win32: use built-in applets for non-existent binaries with Unix-style paths

Shell scripts moved from Unix may contain hard-coded paths to
binaries such as /bin/sh.  A recent commit made it possible to
execute such binaries reliably, but that does require them to be
installed.  As an alternative solution:  if a binary with a
standard Unix path prefix can't be found but is available as a
built-in applet, run the applet.

Add the function unix_path() to detect paths starting with /bin,
/usr/bin, /sbin or /usr/sbin.

Use this function in:

- the 'which' applet
- shellexec(), describe_command() and find_command() in ash
- mingw_spawn_1()

See GitHub issue #195.

2 files changed, 26 insertions, 2 deletions

diff --git a/win32/mingw.c b/win32/mingw.c
index e63ffa0ac..4ffc49e9a 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -1818,6 +1818,19 @@ void *get_proc_addr(const char *dll, const char *function,
         return proc->pfunction;
 }
 
+#if ENABLE_FEATURE_SH_STANDALONE || ENABLE_FEATURE_PREFER_APPLETS
+int unix_path(const char *path)
+{
+        int i;
+        char *p = strdup(path);
+
+#define UNIX_PATHS "/bin\0/usr/bin\0/sbin\0/usr/sbin\0"
+        i = index_in_strings(UNIX_PATHS, dirname(p));
+        free(p);
+        return i >= 0;
+}
+#endif
+
 /* Return true if file is referenced using a path.  This means a path
  * look-up isn't required. */
 int has_path(const char *file)
diff --git a/win32/process.c b/win32/process.c
index cd164e0ed..a050ec11d 100644
--- a/win32/process.c
+++ b/win32/process.c
@@ -350,6 +350,7 @@ mingw_spawn_1(int mode, const char *cmd, char *const *argv, char *const *envp)
 {
         char *prog;
         const char *path;
+        intptr_t ret;
 
 #if ENABLE_FEATURE_PREFER_APPLETS || ENABLE_FEATURE_SH_STANDALONE
         if (find_applet_by_name(cmd) >= 0)
@@ -357,12 +358,22 @@ mingw_spawn_1(int mode, const char *cmd, char *const *argv, char *const *envp)
         else
 #endif
         if (has_path(cmd)) {
+#if ENABLE_FEATURE_PREFER_APPLETS || ENABLE_FEATURE_SH_STANDALONE
+                const char *oldcmd = cmd;
+#endif
                 cmd = auto_add_system_drive(cmd);
                 path = auto_win32_extension(cmd);
-                return mingw_spawn_interpreter(mode, path ? path : cmd, argv, envp, 0);
+                ret = mingw_spawn_interpreter(mode, path ? path : cmd, argv, envp, 0);
+#if ENABLE_FEATURE_PREFER_APPLETS || ENABLE_FEATURE_SH_STANDALONE
+                if (ret == -1 && cmd != oldcmd && unix_path(oldcmd) &&
+                                find_applet_by_name(bb_basename(oldcmd))) {
+                        return mingw_spawn_applet(mode, argv, envp);
+                }
+#endif
+                return ret;
         }
         else if ((prog=find_first_executable(cmd)) != NULL) {
-                intptr_t ret = mingw_spawn_interpreter(mode, prog, argv, envp, 0);
+                ret = mingw_spawn_interpreter(mode, prog, argv, envp, 0);
                 free(prog);
                 return ret;
         }