Provide a public interface to preopened directory lookups. (#10)

* Provide a public interface to preopened directory lookups.

For users of especially non-C compilers, provide an API for looking up
preopened directories. This is used internally in WASI libc to translate
from eg. `open` to `openat`, but it can now also be used by user code.

expected/wasm32-wasi/defined-symbols.txt

@@ -242,6 +242,7 @@ __unlist_locked_file
 __uselocale
 __utc
 __wasilibc_fd_renumber
+__wasilibc_find_relpath
 __wasilibc_init_preopen
 __wasilibc_register_preopened_fd
 __wasilibc_rmdirat

expected/wasm32-wasi/include-all.c

@@ -183,6 +183,7 @@
 #include <utime.h>
 #include <values.h>
 #include <wasi/core.h>
+#include <wasi/libc-find-relpath.h>
 #include <wasi/libc.h>
 #include <wchar.h>
 #include <wctype.h>

expected/wasm32-wasi/predefined-macros.txt

@@ -3622,6 +3622,7 @@
 #define __va_copy(d,s) __builtin_va_copy(d,s)
 #define __wasi__ 1
 #define __wasi_core_h 
+#define __wasi_libc_find_relpath_h 
 #define __wasi_libc_h 
 #define __wasilibc___errno_values_h 
 #define __wasilibc___fd_set_h 

libc-bottom-half/headers/public/wasi/libc-find-relpath.h

@@ -0,0 +1,25 @@
+#ifndef __wasi_libc_find_relpath_h
+#define __wasi_libc_find_relpath_h
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Look up the given path in the preopened directory map. If a suitable
+ * entry is found, return its directory file descriptor, and store the
+ * computed relative path in *relative_path. Ignore preopened directories
+ * which don't provide the specified rights.
+ *
+ * Returns -1 if no directories were suitable.
+ */
+int __wasilibc_find_relpath(const char *path,
+                            __wasi_rights_t rights_base,
+                            __wasi_rights_t rights_inheriting,
+                            const char **relative_path);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

libc-bottom-half/libpreopen/include/libpreopen.h

@@ -82,6 +82,12 @@ struct po_relpath {
 	/** The path, relative to the directory represented by @ref dirfd */
 	const char *relative_path;
 };
+#ifdef __wasilibc_unmodified_upstream
+#else
+// This functionality is exposed in a libc header, so include that header
+// and use its declarations.
+#include <wasi/libc-find-relpath.h>
+#endif
 
 /**
  * Create a @ref po_map of at least the specified capacity.

libc-bottom-half/libpreopen/lib/po_libc_wrappers.c

@@ -682,4 +682,14 @@ __wasilibc_register_preopened_fd(int fd, const char *path)
 
 	return 0;
 }
+
+int __wasilibc_find_relpath(const char *path,
+                            __wasi_rights_t rights_base,
+                            __wasi_rights_t rights_inheriting,
+                            const char **relpath)
+{
+    struct po_relpath rel = find_relative(path, rights_base, rights_inheriting);
+    *relpath = rel.relative_path;
+    return rel.dirfd;
+}
 #endif