasl_gssapi: Added GSS-API based Kerberos V5 variables,
2014-12-03), an SSPI-specific field was added to the kerberos5data
struct without moving the #include "curl_sspi.h" later in the same file.

This broke the build when SSPI was enabled, unless Secure Channel was
used as SSL backend, because it just so happens that Secure Channel also
requires "curl_sspi.h" to be #included.

In f4739f63

 (urldata: include curl_sspi.h when Windows SSPI is enabled,
2017-02-21), this bug was fixed incorrectly: Instead of moving the
appropriate conditional #include, the Secure Channel-conditional part
was now also SSPI-conditional.

Fix this problem by moving the correct #include instead.

This is also required for an upcoming patch that moves all the Secure
Channel-specific stuff out of urldata.h and encapsulates it properly in
vtls/schannel.c instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Since 5017d5ad

 (polarssl: now require 1.3.0+, 2014-03-17), we require
a newer PolarSSL version. No need to keep code trying to support any
older version.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

In the ongoing endeavor to abstract out all SSL backend-specific
functionality, this is the next step: Instead of hard-coding how the
different SSL backends access their internal data in getinfo.c, let's
implement backend-specific functions to do that task.

This will also allow for switching SSL backends as a runtime option.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

There are convenient no-op versions of the init/cleanup functions now,
no need to define private ones for axTLS.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

These functions are all available via the Curl_ssl struct now, no need
to declare them separately anymore.

As the global declarations are removed, the corresponding function
definitions are marked as file-local. The only two exceptions here are
Curl_mbedtls_shutdown() and Curl_polarssl_shutdown(): only the
declarations were removed, there are no function definitions to mark
file-local.

Please note that Curl_nss_force_init() is *still* declared globally, as
the only SSL backend-specific function, because it was introduced
specifically for the use case where cURL was compiled with
`--without-ssl --with-nss`. For details, see f3b77e56

 (http_ntlm: add
support for NSS, 2010-06-27).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The _shutdown() function calls the _session_free() function; While this
is not a problem now (because schannel.h declares both functions), a
patch looming in the immediate future with make all of these functions
file-local.

So let's just move the _session_free() function's definition before it
is called.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The connect_finish() function (like many other functions after it) calls
the Curl_axtls_close() function; While this is not a problem now
(because axtls.h declares the latter function), a patch looming in the
immediate future with make all of these functions file-local.

So let's just move the Curl_axtls_close() function's definition before
it is called.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

That will allow us to choose the SSL backend at runtime.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The entire idea of introducing the Curl_ssl struct to describe SSL
backends is to prepare for choosing the SSL backend at runtime.

To that end, convert all the #ifdef have_curlssl_* style conditionals
to use bit flags instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The SHA-256 checksumming is also an SSL backend-specific function.
Let's include it in the struct declaring the functionality of SSL
backends.

In contrast to MD5, there is no fall-back code. To indicate this, the
respective entries are NULL for those backends that offer no support for
SHA-256 checksumming.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The MD5 summing is also an SSL backend-specific function. So let's
include it, offering the previous fall-back code as a separate function
now: Curl_none_md5sum(). To allow for that, the signature had to be
changed so that an error could be returned from the implementation
(Curl_none_md5sum() can run out of memory).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This is the first step to unify the SSL backend handling. Now all the
SSL backend-specific functionality is accessed via a global instance of
the Curl_ssl struct.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The idea of introducing the Curl_ssl struct was to unify how the SSL
backends are declared and called. To this end, we now provide an
instance of the Curl_ssl struct for each and every SSL backend.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This new struct is similar in nature to Curl_handler: it will define the
functions and capabilities of all the SSL backends (where Curl_handler
defines the functions and capabilities of protocol handlers).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This patch makes the signature of the _sha256sum() functions consistent
among the SSL backends, in preparation for unifying the way all SSL
backends are accessed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This patch makes the signature of the _data_pending() functions
consistent among the SSL backends, in preparation for unifying the way
all SSL backends are accessed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This patch makes the signature of the _cleanup() functions consistent
among the SSL backends, in preparation for unifying the way all SSL
backends are accessed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>