expand: Do not reprocess data when expanding words

Currently various paths will reprocess data when performing word
expansion.  For example, expari will skip backwards looking for
the start of the arithmetic expansion, while evalvar will skip
unexpanded words manually.

This is cumbersome and error-prone.  This patch fixes this by
making word expansions proceed in a linear fashion.  This means
changing argstr and the various expansion functions such as expari
and subevalvar to return the next character to be expanded.

This is inspired by similar code from FreeBSD.  However, we take
things one step further and completely remove the manual word
skipping in evalvar.  This is accomplished by introducing a new
EXP_DISCARD flag that tells argstr to only parse and not produce
any actual expansions.

Incidentally, argstr will now always NUL-terminate the expansion
unless the EXP_WORD flag is set.  This is because all but one
caller of argstr wants the result to be NUL-termianted.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/expand.h b/src/expand.h
index 617b851..c44b848 100644
--- a/src/expand.h
+++ b/src/expand.h
@@ -59,11 +59,11 @@ struct arglist {
 #define EXP_WORD	0x80	/* expand word in parameter expansion */
 #define EXP_QUOTED	0x100	/* expand word in double quotes */
 #define EXP_KEEPNUL	0x200	/* do not skip NUL characters */
+#define EXP_DISCARD	0x400	/* discard result of expansion */
 
 
 union node;
 void expandarg(union node *, struct arglist *, int);
-void expari(int);
 #define rmescapes(p) _rmescapes((p), 0)
 char *_rmescapes(char *, int);
 int casematch(union node *, char *);