FastCGI: Track request IDs during processing

When processing a FastCGI request, save its request ID in thread-local
storage and respond with the same request ID (rather than hard-coding
1).  This fixes FastCGI on picky web servers (nginx).  For more
background, see https://github.com/urweb/urweb/issues/57 and §3.3 of
the FastCGI specification.

Tested with nginx 1.10.2 and lighttpd 1.4.39.

Closes https://github.com/urweb/urweb/issues/57.

3 files changed, 119 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index 8ee3e795..0dda6d3b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -12,6 +12,7 @@ AC_PROG_LIBTOOL()
 AC_CONFIG_HEADERS([include/urweb/config.h])
 
 AX_PTHREAD([echo >/dev/null], [echo "Your C compiler does not support POSIX threads."; exit 1])
+AX_TLS([echo >/dev/null], [echo "Your C compiler does not support thread-local storage."; exit 1])
 
 AX_CHECK_OPENSSL([echo >/dev/null], [echo "You must install OpenSSL development files."; exit 1])
 
diff --git a/m4/ax_tls.m4 b/m4/ax_tls.m4
new file mode 100644
index 00000000..809b761a
--- /dev/null
+++ b/m4/ax_tls.m4
@@ -0,0 +1,74 @@
+# ===========================================================================
+#          http://www.gnu.org/software/autoconf-archive/ax_tls.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_TLS([action-if-found], [action-if-not-found])
+#
+# DESCRIPTION
+#
+#   Provides a test for the compiler support of thread local storage (TLS)
+#   extensions. Defines TLS if it is found. Currently knows about GCC/ICC
+#   and MSVC. I think SunPro uses the same as GCC, and Borland apparently
+#   supports either.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Alan Woodland <ajw05@aber.ac.uk>
+#   Copyright (c) 2010 Diego Elio Petteno` <flameeyes@gmail.com>
+#
+#   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 of the License, 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/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 11
+
+AC_DEFUN([AX_TLS], [
+  AC_MSG_CHECKING([for thread local storage (TLS) class])
+  AC_CACHE_VAL([ac_cv_tls],
+   [for ax_tls_keyword in __thread '__declspec(thread)' none; do
+       AS_CASE([$ax_tls_keyword],
+          [none], [ac_cv_tls=none ; break],
+          [AC_TRY_COMPILE(
+              [#include <stdlib.h>
+               static void
+               foo(void) {
+               static ] $ax_tls_keyword [ int bar;
+               exit(1);
+               }],
+               [],
+               [ac_cv_tls=$ax_tls_keyword ; break],
+               ac_cv_tls=none
+           )])
+    done
+  ])
+  AC_MSG_RESULT([$ac_cv_tls])
+
+  AS_IF([test "$ac_cv_tls" != "none"],
+    [AC_DEFINE_UNQUOTED([TLS],[$ac_cv_tls],[If the compiler supports a TLS storage class define it to that here])
+     m4_ifnblank([$1],[$1])],
+    [m4_ifnblank([$2],[$2])])
+])
diff --git a/src/c/fastcgi.c b/src/c/fastcgi.c
index cda3e1f6..c37debf7 100644
--- a/src/c/fastcgi.c
+++ b/src/c/fastcgi.c
@@ -1,5 +1,7 @@
 #include "config.h"
 
+#include <assert.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
@@ -20,6 +22,8 @@
 
 #include "fastcgi.h"
 
+#define THREAD_LOCAL __thread
+
 extern uw_app uw_application;
 
 typedef struct {
@@ -44,6 +48,21 @@ typedef struct {
   int available, used, sock;
 } FCGI_Input;
 
+// The FastCGI request ID corresponding to the request being handled by the
+// current worker thread.  (Each worker thread can only handle one request at a
+// time.)
+static THREAD_LOCAL int current_request_id;
+
+// Reads the FastCGI request ID from a FastCGI record.  The result is guaranteed
+// to be in the range [0, 2^16); this function returns an int to avoid C type
+// promotion insanity.
+static int fastcgi_request_id(const FCGI_Record* const r) {
+  const int requestid = r->requestIdB1 << 8 | r->requestIdB0;
+  assert(requestid >= 0);
+  assert(requestid <= UINT16_MAX);
+  return requestid;
+}
+
 static FCGI_Output *fastcgi_output() {
   FCGI_Output *o = malloc(sizeof(FCGI_Output));
 
@@ -70,7 +89,9 @@ static int fastcgi_send(FCGI_Output *o,
                         unsigned char type,
                         unsigned short contentLength) {
   o->r.type = type;
-  o->r.requestIdB1 = o->r.requestIdB0 = 0;
+  assert(current_request_id <= UINT16_MAX);
+  o->r.requestIdB1 = current_request_id >> 8;
+  o->r.requestIdB0 = current_request_id & 0x000000ff;
   o->r.contentLengthB1 = contentLength >> 8;
   o->r.contentLengthB0 = contentLength & 255;
   return uw_really_send(o->sock, &o->r, sizeof(o->r) - 65535 + contentLength);
@@ -356,6 +377,10 @@ static void *worker(void *data) {
       goto done;
     }
 
+    // Save the FastCGI request ID this worker is handling so that fastcgi_send
+    // can include it in its response.
+    current_request_id = fastcgi_request_id(r);
+
     if (r->type != FCGI_BEGIN_REQUEST) {
       write_stderr(out, "First message is not BEGIN_REQUEST\n");
       goto done;
@@ -373,6 +398,15 @@ static void *worker(void *data) {
         goto done;
       }
 
+      if (fastcgi_request_id(r) != current_request_id) {
+        write_stderr(out,
+                     "Ignoring environment variables for request %d (current"
+                     " request has id %d)\n",
+                     fastcgi_request_id(r),
+                     current_request_id);
+        continue;
+      }
+
       if (r->type != FCGI_PARAMS) {
         write_stderr(out, "Expected FCGI_PARAMS but got %d\n", r->type);
         goto done;
@@ -428,6 +462,15 @@ static void *worker(void *data) {
         goto done;
       }
 
+      if (fastcgi_request_id(r) != current_request_id) {
+        write_stderr(out,
+                     "Ignoring STDIN for request %d (current request has id"
+                     " %d)\n",
+                     fastcgi_request_id(r),
+                     current_request_id);
+        continue;
+      }
+
       if (r->type != FCGI_STDIN) {
         write_stderr(out, "Expected FCGI_STDIN but got %d\n", r->type);
         goto done;