lib/utils: add unit tests for async-signal-safe execvpe()

Don't try to test async-signal-safety, but strive to exercise as many as
possible paths through nbd_internal_execvpe_init() and
nbd_internal_fork_safe_execvpe().

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20230319094139.285312-3-lersek@redhat.com>

Author: Laszlo Ersek

.gitignore

@@ -130,6 +130,7 @@ Makefile.in
 /lib/states.h
 /lib/test-fork-safe-assert
 /lib/test-fork-safe-assert.err
+/lib/test-fork-safe-execvpe
 /lib/unlocked.h
 /libtool
 /ltmain.sh

lib/Makefile.am

@@ -33,6 +33,7 @@ EXTRA_DIST = \
 	$(generator_built) \
 	libnbd.syms \
 	test-fork-safe-assert.sh \
+	test-fork-safe-execvpe.sh \
 	$(NULL)
 
 lib_LTLIBRARIES = libnbd.la
@@ -105,10 +106,12 @@ pkgconfig_DATA = libnbd.pc
 
 TESTS = \
 	test-fork-safe-assert.sh \
+	test-fork-safe-execvpe.sh \
 	$(NULL)
 
 check_PROGRAMS = \
 	test-fork-safe-assert \
+	test-fork-safe-execvpe \
 	$(NULL)
 
 test_fork_safe_assert_SOURCES = \
@@ -118,3 +121,11 @@ test_fork_safe_assert_SOURCES = \
 	utils.c \
 	$(NULL)
 test_fork_safe_assert_LDADD = $(PTHREAD_LIBS)
+
+test_fork_safe_execvpe_SOURCES = \
+	$(top_srcdir)/common/utils/vector.c \
+	errors.c \
+	test-fork-safe-execvpe.c \
+	utils.c \
+	$(NULL)
+test_fork_safe_execvpe_LDADD = $(PTHREAD_LIBS)

lib/test-fork-safe-execvpe.c

@@ -0,0 +1,117 @@
+  /* nbd client library in userspace
+ * Copyright (C) 2013-2023 Red Hat Inc.
+ *
+ * This 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 of the License, or (at your option) any later version.
+ *
+ * This 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 this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <config.h>
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "internal.h"
+
+extern char **environ;
+
+/* This is a perror() replacement that makes the error message more
+ * machine-readable, for a select few error numbers. Do not use it as a general
+ * error() replacement, only upon nbd_internal_execvpe_init() and
+ * nbd_internal_fork_safe_execvpe() failure.
+ */
+static void
+xperror (const char *s)
+{
+  const char *err;
+
+  if (s != NULL && *s != '\0')
+    (void)fprintf (stderr, "%s: ", s);
+
+  switch (errno) {
+  case EACCES:
+    err = "EACCES";
+    break;
+  case ELOOP:
+    err = "ELOOP";
+    break;
+  case ENOENT:
+    err = "ENOENT";
+    break;
+  case ENOTDIR:
+    err = "ENOTDIR";
+    break;
+  default:
+    err = strerror (errno);
+  }
+  (void)fprintf (stderr, "%s\n", err);
+}
+
+int
+main (int argc, char **argv)
+{
+  struct execvpe ctx;
+  const char *prog_file;
+  string_vector prog_argv;
+  size_t i;
+
+  if (argc < 3) {
+    fprintf (stderr, "%1$s: usage: %1$s program-to-exec argv0 ...\n", argv[0]);
+    return EXIT_FAILURE;
+  }
+
+  prog_file = argv[1];
+
+  /* For the argv of the program to execute, we need to drop our argv[0] (= our
+   * own name) and argv[1] (= the program we need to execute), and to tack on a
+   * terminating null pointer. Note that "argc" does not include the terminating
+   * NULL.
+   */
+  prog_argv = (string_vector)empty_vector;
+  if (string_vector_reserve (&prog_argv, argc - 2 + 1) == -1) {
+    perror ("string_vector_reserve");
+    return EXIT_FAILURE;
+  }
+
+  for (i = 2; i < argc; ++i)
+    (void)string_vector_append (&prog_argv, argv[i]);
+  (void)string_vector_append (&prog_argv, NULL);
+
+  if (nbd_internal_execvpe_init (&ctx, prog_file, prog_argv.len) == -1) {
+    xperror ("nbd_internal_execvpe_init");
+    goto reset_prog_argv;
+  }
+
+  /* Print out the generated candidates. */
+  for (i = 0; i < ctx.pathnames.len; ++i)
+    (void)fprintf (stdout, "%s\n", ctx.pathnames.ptr[i]);
+
+  if (fflush (stdout) == EOF) {
+    perror ("fflush");
+    goto uninit_execvpe;
+  }
+
+  (void)nbd_internal_fork_safe_execvpe (&ctx, &prog_argv, environ);
+  xperror ("nbd_internal_fork_safe_execvpe");
+  /* fall through */
+
+uninit_execvpe:
+  nbd_internal_execvpe_uninit (&ctx);
+
+reset_prog_argv:
+  string_vector_reset (&prog_argv);
+
+  return EXIT_FAILURE;
+}