[PATCH 3/4] Enhance --enable-tunables to select tunables frontend at build time

[Siddhesh Poyarekar <siddhesh at sourceware dot org>]

At the GNU Tools Cauldron 2016, the state of the current tunables
patchset was considered OK with the addition of a way to select the
frontend to be used for the tunables.  That is, to avoid being locked
in to one type of frontend initially, it should be possible to build
tunables with a different frontend with something as simple as a
configure switch.

To that effect, this patch enhances the --enable-tunables option to
accept more values than just 'yes' or 'no'.  The current frontend (and
default when enable-tunables is 'yes') is called 'valstring', to
select the frontend where a single environment variable is set to a
colon-separated value string.  More such frontends can be added in
future.

	* Makeconfig (have-tunables): Check for non-negative instead
	of positive.
	* configure.ac: Add 'valstring' as a valid value for
	--enable-tunables.
	* configure: Regenerate.
	* elf/Makefile (have-tunables): Check for non-negative instead
	of positive.
	(CPPFLAGS-dl-tunables.c): Define TUNABLES_FRONTEND for
	dl-tunables.c.
	* elf/dl-tunables.c (GLIBC_TUNABLES): Define only when
	TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring.
	(tunables_strdup): Likewise.
	(disable_tunables): Likewise.
	(parse_tunables): Likewise.
	(__tunables_init): Process GLIBC_TUNABLES envvar only when.
	TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring.
	* elf/dl-tunables.h (TUNABLES_FRONTEND_valstring): New macro.
	(TUNABLES_FRONTEND_yes): New macro, define as
	TUNABLES_FRONTEND_valstring by default.
	* manual/install.texi: Document new acceptable values for
	--enable-tunables.
	* INSTALL: Regenerate.
---
 INSTALL             | 18 +++++++++++++++++-
 Makeconfig          |  4 ++--
 configure           |  3 ++-
 configure.ac        |  2 +-
 elf/Makefile        |  4 +++-
 elf/dl-tunables.c   | 12 +++++++++++-
 elf/dl-tunables.h   |  4 ++++
 manual/install.texi | 17 ++++++++++++++++-
 8 files changed, 56 insertions(+), 8 deletions(-)

diff --git a/INSTALL b/INSTALL
index 25619fc..55d52c5 100644
--- a/INSTALL
+++ b/INSTALL
@@ -172,7 +172,23 @@ will be used, and CFLAGS sets optimization options for the compiler.
 '--enable-tunables'
      Tunables support allows additional library parameters to be
      customized at runtime.  This is an experimental feature and affects
-     startup time and is thus disabled by default.
+     startup time and is thus disabled by default.  This option can take
+     the following values:
+
+     'no'
+          This is the default if the option is not passed to configure.
+          This disables tunables.
+
+     'yes'
+          This is the default if the option is passed to configure.
+          This enables tunables and selects the default frontend
+          (currently 'valstring').
+
+     'valstring'
+          This enables tunables and selects the 'valstring' frontend for
+          tunables.  This frontend allows users to specify tunables as a
+          colon-separated list in a single environment variable
+          'GLIBC_TUNABLES'.
 
 '--build=BUILD-SYSTEM'
 '--host=HOST-SYSTEM'
diff --git a/Makeconfig b/Makeconfig
index b173e4c..1a2db6d 100644
--- a/Makeconfig
+++ b/Makeconfig
@@ -935,7 +935,7 @@ CPPFLAGS = $(config-extra-cppflags) $(CPPUNDEFS) $(CPPFLAGS-config) \
 			 $(libof-$(<F)) $(libof-$(@F)),$(CPPFLAGS-$(lib))) \
 	   $(CPPFLAGS-$(<F)) $(CPPFLAGS-$(@F)) $(CPPFLAGS-$(basename $(@F)))
 
-ifeq (yes,$(have-tunables))
+ifneq (no,$(have-tunables))
 CPPFLAGS += -DTOP_NAMESPACE=glibc
 endif
 
@@ -1115,7 +1115,7 @@ endif
 
 # Build the tunables list header early since it could be used by any module in
 # glibc.
-ifeq (yes,$(have-tunables))
+ifneq (no,$(have-tunables))
 before-compile += $(common-objpfx)dl-tunable-list.h
 
 $(common-objpfx)dl-tunable-list.h: $(..)scripts/gen-tunables.awk \
diff --git a/configure b/configure
index d80d738..eecd0ac 100755
--- a/configure
+++ b/configure
@@ -1454,7 +1454,8 @@ Optional Features:
   --disable-build-nscd    disable building and installing the nscd daemon
   --disable-nscd          library functions will not contact the nscd daemon
   --enable-pt_chown       Enable building and installing pt_chown
-  --enable-tunables       Enable tunables support
+  --enable-tunables       Enable tunables support. Known values are 'yes',
+                          'no' and 'valstring'
   --enable-mathvec        Enable building and installing mathvec [default
                           depends on architecture]
 
diff --git a/configure.ac b/configure.ac
index 22f5cab..4a77411 100644
--- a/configure.ac
+++ b/configure.ac
@@ -423,7 +423,7 @@ fi
 
 AC_ARG_ENABLE([tunables],
 	      [AS_HELP_STRING([--enable-tunables],
-	       [Enable tunables support])],
+	       [Enable tunables support. Known values are 'yes', 'no' and 'valstring'])],
 	      [have_tunables=$enableval],
 	      [have_tunables=no])
 AC_SUBST(have_tunables)
diff --git a/elf/Makefile b/elf/Makefile
index 3cda2c9..c3636a6 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -36,8 +36,10 @@ ifeq (yes,$(use-ldconfig))
 dl-routines += dl-cache
 endif
 
-ifeq (yes,$(have-tunables))
+ifneq (no,$(have-tunables))
 dl-routines += dl-tunables
+tunables-type = $(addprefix TUNABLES_FRONTEND_,$(have-tunables))
+CPPFLAGS-dl-tunables.c = -DTUNABLES_FRONTEND=$(tunables-type)
 
 # Make sure that the compiler does not insert any library calls in tunables
 # code paths.
diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index bbdaa15..a970e1e 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -30,7 +30,9 @@
 #define TUNABLES_INTERNAL 1
 #include "dl-tunables.h"
 
-#define GLIBC_TUNABLES "GLIBC_TUNABLES"
+#if TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring
+# define GLIBC_TUNABLES "GLIBC_TUNABLES"
+#endif
 
 /* Compare environment or tunable names, bounded by the name hardcoded in
    glibc.  */
@@ -48,6 +50,7 @@ is_name (const char *orig, const char *envname)
     return false;
 }
 
+#if TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring
 static char *
 tunables_strdup (const char *in)
 {
@@ -70,6 +73,7 @@ tunables_strdup (const char *in)
 
   return out;
 }
+#endif
 
 static char **
 get_next_env (char **envp, char **name, size_t *namelen, char **val)
@@ -244,6 +248,7 @@ tunable_initialize (tunable_t *cur, const char *strval)
     }
 }
 
+#if TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring
 static void
 parse_tunables (char *tunestr)
 {
@@ -309,6 +314,7 @@ parse_tunables (char *tunestr)
 	return;
     }
 }
+#endif
 
 static size_t
 min_strlen (const char *s)
@@ -329,6 +335,7 @@ disable_tunable (tunable_id_t id, char **envp)
   if (env_alias != NULL)
     tunables_unsetenv (envp, tunable_list[id].env_alias);
 
+#if TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring
   char *tunable = getenv (GLIBC_TUNABLES);
   const char *cmp = tunable_list[id].name;
   const size_t len = min_strlen (cmp);
@@ -345,6 +352,7 @@ disable_tunable (tunable_id_t id, char **envp)
 	}
       tunable++;
     }
+#endif
 }
 
 /* Disable the glibc.malloc.check tunable in SETUID/SETGID programs unless
@@ -375,6 +383,7 @@ __tunables_init (char **envp)
 
   while ((envp = get_next_env (envp, &envname, &len, &envval)) != NULL)
     {
+#if TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring
       if (is_name (GLIBC_TUNABLES, envname))
 	{
 	  char *val = tunables_strdup (envval);
@@ -382,6 +391,7 @@ __tunables_init (char **envp)
 	    parse_tunables (val);
 	  continue;
 	}
+#endif
 
       for (int i = 0; i < sizeof (tunable_list) / sizeof (tunable_t); i++)
 	{
diff --git a/elf/dl-tunables.h b/elf/dl-tunables.h
index 7762c46..785a9b0 100644
--- a/elf/dl-tunables.h
+++ b/elf/dl-tunables.h
@@ -84,5 +84,9 @@ extern void __tunable_set_val (tunable_id_t, void *, tunable_callback_t);
 /* Namespace sanity for callback functions.  Use this macro to keep the
    namespace of the modules clean.  */
 #  define DL_TUNABLE_CALLBACK(__name) _dl_tunable_ ## __name
+
+#  define TUNABLES_FRONTEND_valstring 1
+/* The default value for TUNABLES_FRONTEND.  */
+#  define TUNABLES_FRONTEND_yes TUNABLES_FRONTEND_valstring
 # endif
 #endif
diff --git a/manual/install.texi b/manual/install.texi
index d412962..2657da1 100644
--- a/manual/install.texi
+++ b/manual/install.texi
@@ -203,7 +203,22 @@ Use this option to disable the vector math library.
 @item --enable-tunables
 Tunables support allows additional library parameters to be customized at
 runtime.  This is an experimental feature and affects startup time and is thus
-disabled by default.
+disabled by default.  This option can take the following values:
+
+@table @code
+@item no
+This is the default if the option is not passed to configure. This disables
+tunables.
+
+@item yes
+This is the default if the option is passed to configure. This enables tunables
+and selects the default frontend (currently @samp{valstring}).
+
+@item valstring
+This enables tunables and selects the @samp{valstring} frontend for tunables.
+This frontend allows users to specify tunables as a colon-separated list in a
+single environment variable @env{GLIBC_TUNABLES}.
+@end table
 
 @item --build=@var{build-system}
 @itemx --host=@var{host-system}
-- 
2.7.4