vtls: allow selecting which SSL backend to use at runtime

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>

1 files changed, 50 insertions, 27 deletions

diff --git a/configure.ac b/configure.ac
index 1b3f82184..a84974b7b 100755
--- a/configure.ac
+++ b/configure.ac
@@ -165,7 +165,7 @@ curl_verbose_msg="enabled (--disable-verbose)"
   curl_mtlnk_msg="no      (--with-libmetalink)"
     curl_psl_msg="no      (--with-libpsl)"
 
-    init_ssl_msg=${curl_ssl_msg}
+    ssl_backends=
 
 dnl
 dnl Save some initial values the user might have provided
@@ -1345,13 +1345,14 @@ AC_HELP_STRING([--without-winssl], [disable Windows native SSL/TLS]),
   OPT_WINSSL=$withval)
 
 AC_MSG_CHECKING([whether to enable Windows native SSL/TLS (Windows native builds only)])
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_WINSSL" != xno; then
+  ssl_msg=
   if test "x$OPT_WINSSL" != "xno"  &&
      test "x$curl_cv_native_windows" = "xyes"; then
     AC_MSG_RESULT(yes)
     AC_DEFINE(USE_SCHANNEL, 1, [to enable Windows native SSL/TLS support])
     AC_SUBST(USE_SCHANNEL, [1])
-    curl_ssl_msg="enabled (Windows-native)"
+    ssl_msg="Windows-native"
     WINSSL_ENABLED=1
     # --with-winssl implies --enable-sspi
     AC_DEFINE(USE_WINDOWS_SSPI, 1, [to enable SSPI support])
@@ -1361,6 +1362,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
   else
     AC_MSG_RESULT(no)
   fi
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 else
   AC_MSG_RESULT(no)
 fi
@@ -1372,18 +1374,19 @@ AC_HELP_STRING([--without-darwinssl], [disable Apple OS native SSL/TLS]),
   OPT_DARWINSSL=$withval)
 
 AC_MSG_CHECKING([whether to enable Apple OS native SSL/TLS])
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_DARWINSSL" != xno; then
   if test "x$OPT_DARWINSSL" != "xno" &&
      test -d "/System/Library/Frameworks/Security.framework"; then
     AC_MSG_RESULT(yes)
     AC_DEFINE(USE_DARWINSSL, 1, [to enable Apple OS native SSL/TLS support])
     AC_SUBST(USE_DARWINSSL, [1])
-    curl_ssl_msg="enabled (Apple OS-native)"
+    ssl_msg="$ssh_backends, Apple OS-native"
     DARWINSSL_ENABLED=1
     LDFLAGS="$LDFLAGS -framework CoreFoundation -framework Security"
   else
     AC_MSG_RESULT(no)
   fi
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 else
   AC_MSG_RESULT(no)
 fi
@@ -1401,7 +1404,10 @@ AC_HELP_STRING([--with-ssl=PATH],[Where to look for OpenSSL, PATH points to the
 AC_HELP_STRING([--without-ssl], [disable OpenSSL]),
   OPT_SSL=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg" && test X"$OPT_SSL" != Xno; then
+if test -z "$ssl_backends" -o "x$OPT_SSL" != xno &&
+   test X"$OPT_SSL" != Xno; then
+  ssl_msg=
+
   dnl backup the pre-ssl variables
   CLEANLDFLAGS="$LDFLAGS"
   CLEANCPPFLAGS="$CPPFLAGS"
@@ -1582,7 +1588,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg" && test X"$OPT_SSL" != Xno; then
       dnl Have the libraries--check for OpenSSL headers
       AC_CHECK_HEADERS(openssl/x509.h openssl/rsa.h openssl/crypto.h \
                        openssl/pem.h openssl/ssl.h openssl/err.h,
-        curl_ssl_msg="enabled (OpenSSL)"
+        ssl_msg="OpenSSL"
         OPENSSL_ENABLED=1
         AC_DEFINE(USE_OPENSSL, 1, [if OpenSSL is in use]))
 
@@ -1596,7 +1602,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg" && test X"$OPT_SSL" != Xno; then
            test $ac_cv_header_crypto_h = yes &&
            test $ac_cv_header_ssl_h = yes; then
           dnl three matches
-          curl_ssl_msg="enabled (OpenSSL)"
+          ssl_msg="OpenSSL"
           OPENSSL_ENABLED=1
         fi
       fi
@@ -1647,7 +1653,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg" && test X"$OPT_SSL" != Xno; then
         AC_MSG_RESULT([yes])
         AC_DEFINE_UNQUOTED(HAVE_BORINGSSL, 1,
                            [Define to 1 if using BoringSSL.])
-        curl_ssl_msg="enabled (BoringSSL)"
+        ssl_msg="BoringSSL"
     ],[
         AC_MSG_RESULT([no])
     ])
@@ -1663,7 +1669,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg" && test X"$OPT_SSL" != Xno; then
       AC_MSG_RESULT([yes])
       AC_DEFINE_UNQUOTED(HAVE_LIBRESSL, 1,
         [Define to 1 if using libressl.])
-      curl_ssl_msg="enabled (libressl)"
+      ssl_msg="libressl"
     ],[
       AC_MSG_RESULT([no])
     ])
@@ -1683,6 +1689,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg" && test X"$OPT_SSL" != Xno; then
     CURL_CHECK_OPENSSL_API
   fi
 
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
 dnl **********************************************************************
@@ -1744,7 +1751,8 @@ AC_HELP_STRING([--with-gnutls=PATH],[where to look for GnuTLS, PATH points to th
 AC_HELP_STRING([--without-gnutls], [disable GnuTLS detection]),
   OPT_GNUTLS=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_GNUTLS" != xno; then
+  ssl_msg=
 
   if test X"$OPT_GNUTLS" != Xno; then
 
@@ -1818,7 +1826,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
        AC_SUBST(USE_GNUTLS, [1])
        GNUTLS_ENABLED=1
        USE_GNUTLS="yes"
-       curl_ssl_msg="enabled (GnuTLS)"
+       ssl_msg="GnuTLS"
        ],
        [
          LIBS="$CLEANLIBS"
@@ -1846,6 +1854,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
 
   fi dnl GNUTLS not disabled
 
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
 dnl ---
@@ -1903,7 +1912,8 @@ AC_HELP_STRING([--with-polarssl=PATH],[where to look for PolarSSL, PATH points t
 AC_HELP_STRING([--without-polarssl], [disable PolarSSL detection]),
   OPT_POLARSSL=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_POLARSSL" != xno; then
+  ssl_msg=
 
   if test X"$OPT_POLARSSL" != Xno; then
 
@@ -1921,7 +1931,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
          AC_SUBST(USE_POLARSSL, [1])
          POLARSSL_ENABLED=1
          USE_POLARSSL="yes"
-         curl_ssl_msg="enabled (PolarSSL)"
+         ssl_msg="PolarSSL"
         ])
     fi
 
@@ -1947,7 +1957,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
        AC_SUBST(USE_POLARSSL, [1])
        POLARSSL_ENABLED=1
        USE_POLARSSL="yes"
-       curl_ssl_msg="enabled (PolarSSL)"
+       ssl_msg="PolarSSL"
        ],
        [
          CPPFLAGS=$_cppflags
@@ -1975,6 +1985,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
 
   fi dnl PolarSSL not disabled
 
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
 dnl ----------------------------------------------------
@@ -1990,7 +2001,8 @@ AC_HELP_STRING([--with-mbedtls=PATH],[where to look for mbedTLS, PATH points to
 AC_HELP_STRING([--without-mbedtls], [disable mbedTLS detection]),
   OPT_MBEDTLS=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_MBEDTLS" != xno; then
+  ssl_msg=
 
   if test X"$OPT_MBEDTLS" != Xno; then
 
@@ -2008,7 +2020,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
          AC_SUBST(USE_MBEDTLS, [1])
          MBEDTLS_ENABLED=1
          USE_MBEDTLS="yes"
-         curl_ssl_msg="enabled (mbedTLS)"
+         ssl_msg="mbedTLS"
         ], [], -lmbedx509 -lmbedcrypto)
     fi
 
@@ -2034,7 +2046,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
        AC_SUBST(USE_MBEDTLS, [1])
        MBEDTLS_ENABLED=1
        USE_MBEDTLS="yes"
-       curl_ssl_msg="enabled (mbedTLS)"
+       ssl_msg="mbedTLS"
        ],
        [
          CPPFLAGS=$_cppflags
@@ -2062,6 +2074,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
 
   fi dnl mbedTLS not disabled
 
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
 dnl ----------------------------------------------------
@@ -2078,7 +2091,8 @@ AC_HELP_STRING([--with-cyassl=PATH],[where to look for CyaSSL, PATH points to th
 AC_HELP_STRING([--without-cyassl], [disable CyaSSL detection]),
   OPT_CYASSL=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_CYASSL" != xno; then
+  ssl_msg=
 
   if test X"$OPT_CYASSL" != Xno; then
 
@@ -2100,7 +2114,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
          AC_SUBST(USE_CYASSL, [1])
          CYASSL_ENABLED=1
          USE_CYASSL="yes"
-         curl_ssl_msg="enabled (CyaSSL)"
+         ssl_msg="CyaSSL"
         ])
     fi
 
@@ -2126,7 +2140,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
        AC_SUBST(USE_CYASSL, [1])
        CYASSL_ENABLED=1
        USE_CYASSL="yes"
-       curl_ssl_msg="enabled (CyaSSL)"
+       ssl_msg="CyaSSL"
        ],
        [
          CPPFLAGS=$_cppflags
@@ -2171,7 +2185,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
          AC_SUBST(USE_CYASSL, [1])
          CYASSL_ENABLED=1
          USE_CYASSL="yes"
-         curl_ssl_msg="enabled (WolfSSL)"
+         ssl_msg="WolfSSL"
        ],
        [
          AC_MSG_RESULT(no)
@@ -2225,6 +2239,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
 
   fi dnl CyaSSL not disabled
 
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
 dnl ----------------------------------------------------
@@ -2239,7 +2254,8 @@ AC_HELP_STRING([--with-nss=PATH],[where to look for NSS, PATH points to the inst
 AC_HELP_STRING([--without-nss], [disable NSS detection]),
   OPT_NSS=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_NSS" != xno; then
+  ssl_msg=
 
   if test X"$OPT_NSS" != Xno; then
 
@@ -2314,7 +2330,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
      AC_SUBST(USE_NSS, [1])
      USE_NSS="yes"
      NSS_ENABLED=1
-     curl_ssl_msg="enabled (NSS)"
+     ssl_msg="NSS"
      ],
      [
        LDFLAGS="$CLEANLDFLAGS"
@@ -2343,7 +2359,8 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
 
   fi dnl NSS not disabled
 
-fi dnl curl_ssl_msg = init_ssl_msg
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
+fi
 
 OPT_AXTLS=off
 
@@ -2352,7 +2369,8 @@ AC_HELP_STRING([--with-axtls=PATH],[Where to look for axTLS, PATH points to the
 AC_HELP_STRING([--without-axtls], [disable axTLS]),
   OPT_AXTLS=$withval)
 
-if test "$curl_ssl_msg" = "$init_ssl_msg"; then
+if test -z "$ssl_backends" -o "x$OPT_AXTLS" != xno; then
+  ssl_msg=
   if test X"$OPT_AXTLS" != Xno; then
     dnl backup the pre-axtls variables
     CLEANLDFLAGS="$LDFLAGS"
@@ -2386,7 +2404,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
       AC_SUBST(USE_AXTLS, [1])
       AXTLS_ENABLED=1
       USE_AXTLS="yes"
-      curl_ssl_msg="enabled (axTLS)"
+      ssl_msg="axTLS"
 
       if test "x$cross_compiling" != "xyes"; then
         LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$LIB_AXTLS"
@@ -2399,6 +2417,7 @@ if test "$curl_ssl_msg" = "$init_ssl_msg"; then
       LIBS="$CLEANLIBS"
     ])
   fi
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
 if test "x$OPENSSL_ENABLED$GNUTLS_ENABLED$NSS_ENABLED$POLARSSL_ENABLED$MBEDTLS_ENABLED$AXTLS_ENABLED$CYASSL_ENABLED$WINSSL_ENABLED$DARWINSSL_ENABLED" = "x"; then
@@ -2410,6 +2429,10 @@ else
   SSL_ENABLED="1"
 fi
 
+if test -n "$ssl_backends"; then
+  curl_ssl_msg="enabled ($ssl_backends)"
+fi
+
 dnl **********************************************************************
 dnl Check for the CA bundle
 dnl **********************************************************************