libbb: modify find_executable() to not temporarily write to PATH

This allows to simplify "which" applet code

function                                             old     new   delta
find_executable                                       93     111     +18
which_main                                           191     177     -14
builtin_source                                       316     294     -22
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 18/-36)            Total: -18 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 30 insertions, 26 deletions

diff --git a/libbb/executable.c b/libbb/executable.c
index a033b74d9..09bed1eaf 100644
--- a/libbb/executable.c
+++ b/libbb/executable.c
@@ -21,14 +21,11 @@ int FAST_FUNC file_is_executable(const char *name)
 /* search (*PATHp) for an executable file;
  * return allocated string containing full path if found;
  *  PATHp points to the component after the one where it was found
- *  (or NULL),
+ *  (or NULL if found in last component),
  *  you may call find_executable again with this PATHp to continue
- *  (if it's not NULL).
- * return NULL otherwise; (PATHp is undefined)
- * in all cases (*PATHp) contents are temporarily modified
- * but are restored on return (s/:/NUL/ and back).
+ * return NULL otherwise (PATHp is undefined)
  */
-char* FAST_FUNC find_executable(const char *filename, char **PATHp)
+char* FAST_FUNC find_executable(const char *name, const char **PATHp)
 {
         /* About empty components in $PATH:
          * http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html
@@ -38,38 +35,45 @@ char* FAST_FUNC find_executable(const char *filename, char **PATHp)
          * initial colon preceding the rest of the list, or as a trailing colon
          * following the rest of the list.
          */
-        char *p, *n;
+        char *p = (char*) *PATHp;
 
-        p = *PATHp;
-        while (p) {
-                int ex;
+        if (!p)
+                return NULL;
+        while (1) {
+                const char *end = strchrnul(p, ':');
+                int sz = end - p;
 
-                n = strchr(p, ':');
-                if (n) *n = '\0';
-                p = concat_path_file(
-                        p[0] ? p : ".", /* handle "::" case */
-                        filename
-                );
-                ex = file_is_executable(p);
-                if (n) *n++ = ':';
-                if (ex) {
-                        *PATHp = n;
+                if (sz != 0) {
+                        p = xasprintf("%.*s/%s", sz, p, name);
+                } else {
+                        /* We have xxx::yyy in $PATH,
+                         * it means "use current dir" */
+                        p = xstrdup(name);
+// A bit of discrepancy wrt the path used if file is found here.
+// bash 5.2.15 "type" returns "./NAME".
+// GNU which v2.21 returns "/CUR/DIR/NAME".
+// With -a, both skip over all colons: xxx::::yyy is the same as xxx::yyy,
+// current dir is not tried the second time.
+                }
+                if (file_is_executable(p)) {
+                        *PATHp = (*end ? end+1 : NULL);
                         return p;
                 }
                 free(p);
-                p = n;
-        } /* on loop exit p == NULL */
-        return p;
+                if (*end == '\0')
+                        return NULL;
+                p = (char *) end + 1;
+        }
 }
 
 /* search $PATH for an executable file;
  * return 1 if found;
  * return 0 otherwise;
  */
-int FAST_FUNC executable_exists(const char *filename)
+int FAST_FUNC executable_exists(const char *name)
 {
-        char *path = getenv("PATH");
-        char *ret = find_executable(filename, &path);
+        const char *path = getenv("PATH");
+        char *ret = find_executable(name, &path);
         free(ret);
         return ret != NULL;
 }