Add a new function child_path.

child_path returns a pointer to the first component in a child path
that comes after a parent path.  This does not depend on trying to
stat() the paths since they may describe remote paths but instead
relies on filename parsing.  The function requires that the child path
describe a filename that contains at least one component below the
parent path and returns a pointer to the first component.

gdb/ChangeLog:

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/child-path-selftests.c.
	* common/pathstuff.c (child_path): New function.
	* common/pathstuff.h (child_path): New prototype.
	* unittests/child-path-selftests.c: New file.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2019-02-12  John Baldwin  <jhb@FreeBSD.org>
+
+	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
+	unittests/child-path-selftests.c.
+	* common/pathstuff.c (child_path): New function.
+	* common/pathstuff.h (child_path): New prototype.
+	* unittests/child-path-selftests.c: New file.
+
 2019-02-12  John Baldwin  <jhb@FreeBSD.org>
 
 	* symfile.c (find_separate_debug_file): Look for separate debug

gdb/Makefile.in

@@ -411,6 +411,7 @@ SUBDIR_PYTHON_CFLAGS =
 
 SUBDIR_UNITTESTS_SRCS = \
 	unittests/array-view-selftests.c \
+	unittests/child-path-selftests.c \
 	unittests/cli-utils-selftests.c \
 	unittests/common-utils-selftests.c \
 	unittests/copy_bitwise-selftests.c \

gdb/common/pathstuff.c

@@ -147,6 +147,56 @@ gdb_abspath (const char *path)
 
 /* See common/pathstuff.h.  */
 
+const char *
+child_path (const char *parent, const char *child)
+{
+  /* The child path must start with the parent path.  */
+  size_t parent_len = strlen (parent);
+  if (filename_ncmp (parent, child, parent_len) != 0)
+    return NULL;
+
+  /* The parent path must be a directory and the child must contain at
+     least one component underneath the parent.  */
+  const char *child_component;
+  if (IS_DIR_SEPARATOR (parent[parent_len - 1]))
+    {
+      /* The parent path ends in a directory separator, so it is a
+	 directory.  The first child component starts after the common
+	 prefix.  */
+      child_component = child + parent_len;
+    }
+  else
+    {
+      /* The parent path does not end in a directory separator.  The
+	 first character in the child after the common prefix must be
+	 a directory separator.
+
+	 Note that CHILD must hold at least parent_len characters for
+	 filename_ncmp to return zero.  If the character at parent_len
+	 is nul due to CHILD containing the same path as PARENT, the
+	 IS_DIR_SEPARATOR check will fail here.  */
+      if (!IS_DIR_SEPARATOR (child[parent_len]))
+	return NULL;
+
+      /* The first child component starts after the separator after the
+	 common prefix.  */
+      child_component = child + parent_len + 1;
+    }
+
+  /* The child must contain at least one non-separator character after
+     the parent.  */
+  while (*child_component != '\0')
+    {
+      if (!IS_DIR_SEPARATOR (*child_component))
+	return child_component;
+
+      child_component++;
+    }
+  return NULL;
+}
+
+/* See common/pathstuff.h.  */
+
 bool
 contains_dir_separator (const char *path)
 {

gdb/common/pathstuff.h

@@ -48,6 +48,12 @@ extern gdb::unique_xmalloc_ptr<char>
 
 extern gdb::unique_xmalloc_ptr<char> gdb_abspath (const char *path);
 
+/* If the path in CHILD is a child of the path in PARENT, return a
+   pointer to the first component in the CHILD's pathname below the
+   PARENT.  Otherwise, return NULL.  */
+
+extern const char *child_path (const char *parent, const char *child);
+
 /* Return whether PATH contains a directory separator character.  */
 
 extern bool contains_dir_separator (const char *path);

gdb/unittests/child-path-selftests.c

@@ -0,0 +1,68 @@
+/* Self tests for child_path for GDB, the GNU debugger.
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+#include "defs.h"
+#include "common/pathstuff.h"
+#include "common/selftest.h"
+
+namespace selftests {
+namespace child_path {
+
+/* Verify the result of a single child_path test.  */
+
+static bool
+child_path_check (const char *parent, const char *child, const char *expected)
+{
+  const char *result = ::child_path (parent, child);
+  if (result == NULL || expected == NULL)
+    return result == expected;
+  return strcmp (result, expected) == 0;
+}
+
+/* Test child_path.  */
+
+static void
+test ()
+{
+  SELF_CHECK (child_path_check ("/one", "/two", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one/", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one//", NULL));
+  SELF_CHECK (child_path_check ("/one", "/one/two", "two"));
+  SELF_CHECK (child_path_check ("/one/", "/two", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one/", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one//", NULL));
+  SELF_CHECK (child_path_check ("/one/", "/one/two", "two"));
+  SELF_CHECK (child_path_check ("/one/", "/one//two", "two"));
+  SELF_CHECK (child_path_check ("/one/", "/one//two/", "two/"));
+  SELF_CHECK (child_path_check ("/one", "/onetwo", NULL));
+  SELF_CHECK (child_path_check ("/one", "/onetwo/three", NULL));
+}
+
+}
+}
+
+void
+_initialize_child_path_selftests ()
+{
+  selftests::register_test ("child_path",
+			    selftests::child_path::test);
+}
+