Update get_standard_cache_dir for macOS

On macOS the usual cache directory is ~/Library/Caches.  This patch
changes get_standard_cache_dir to use that instead of XDG.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* common/pathstuff.c (get_standard_cache_dir): Use
	~/Library/Caches on macOS.
	* common/pathstuff.h (get_standard_cache_dir): Update comment.

gdb/doc/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Index Files): Update for cache directory change on
	macOS.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2018-09-17  Tom Tromey  <tom@tromey.com>
+
+	* common/pathstuff.c (get_standard_cache_dir): Use
+	~/Library/Caches on macOS.
+	* common/pathstuff.h (get_standard_cache_dir): Update comment.
+
 2018-09-17  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	PR python/23669

gdb/common/pathstuff.c

@@ -164,6 +164,13 @@ contains_dir_separator (const char *path)
 std::string
 get_standard_cache_dir ()
 {
+#ifdef __APPLE__
+#define HOME_CACHE_DIR "Library/Caches"
+#else
+#define HOME_CACHE_DIR ".cache"
+#endif
+
+#ifndef __APPLE__
   char *xdg_cache_home = getenv ("XDG_CACHE_HOME");
   if (xdg_cache_home != NULL)
     {
@@ -171,13 +178,14 @@ get_standard_cache_dir ()
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (xdg_cache_home));
       return string_printf ("%s/gdb", abs.get ());
     }
+#endif
 
   char *home = getenv ("HOME");
   if (home != NULL)
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (home));
-      return string_printf ("%s/.cache/gdb", abs.get ());
+      return string_printf ("%s/" HOME_CACHE_DIR "/gdb", abs.get ());
     }
 
   return {};

gdb/common/pathstuff.h

@@ -53,10 +53,14 @@ extern bool contains_dir_separator (const char *path);
 /* Get the usual user cache directory for the current platform.
 
    On Linux, it follows the XDG Base Directory specification: use
-   $XDG_CACHE_HOME/gdb if the XDG_CACHE_HOME environment variable is defined,
-   otherwise $HOME/.cache.  The return value is absolute and tilde-expanded.
+   $XDG_CACHE_HOME/gdb if the XDG_CACHE_HOME environment variable is
+   defined, otherwise $HOME/.cache.
 
-   Return an empty string if neither XDG_CACHE_HOME or HOME are defined.  */
+   On macOS, it follows the local convention and uses
+   ~/Library/Caches/gdb.
+
+  The return value is absolute and tilde-expanded.  Return an empty
+  string if neither XDG_CACHE_HOME (on Linux) or HOME are defined.  */
 
 extern std::string get_standard_cache_dir ();
 

gdb/doc/ChangeLog

@@ -1,3 +1,8 @@
+2018-09-17  Tom Tromey  <tom@tromey.com>
+
+	* gdb.texinfo (Index Files): Update for cache directory change on
+	macOS.
+
 2018-09-16  Tom Tromey  <tom@tromey.com>
 
 	* python.texi (Basic Python): Update docs for find_pc_line,

gdb/doc/gdb.texinfo

@@ -20128,10 +20128,14 @@ Enable or disable the use of the symbol index cache.
 
 @item set index-cache directory @var{directory}
 @itemx show index-cache directory
-Set/show the directory where index files will be saved.  By default, the index
-is cached in the @file{gdb} subdirectory of the directory pointed to by the
-@env{XDG_CACHE_HOME} environment variable, if it is defined, else in the
-@file{.cache/gdb} subdirectory of your home directory.
+Set/show the directory where index files will be saved.
+
+The default value for this directory depends on the host platform.  On
+most systems, the index is cached in the @file{gdb} subdirectory of
+the directory pointed to by the @env{XDG_CACHE_HOME} environment
+variable, if it is defined, else in the @file{.cache/gdb} subdirectory
+of your home directory.  However, on some systems, the default may
+differ according to local convention.
 
 There is no limit on the disk space used by index cache.  It is perfectly safe
 to delete the content of that directory to free up disk space.