diagnostics: add experimental SARIF JSON-RPC notifications for IDEs [PR115970]

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3358r0.html#msvc
describes a feature of Visual Studio 2022 version 17.8. which can send its
diagnostics in SARIF form to a pipe when setting the environment variable
SARIF_OUTPUT_PIPE:
  https://learn.microsoft.com/en-us/cpp/build/reference/sarif-output?view=msvc-170#retrieving-sarif-through-a-pipe

The precise mechanism above involves Windows-specific details (windows pipes
and HANDLEs).

The following patch implements an analogous feature for GCC, using Unix
domain sockets rather than the Windows-specific details.

With this patch, GCC's cc1, cc1plus, etc will check if
EXPERIMENTAL_SARIF_SOCKET is set in the environment, and if so,
will attempt to connect to that socket.  It will send a JSON-RPC
notification to the socket for every diagnostic emitted.  Like the
MSVC feature, the diagnostics are sent one-at-a-time as SARIF
"result" objects, rather than sending a full SARIF "log" object.

The patch includes a python test script which runs a server.
Tested by running the script in one terminal:

$ ../../src/contrib/sarif-listener.py
listening on socket: /tmp/tmpjgts0u0i/socket

and then invoking a build in another terminal with the envvar
set to the pertinent socket:

$ EXPERIMENTAL_SARIF_SOCKET=/tmp/tmpjgts0u0i/socket \
    make check-gcc RUNTESTFLAGS="analyzer.exp=*"

and watching as all the diagnostics generated during the build
get sent to the listener.

The idea is that an IDE ought to be able to create a socket and
set the environment variable when invoking a build, and then listen
for all the diagnostics, without needing to manually set build flags
to inject SARIF output.

This feature is experimental and subject to change or removal
without notice; I'm adding it to make it easier for IDE developers to
try it out and give feedback.

contrib/ChangeLog:
	PR diagnostics/115970
	* sarif-listener.py: New file.

gcc/ChangeLog:
	PR diagnostics/115970
	* diagnostics/sarif-sink.cc: Include <sys/un.h> and <sys/socket.h>.
	(sarif_builder::end_group): Update comment.
	(sarif_sink::on_end_group): Drop "final".
	(class sarif_socket_sink): New subclass.
	(maybe_open_sarif_sink_for_socket): New function.
	* diagnostics/sarif-sink.h: (maybe_open_sarif_sink_for_socket):
	New decl.
	* doc/invoke.texi (EXPERIMENTAL_SARIF_SOCKET): Document new
	environment variable.
	* toplev.cc: Define INCLUDE_VECTOR. Add include of
	"diagnostics/sarif-sink.h".
	(toplev::main): Call
	diagnostics::maybe_open_sarif_sink_for_socket.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

contrib/sarif-listener.py

@@ -0,0 +1,41 @@
+#!/usr/bin/env python3
+
+# Example listener for SARIF output to a socket.
+# Copyright (C) 2025 Free Software Foundation, Inc.
+# Contributed by David Malcolm <dmalcolm@redhat.com>.
+#
+# This file is part of GCC.
+#
+# GCC 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, or (at your option) any later
+# version.
+#
+# GCC 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 GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.  */
+
+from pathlib import Path
+from socketserver import UnixStreamServer, StreamRequestHandler, ThreadingMixIn
+import tempfile
+
+class MyHandler(StreamRequestHandler):
+    def handle(self):
+        notification = self.rfile.read()
+        print(notification)
+
+class ThreadedUnixStreamServer(ThreadingMixIn, UnixStreamServer):
+    pass
+
+if __name__ == '__main__':
+    with tempfile.TemporaryDirectory() as tmpdir:
+        socket_path = Path(tmpdir) / 'socket'
+        with ThreadedUnixStreamServer(socket_path.as_posix(),
+                                      MyHandler) as server:
+            print('listening on socket: %s' % socket_path)
+            server.serve_forever()

gcc/diagnostics/sarif-sink.cc

@@ -55,6 +55,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "backtrace.h"
 #include "xml.h"
 #include "intl.h"
+#include <sys/un.h>
+#include <sys/socket.h>
 
 namespace diagnostics {
 
@@ -1917,7 +1919,8 @@ sarif_builder::emit_diagram (const diagram &d)
   m_cur_group_result->on_diagram (d, *this);
 }
 
-/* Implementation of "end_group_cb" for SARIF output.  */
+/* Implementation of "end_group_cb" for SARIF output.
+   Append the current sarifResult to results, and set it to nullptr.  */
 
 void
 sarif_builder::end_group ()
@@ -4011,7 +4014,7 @@ public:
   {
     /* No-op,  */
   }
-  void on_end_group () final override
+  void on_end_group () override
   {
     m_builder.end_group ();
   }
@@ -4123,6 +4126,93 @@ private:
   output_file m_output_file;
 };
 
+class sarif_socket_sink : public sarif_sink
+{
+public:
+  sarif_socket_sink (context &dc,
+		     const line_maps *line_maps,
+		     std::unique_ptr<sarif_serialization_format> serialization_format,
+		     const sarif_generation_options &sarif_gen_opts,
+		     int fd)
+  : sarif_sink (dc, line_maps,
+		std::move (serialization_format),
+		sarif_gen_opts),
+    m_fd (fd)
+  {
+  }
+  ~sarif_socket_sink ()
+  {
+    close (m_fd);
+  }
+  void dump_kind (FILE *out) const override
+  {
+    fprintf (out, "sarif_socket_sink: fd=%i", m_fd);
+  }
+  bool machine_readable_stderr_p () const final override
+  {
+    return false;
+  }
+
+  /* Rather than appending it to the results array, instead
+     send it to the output socket as a JSON-RPC 2.0 notification.  */
+  void on_end_group () final override
+  {
+    // TODO: what about buffering?
+
+    std::unique_ptr<sarif_result> result = m_builder.take_current_result ();
+    if (!result)
+      return;
+
+    auto notification = std::make_unique<json::object> ();
+    notification->set_string ("jsonrpc", "2.0");
+    notification->set_string ("method", "OnSarifResult");
+    {
+      auto params = std::make_unique<json::object> ();
+      params->set ("result", std::move (result));
+      notification->set ("params", std::move (params));
+    }
+
+    send_rpc_notification (*notification);
+  }
+
+private:
+  void
+  send_rpc_notification (const json::object &notification)
+  {
+    DIAGNOSTICS_LOG_SCOPE_PRINTF0 (m_context.get_logger (),
+				   "sarif_socket_sink::send_rpc_notification");
+    const bool formatted = false;
+    pretty_printer pp_content;
+    notification.print (&pp_content, formatted);
+    size_t content_length = strlen (pp_formatted_text (&pp_content));
+
+    pretty_printer pp_header;
+#if __GNUC__ >= 10
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wformat-diag"
+#endif
+    pp_printf (&pp_header, "Content-Length: %li\n\n", content_length);
+#if __GNUC__ >= 10
+#  pragma GCC diagnostic pop
+#endif
+    size_t header_length = strlen (pp_formatted_text (&pp_header));
+
+    size_t output_size = header_length + content_length;
+    char *buf = (char *)xmalloc (output_size);
+    memcpy (buf, pp_formatted_text (&pp_header), header_length);
+    memcpy (buf + header_length,
+	    pp_formatted_text (&pp_content),
+	    content_length);
+
+    /* TODO: should we attempt to handle partial writes here?  */
+    write (m_fd, buf, output_size);
+
+    free (buf);
+  }
+
+  int m_fd;
+};
+
 /* Print the start of an embedded link to PP, as per 3.11.6.  */
 
 static void
@@ -4448,6 +4538,47 @@ sarif_generation_options::dump (FILE *outfile, int indent) const
   DIAGNOSTICS_DUMPING_EMIT_BOOL_FIELD (m_state_graph);
 }
 
+void
+maybe_open_sarif_sink_for_socket (context &dc)
+{
+  gcc_assert (line_table);
+
+  const char * const env_var_name = "EXPERIMENTAL_SARIF_SOCKET";
+  const char * const socket_name = getenv (env_var_name);
+  if (!socket_name)
+    return;
+
+  int sfd = socket (AF_UNIX, SOCK_STREAM, 0);
+  if (sfd == -1)
+    fatal_error (UNKNOWN_LOCATION,
+		 "unable to create socket");
+
+  struct sockaddr_un addr;
+  memset (&addr, 0, sizeof (addr));
+  addr.sun_family = AF_UNIX;
+  strncpy (addr.sun_path, socket_name, sizeof (addr.sun_path) - 1);
+
+  if (connect (sfd, (struct sockaddr *)&addr, sizeof (addr)) == -1)
+    fatal_error (UNKNOWN_LOCATION,
+		 "unable to connect to %qs",
+		 socket_name);
+
+  /* TODO: should there be a way to specify other key/value
+     pairs here?  (as per -fdiagnostics-add-output, but as an
+     environment variable, perhaps).  */
+  sarif_generation_options sarif_gen_opts;
+  sarif_gen_opts.m_version = sarif_version::v2_1_0;
+
+  auto sink_ = std::make_unique<sarif_socket_sink>
+    (dc,
+     line_table,
+     std::make_unique <sarif_serialization_format_json> (true),
+     sarif_gen_opts,
+     sfd);
+  sink_->update_printer ();
+  dc.add_sink (std::move (sink_));
+}
+
 #if CHECKING_P
 
 namespace selftest {

gcc/diagnostics/sarif-sink.h

@@ -188,6 +188,9 @@ extern std::unique_ptr<sarif_edge>
 make_sarif_edge (const digraphs::edge &e,
 		 sarif_builder *builder);
 
+extern void
+maybe_open_sarif_sink_for_socket (context &ctxt);
+
 } // namespace diagnostics
 
 #endif /* ! GCC_DIAGNOSTICS_SARIF_SINK_H */

gcc/doc/invoke.texi

@@ -38425,6 +38425,19 @@ As @code{fixits-v1}, but columns are expressed as display columns,
 as per @option{-fdiagnostics-column-unit=display}.
 @end table
 
+@vindex EXPERIMENTAL_SARIF_SOCKET
+@item EXPERIMENTAL_SARIF_SOCKET
+If @env{EXPERIMENTAL_SARIF_SOCKET} is set in the environment, then the
+compiler will attempt to connect to a UNIX domain stream socket with
+that name, and send an @code{OnSarifResult} JSON-RPC 2.0 notification to
+it for each diagnostic that occurs, where the value of the notification
+is a SARIF @code{result} object.
+
+The compiler will fail immediately if @env{EXPERIMENTAL_SARIF_SOCKET} is
+set and it cannot connect to it.
+
+This feature is experimental and subject to change or removal without
+notice.
 @end table
 
 @noindent

gcc/toplev.cc

@@ -22,6 +22,7 @@ along with GCC; see the file COPYING3.  If not see
    in the proper order, and counts the time used by each.
    Error messages and low-level interface to malloc also handled here.  */
 
+#define INCLUDE_VECTOR
 #include "config.h"
 #include "system.h"
 #include "coretypes.h"
@@ -83,6 +84,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "omp-offload.h"
 #include "diagnostics/changes.h"
 #include "diagnostics/file-cache.h"
+#include "diagnostics/sarif-sink.h"
 #include "tree-pass.h"
 #include "dumpfile.h"
 #include "ipa-fnsummary.h"
@@ -2352,6 +2354,8 @@ toplev::main (int argc, char **argv)
 
   handle_common_deferred_options ();
 
+  diagnostics::maybe_open_sarif_sink_for_socket (*global_dc);
+
   init_local_tick ();
 
   initialize_plugins ();