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Zuo: fix `{file,directory,link}-exists?`

Treat failure at the `stat` level as non-existence.

Closes #4337

--- a/lib/zuo/private/more.zuo

+++ b/lib/zuo/private/more.zuo

@@ -131,17 +131,17 @@

 (define (file-exists? p)

   (unless (path-string? p) (arg-error 'file-exists? "path string" p))

-  (let ([s (stat p)])

+  (let ([s (stat p #t #t)])

     (and s (eq? (hash-ref s 'type) 'file))))

 (define (directory-exists? p)

   (unless (path-string? p) (arg-error 'directory-exists?: "path string" p))

-  (let ([s (stat p)])

+  (let ([s (stat p #t #t)])

     (and s (eq? (hash-ref s 'type) 'dir))))

 (define (link-exists? p)

   (unless (path-string? p) (arg-error 'link-exists? "path string" p))

-  (let ([s (stat p #f)])

+  (let ([s (stat p #f #t)])

     (and s (eq? (hash-ref s 'type) 'link))))

 (define (explode-path p)

--- a/zuo-doc/lang-zuo.scrbl

+++ b/zuo-doc/lang-zuo.scrbl

@@ -1149,7 +1149,10 @@

 @section{Filesystem}

-@defproc[(stat [name path-string?] [follow-links? any/c #t]) (or/c hash? #f)]{

+@defproc[(stat [name path-string?]

+               [follow-links? any/c #t]

+               [false-on-error? any/c #f])

+         (or/c hash? #f)]{

 Returns information about the file, directory, or link referenced by

 @racket[name]. If @racket[follow-links?] is @racket[#f], then when

@@ -1156,7 +1159,10 @@

 @racket[name] refers to a link, information is reported about the

 link; otherwise, information is reported about the target of a link.

-If no such file, directory, or link exists, the result is @racket[#f].

+If @racket[name] is a valid path but such file, directory, or link exists,

+the result is @racket[#f]. If accessing @racket[name] encounters an error

+(e.g., @racket[name] uses a file as a directory or permission is denied),

+then @racket[#f] is reported instead of an error if @racket[false-on-error?].

 Otherwise, the hash table has similar keys and values as

 @realracket[file-or-directory-stat] from @racketmodname[racket], but

 with only certain keys per platform:

@@ -1256,7 +1262,9 @@

 )]{

 Uses @racket[stat] to check for a file, directory, or link,

-respectively.}

+respectively. If @racket[stat] encounters an error (e.g., access

+permission), then the file, directory, or link is treated as

+non-existent.}

 @section{Run Time Configuration}

--- a/zuo.c

+++ b/zuo.c

@@ -4788,11 +4788,12 @@

 # define TO_INT64(a, b) (((zuo_int_t)(a) << 32) | (((zuo_int_t)b) & (zuo_int_t)0xFFFFFFFF))

 #endif

-static zuo_t *zuo_stat(zuo_t *path, zuo_t *follow_links) {

+static zuo_t *zuo_stat(zuo_t *path, zuo_t *follow_links, zuo_t *false_on_error) {

   const char *who = "stat";

   zuo_t *result = z.o_empty_hash;

   if (follow_links == z.o_undefined) follow_links = z.o_true;

+  if (false_on_error == z.o_undefined) false_on_error = z.o_false;

   check_path_string(who, path);

@@ -4807,7 +4808,7 @@

       stat_result = stat(ZUO_STRING_PTR(path), &stat_buf);

     if (stat_result != 0) {

-      if (errno != ENOENT)

+      if ((errno != ENOENT) && (false_on_error == z.o_false))

 	zuo_fail1w_errno(who, "failed", path);

       return z.o_false;

     }

@@ -4853,13 +4854,17 @@

     if (fdh == INVALID_HANDLE_VALUE) {

       DWORD err = GetLastError();

-      if ((err != ERROR_FILE_NOT_FOUND) && (err != ERROR_PATH_NOT_FOUND))

+      if ((err != ERROR_FILE_NOT_FOUND) && (err != ERROR_PATH_NOT_FOUND)

+          && (false_on_error == z.o_false))

 	zuo_fail1w(who, "failed", path);

       return z.o_false;

     }

-    if (!GetFileInformationByHandle(fdh, &info))

+    if (!GetFileInformationByHandle(fdh, &info)) {

+      if (false_on_error != z.o_false)

+        return z.o_false;

       zuo_fail1w(who, "failed", path);

+    }

     CloseHandle(fdh);

@@ -6925,7 +6930,7 @@

   ZUO_TOP_ENV_SET_PRIMITIVEb("fd-terminal?", zuo_fd_terminal_p);

   ZUO_TOP_ENV_SET_PRIMITIVE1("fd-valid?", zuo_fd_valid_p);

-  ZUO_TOP_ENV_SET_PRIMITIVEb("stat", zuo_stat);

+  ZUO_TOP_ENV_SET_PRIMITIVEC("stat", zuo_stat);

   ZUO_TOP_ENV_SET_PRIMITIVE1("rm", zuo_rm);

   ZUO_TOP_ENV_SET_PRIMITIVE2("mv", zuo_mv);

   ZUO_TOP_ENV_SET_PRIMITIVE1("mkdir", zuo_mkdir);

@@ -7152,7 +7157,7 @@

   if (load_file == NULL) {

     load_file = "main.zuo";

     load_path = zuo_string(load_file);

-    if (zuo_stat(load_path, z.o_true) == z.o_false) {

+    if (zuo_stat(load_path, z.o_true, z.o_true) == z.o_false) {

       zuo_error_color();

       fprintf(stderr, "%s: no file specified, and no \"main.zuo\" found", argv0);

       zuo_fail("");

@@ -7161,7 +7166,7 @@

     zuo_t *st;

     load_path = zuo_string(load_file);

     load_path = zuo_normalize_input_path(load_path);

-    st = zuo_stat(load_path, z.o_true);

+    st = zuo_stat(load_path, z.o_true, z.o_true);

     if ((st != z.o_false)

         && (zuo_trie_lookup(st, zuo_symbol("type")) == zuo_symbol("dir"))) {

       if (!strcmp(load_file, "."))