libc: simplify access; libposix: let access lie

There are a few issues with Plan 9's `access`:

- it has side effects: to test the actual access (that the file
  servers can allow or deny according to complex custom rules)
  it opens and then closes the file, allocating (and disposing) the fd
- it does not work on directories, since
  - they cannot be opened for writing, despite the fact that to
    create a file in a directory you must be granted write access on
    that directory
  - they cannot be opened for execution, despite the fact that to
    access a file in a directory you must be granted execution access
    on that directory

Despite the fact that `access` (even on UNIX) is a violation of the
"tell, don't ask" principle (the access could be forbidden just after
its successful return, making subsequent `open` fail anyway), this
fact smells of a little design error in the file interface.

So, right now we choose to let the libposix's `access` lie on directories:
it will always return 0 on AWRITE and AEXEC for them, accepting that
a successive create/mkdir may fail.

However, a cleaner file API and protocol should allow a simpler `access`
to be implemented for directories too.

qa/lib/c/access.c

@@ -26,10 +26,15 @@ main(void)
 		err = "/tmp does not exists";
 	else if(access("/tmp", AREAD) != 0)
 		err = "/tmp is not readable";
+/* NOTE:
+ * In Plan 9 access(AWRITE) and access(AEXEC) in directories
+ * fail despite the actual permission of the directory.
+ *
 	else if(access("/tmp", AWRITE) != 0)
 		err = "/tmp is not writeable";
 	else if(access("/tmp", AEXEC) != 0)
 		err = "/tmp is not traversable";
+*/
 	if(err == nil){
 		print("PASS\n");
 		exits("PASS");

sys/src/lib/c/9sys/access.c

@@ -22,9 +22,7 @@
 int
 jehanne_access(const char *name, int mode)
 {
-	int tmp, reqmode;
+	int fd, reqmode;
-	Dir *db;
-	char *user;
 
 	static char omode[] = {
 		OSTAT,
@@ -38,35 +36,25 @@ jehanne_access(const char *name, int mode)
 	};
 
 	reqmode = omode[mode&AMASK];
-	tmp = open(name, reqmode);
+	fd = open(name, reqmode);
-	if(tmp >= 0){
+	if(fd >= 0){
-		close(tmp);
+		close(fd);
-		return 0;
-	}
-	db = jehanne_dirstat(name);
-	if(db != nil){
-		if(db->mode & DMDIR){
-			/* check other first */
-			tmp = db->mode & reqmode;
-			if(tmp != reqmode){
-				/* TODO: make something better */
-				user = jehanne_getuser();
-				if(jehanne_strcmp(user, db->gid) == 0){
-					/* check group */
-					tmp |= (db->mode & (reqmode << 3)) >> 3;
-					if(tmp != reqmode && jehanne_strcmp(user, db->uid)== 0){
-						/* check user */
-						tmp |= (db->mode & (reqmode << 6)) >> 6;
-					}
-				} else if (jehanne_strcmp(user, db->uid)== 0){
-					/* check user */
-					tmp |= (db->mode & (reqmode << 6)) >> 6;
-				}
-			}
-		}
-		jehanne_free(db);
-		if(tmp == reqmode)
 		return 0;
 	}
+
+	/* WARNING:
+	 *
+	 * In Plan 9 access(AWRITE) and access(AEXEC) in directories
+	 * fail despite the actual permission of the directory.
+	 *
+	 * This is well understood in Plan 9, but it's counter intuitive.
+	 *
+	 * In Plan 9, to create a file in a directory you need write
+	 * permission in the directory. Still you don't need to (and you
+	 * cannot) open the directory for writing before calling create.
+	 *
+	 * To my eyes this is a UNIX inheritance that could be "fixed"
+	 * but there are some trade off.
+	 */
 	return -1;
 }

sys/src/lib/posix/files.c

@@ -119,25 +119,36 @@ find_seek_type(int whence)
 int
 POSIX_access(int *errnop, const char *path, int amode)
 {
+	Dir *d;
 	PosixError e = 0;
 	
+	d = dirstat(path);
 	if(path == nil || path[0] == '\0'){
 		e = PosixENOENT;
- 	} else if(amode == __libposix_F_OK){
+	} else if(d == nil){
-		if(access(path, AEXIST) == 0)
-			return 0;
 		e = PosixENOENT;
-	} else if(amode & ~(__libposix_R_OK|__libposix_W_OK|__libposix_X_OK))
+	} else if(amode == __libposix_F_OK){
+		goto AccessDone;
+	} else if(amode & ~(__libposix_R_OK|__libposix_W_OK|__libposix_X_OK)){
 		e = PosixEINVAL;
-	else if((amode & __libposix_R_OK) && access(path, AREAD) != 0)
+	} else if((amode & __libposix_R_OK) && access(path, AREAD) != 0){
 		e = PosixEACCES;
-	else if((amode & __libposix_W_OK) && access(path, AWRITE) != 0)
+	} else if((d->mode & DMDIR) != 0){
+		/* we lie, but on Plan 9 access(AWRITE) and access(AEXEC)
+		 * will always fail on directories.
+		 */
+		goto AccessDone;
+	} else if((amode & __libposix_W_OK) && access(path, AWRITE) != 0){
 		e = PosixEACCES;
-	else if((amode & __libposix_X_OK) && access(path, AEXEC) != 0)
+	} else if((amode & __libposix_X_OK) && access(path, AEXEC) != 0){
 		e = PosixEACCES;
-	else
+	}else{
+AccessDone:
+		free(d);
 		return 0;
+	}
 
+	free(d);
 	*errnop = __libposix_get_errno(e);
 	return -1;
 }