ash: add bash-compatible EPOCH variables

Bash 5.0 added the dynamic variable EPOCHSECONDS and EPOCHREALTIME
which return the number of seconds since the Unix Epoch as an
integer or float.  These are useful for logging or tracing.

function                                             old     new   delta
change_epoch                                           -      78     +78
.rodata                                           175167  175235     +68
varinit_data                                         264     312     +48
change_seconds                                         -      24     +24
change_realtime                                        -      24     +24
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 2/0 up/down: 242/0)             Total: 242 bytes
   text	   data	    bss	    dec	    hex	filename
 938508	   4203	   1888	 944599	  e69d7	busybox_old
 938702	   4203	   1888	 944793	  e6a99	busybox_unstripped

v2: Cast tv_sec and tv_usec to unsigned quantities.
    Add brackets to macros.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -220,6 +220,7 @@
 #define    BASH_SOURCE          ENABLE_ASH_BASH_COMPAT
 #define    BASH_PIPEFAIL        ENABLE_ASH_BASH_COMPAT
 #define    BASH_HOSTNAME_VAR    ENABLE_ASH_BASH_COMPAT
+#define    BASH_EPOCH_VARS      ENABLE_ASH_BASH_COMPAT
 #define    BASH_SHLVL_VAR       ENABLE_ASH_BASH_COMPAT
 #define    BASH_XTRACEFD        ENABLE_ASH_BASH_COMPAT
 #define    BASH_READ_D          ENABLE_ASH_BASH_COMPAT
@@ -2053,6 +2054,10 @@ static void changepath(const char *) FAST_FUNC;
 #if ENABLE_ASH_RANDOM_SUPPORT
 static void change_random(const char *) FAST_FUNC;
 #endif
+#if BASH_EPOCH_VARS
+static void change_seconds(const char *) FAST_FUNC;
+static void change_realtime(const char *) FAST_FUNC;
+#endif
 
 static const struct {
 	int flags;
@@ -2079,6 +2084,10 @@ static const struct {
 #if ENABLE_ASH_RANDOM_SUPPORT
 	{ VSTRFIXED|VTEXTFIXED|VUNSET|VDYNAMIC, "RANDOM", change_random },
 #endif
+#if BASH_EPOCH_VARS
+	{ VSTRFIXED|VTEXTFIXED|VUNSET|VDYNAMIC, "EPOCHSECONDS", change_seconds },
+	{ VSTRFIXED|VTEXTFIXED|VUNSET|VDYNAMIC, "EPOCHREALTIME", change_realtime },
+#endif
 #if ENABLE_LOCALE_SUPPORT
 	{ VSTRFIXED|VTEXTFIXED|VUNSET, "LC_ALL"    , change_lc_all   },
 	{ VSTRFIXED|VTEXTFIXED|VUNSET, "LC_CTYPE"  , change_lc_ctype },
@@ -2110,26 +2119,26 @@ extern struct globals_var *BB_GLOBAL_CONST ash_ptr_to_globals_var;
 #define linenovar     (G_var.linenovar    )
 #define vifs      varinit[0]
 #if ENABLE_ASH_MAIL
-# define vmail    (&vifs)[1]
-# define vmpath   (&vmail)[1]
-# define vpath    (&vmpath)[1]
-#else
-# define vpath    (&vifs)[1]
-#endif
-#define vps1      (&vpath)[1]
-#define vps2      (&vps1)[1]
-#define vps4      (&vps2)[1]
+# define vmail    varinit[1]
+# define vmpath   varinit[2]
+#endif
+#define VAR_OFFSET1 (ENABLE_ASH_MAIL*2)
+#define vpath     varinit[VAR_OFFSET1 + 1]
+#define vps1      varinit[VAR_OFFSET1 + 2]
+#define vps2      varinit[VAR_OFFSET1 + 3]
+#define vps4      varinit[VAR_OFFSET1 + 4]
 #if ENABLE_ASH_GETOPTS
-# define voptind  (&vps4)[1]
-# define vlineno  (&voptind)[1]
-# if ENABLE_ASH_RANDOM_SUPPORT
-#  define vrandom (&vlineno)[1]
-# endif
-#else
-# define vlineno  (&vps4)[1]
-# if ENABLE_ASH_RANDOM_SUPPORT
-#  define vrandom (&vlineno)[1]
-# endif
+# define voptind  varinit[VAR_OFFSET1 + 5]
+#endif
+#define VAR_OFFSET2 (VAR_OFFSET1 + ENABLE_ASH_GETOPTS)
+#define vlineno   varinit[VAR_OFFSET2 + 5]
+#if ENABLE_ASH_RANDOM_SUPPORT
+# define vrandom  varinit[VAR_OFFSET2 + 6]
+#endif
+#define VAR_OFFSET3 (VAR_OFFSET2 + ENABLE_ASH_RANDOM_SUPPORT)
+#if BASH_EPOCH_VARS
+# define vepochs  varinit[VAR_OFFSET3 + 6]
+# define vepochr  varinit[VAR_OFFSET3 + 7]
 #endif
 #define INIT_G_var() do { \
 	unsigned i; \
@@ -2268,7 +2277,7 @@ lookupvar(const char *name)
 
 	v = *findvar(hashvar(name), name);
 	if (v) {
-#if ENABLE_ASH_RANDOM_SUPPORT
+#if ENABLE_ASH_RANDOM_SUPPORT || BASH_EPOCH_VARS
 	/*
 	 * Dynamic variables are implemented roughly the same way they are
 	 * in bash. Namely, they're "special" so long as they aren't unset.
@@ -2364,7 +2373,7 @@ setvareq(char *s, int flags)
 		}
 
 		flags |= vp->flags & ~(VTEXTFIXED|VSTACK|VNOSAVE|VUNSET);
-#if ENABLE_ASH_RANDOM_SUPPORT
+#if ENABLE_ASH_RANDOM_SUPPORT || BASH_EPOCH_VARS
 		if (flags & VUNSET)
 			flags &= ~VDYNAMIC;
 #endif
@@ -11249,6 +11258,32 @@ change_random(const char *value)
 }
 #endif
 
+#if BASH_EPOCH_VARS
+static void FAST_FUNC
+change_epoch(struct var *vepoch, const char *fmt)
+{
+	struct timeval tv;
+	char buffer[sizeof("%lu.nnnnnn") + sizeof(long)*3];
+
+	gettimeofday(&tv, NULL);
+	sprintf(buffer, fmt, (unsigned long)tv.tv_sec, (unsigned)tv.tv_usec);
+	setvar(vepoch->var_text, buffer, VNOFUNC);
+	vepoch->flags &= ~VNOFUNC;
+}
+
+static void FAST_FUNC
+change_seconds(const char *value UNUSED_PARAM)
+{
+	change_epoch(&vepochs, "%lu");
+}
+
+static void FAST_FUNC
+change_realtime(const char *value UNUSED_PARAM)
+{
+	change_epoch(&vepochr, "%lu.%06u");
+}
+#endif
+
 #if ENABLE_ASH_GETOPTS
 static int
 getopts(char *optstr, char *optvar, char **optfirst)