Add internal facility for dynamic array handling

This is intended as a type-safe alternative to obstacks and
hand-written realloc constructs.  The implementation avoids
writing function pointers to the heap.

support/support_capture_subprocess.c

@@ -0,0 +1,108 @@
+/* Capture output from a subprocess.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/capture_subprocess.h>
+
+#include <errno.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/xunistd.h>
+#include <support/xsocket.h>
+
+static void
+transfer (const char *what, struct pollfd *pfd, struct xmemstream *stream)
+{
+  if (pfd->revents != 0)
+    {
+      char buf[1024];
+      ssize_t ret = TEMP_FAILURE_RETRY (read (pfd->fd, buf, sizeof (buf)));
+      if (ret < 0)
+        {
+          support_record_failure ();
+          printf ("error: reading from subprocess %s: %m", what);
+          pfd->events = 0;
+          pfd->revents = 0;
+        }
+      else if (ret == 0)
+        {
+          /* EOF reached.  Stop listening.  */
+          pfd->events = 0;
+          pfd->revents = 0;
+        }
+      else
+        /* Store the data just read.   */
+        TEST_VERIFY (fwrite (buf, ret, 1, stream->out) == 1);
+    }
+}
+
+struct support_capture_subprocess
+support_capture_subprocess (void (*callback) (void *), void *closure)
+{
+  struct support_capture_subprocess result;
+  xopen_memstream (&result.out);
+  xopen_memstream (&result.err);
+
+  int stdout_pipe[2];
+  xpipe (stdout_pipe);
+  int stderr_pipe[2];
+  xpipe (stderr_pipe);
+
+  TEST_VERIFY (fflush (stdout) == 0);
+  TEST_VERIFY (fflush (stderr) == 0);
+
+  pid_t pid = xfork ();
+  if (pid == 0)
+    {
+      xclose (stdout_pipe[0]);
+      xclose (stderr_pipe[0]);
+      xdup2 (stdout_pipe[1], STDOUT_FILENO);
+      xdup2 (stderr_pipe[1], STDERR_FILENO);
+      callback (closure);
+      _exit (0);
+    }
+  xclose (stdout_pipe[1]);
+  xclose (stderr_pipe[1]);
+
+  struct pollfd fds[2] =
+    {
+      { .fd = stdout_pipe[0], .events = POLLIN },
+      { .fd = stderr_pipe[0], .events = POLLIN },
+    };
+
+  do
+    {
+      xpoll (fds, 2, -1);
+      transfer ("stdout", &fds[0], &result.out);
+      transfer ("stderr", &fds[1], &result.err);
+    }
+  while (fds[0].events != 0 || fds[1].events != 0);
+  xclose (stdout_pipe[0]);
+  xclose (stderr_pipe[0]);
+
+  xfclose_memstream (&result.out);
+  xfclose_memstream (&result.err);
+  xwaitpid (pid, &result.status, 0);
+  return result;
+}
+
+void
+support_capture_subprocess_free (struct support_capture_subprocess *p)
+{
+  free (p->out.buffer);
+  free (p->err.buffer);
+}