get_filename_component: Teach new BASE_DIR parameter.

In the get_filename_component command, add a new BASE_DIR parameter to
use with the ABSOLUTE and REALPATH options.  This will be used when
finding an absolute path from a relative path.

Help/command/get_filename_component.rst

@@ -3,6 +3,8 @@ get_filename_component
 
 Get a specific component of a full filename.
 
+------------------------------------------------------------------------------
+
 ::
 
   get_filename_component(<VAR> <FileName> <COMP> [CACHE])
@@ -15,8 +17,6 @@ Set ``<VAR>`` to a component of ``<FileName>``, where ``<COMP>`` is one of:
  NAME      = File name without directory
  EXT       = File name longest extension (.b.c from d/a.b.c)
  NAME_WE   = File name without directory or longest extension
- ABSOLUTE  = Full path to file
- REALPATH  = Full path to existing file with symlinks resolved
  PATH      = Legacy alias for DIRECTORY (use for CMake <= 2.8.11)
 
 Paths are returned with forward slashes and have no trailing slahes.
@@ -24,14 +24,41 @@ The longest file extension is always considered.  If the optional
 ``CACHE`` argument is specified, the result variable is added to the
 cache.
 
+------------------------------------------------------------------------------
+
+::
+
+  get_filename_component(<VAR> <FileName>
+                         <COMP> [BASE_DIR <BASE_DIR>]
+                         [CACHE])
+
+Set ``<VAR>`` to the absolute path of ``<FileName>``, where ``<COMP>`` is one
+of:
+
+::
+
+ ABSOLUTE  = Full path to file
+ REALPATH  = Full path to existing file with symlinks resolved
+
+If the provided ``<FileName>`` is a relative path, it is evaluated relative
+to the given base directory ``<BASE_DIR>``.  If no base directory is
+provided, the default base directory will be
+:variable:`CMAKE_CURRENT_SOURCE_DIR`.
+
+Paths are returned with forward slashes and have no trailing slahes.  If the
+optional ``CACHE`` argument is specified, the result variable is added to the
+cache.
+
+------------------------------------------------------------------------------
+
 ::
 
-  get_filename_component(<VAR> FileName
+  get_filename_component(<VAR> <FileName>
                          PROGRAM [PROGRAM_ARGS <ARG_VAR>]
                          [CACHE])
 
-The program in ``FileName`` will be found in the system search path or
+The program in ``<FileName>`` will be found in the system search path or
 left as a full path.  If ``PROGRAM_ARGS`` is present with ``PROGRAM``, then
-any command-line arguments present in the ``FileName`` string are split
+any command-line arguments present in the ``<FileName>`` string are split
 from the program name and stored in ``<ARG_VAR>``.  This is used to
 separate a program name from its arguments in a command line string.

Help/release/dev/get-filename-component-base-dir.rst

@@ -0,0 +1,6 @@
+get-filename-component-base-dir
+-------------------------------
+
+* The :command:`get_filename_component` command learned a new ``BASE_DIR``
+  subcommand.  This is used to specify a base directory when calculating an
+  absolute path from a relative path.

Source/cmGetFilenameComponentCommand.cxx

@@ -93,11 +93,23 @@ bool cmGetFilenameComponentCommand
   else if (args[2] == "ABSOLUTE" ||
            args[2] == "REALPATH")
     {
+    // If the path given is relative, evaluate it relative to the
+    // current source directory unless the user passes a different
+    // base directory.
+    std::string baseDir = this->Makefile->GetCurrentSourceDirectory();
+    for(unsigned int i=3; i < args.size(); ++i)
+      {
+      if(args[i] == "BASE_DIR")
+        {
+        ++i;
+        if(i < args.size())
+          {
+          baseDir = args[i];
+          }
+        }
+      }
     // Collapse the path to its simplest form.
-    // If the path given is relative evaluate it relative to the
-    // current source directory.
-    result = cmSystemTools::CollapseFullPath(
-      filename, this->Makefile->GetCurrentSourceDirectory());
+    result = cmSystemTools::CollapseFullPath(filename, baseDir);
     if(args[2] == "REALPATH")
       {
       // Resolve symlinks if possible

Tests/RunCMake/get_filename_component/KnownComponents.cmake

@@ -38,6 +38,39 @@ check("ABSOLUTE .. in windows root" "${test_absolute}" "c:/path/to/filename.ext.
 
 list(APPEND non_cache_vars test_absolute)
 
+# Test finding absolute paths from various base directories.
+
+get_filename_component(test_abs_base "testdir1" ABSOLUTE)
+check("ABSOLUTE .. from default base" "${test_abs_base}"
+      "${CMAKE_CURRENT_SOURCE_DIR}/testdir1")
+
+get_filename_component(test_abs_base "../testdir2" ABSOLUTE
+                       BASE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/dummydir")
+check("ABSOLUTE .. from dummy base to parent" "${test_abs_base}"
+      "${CMAKE_CURRENT_SOURCE_DIR}/testdir2")
+
+get_filename_component(test_abs_base "testdir3" ABSOLUTE
+                       BASE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/dummydir")
+check("ABSOLUTE .. from dummy base to child" "${test_abs_base}"
+      "${CMAKE_CURRENT_SOURCE_DIR}/dummydir/testdir3")
+
+list(APPEND non_cache_vars test_abs_base)
+
+# Test finding absolute paths with CACHE parameter.  (Note that more
+# rigorous testing of the CACHE parameter comes later with PROGRAM).
+
+get_filename_component(test_abs_base_1 "testdir4" ABSOLUTE CACHE)
+check("ABSOLUTE CACHE 1" "${test_abs_base_1}"
+      "${CMAKE_CURRENT_SOURCE_DIR}/testdir4")
+list(APPEND cache_vars test_abs_base_1)
+
+get_filename_component(test_abs_base_2 "testdir5" ABSOLUTE
+                       BASE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/dummydir"
+                       CACHE)
+check("ABSOLUTE CACHE 2" "${test_abs_base_2}"
+      "${CMAKE_CURRENT_SOURCE_DIR}/dummydir/testdir5")
+list(APPEND cache_vars test_abs_base_2)
+
 # Test the PROGRAM component type.
 get_filename_component(test_program_name "/ arg1 arg2" PROGRAM)
 check("PROGRAM with no args output" "${test_program_name}" "/")