Stop returning NOTCAPABLE errors from WASI calls.

`ENOTCAPABLE` was an error code that is used as part of the rights
system, from CloudABI. There is a set of flags associated with each file
descriptor listing which operations can be performed with the file
descriptor, and if an attempt is made to perform an operation with a
file descriptor that isn't permitted by its rights flags, it fails with
`ENOTCAPABLE`.

WASI is removing the rights system. For example, WebAssembly/wasi-libc#294
removed support for translating `ENOTCAPABLE` into POSIX error codes, on
the assumption that engines should stop using it.

So as another step to migrating away from the rights system, remove uses
of the `ENOTCAPABLE` error.

crates/test-programs/wasi-tests/src/bin/path_open_read_without_rights.rs

@@ -30,7 +30,7 @@ unsafe fn try_read_file(dir_fd: wasi::Fd) {
         wasi::fd_read(fd, &[iovec])
             .expect_err("reading bytes from file should fail")
             .raw_error(),
-        wasi::ERRNO_NOTCAPABLE
+        wasi::ERRNO_BADF
     );
 }
 

crates/test-programs/wasi-tests/src/bin/truncation_rights.rs

@@ -68,7 +68,7 @@ unsafe fn test_truncation_rights(dir_fd: wasi::Fd) {
             wasi::path_open(dir_fd, 0, "file", wasi::OFLAGS_TRUNC, 0, 0, 0)
                 .expect_err("truncating a file without path_filestat_set_size right")
                 .raw_error(),
-            wasi::ERRNO_NOTCAPABLE
+            wasi::ERRNO_PERM
         );
     }
 

crates/wasi-common/src/dir.rs

@@ -76,15 +76,24 @@ impl DirEntry {
         if self.caps.contains(caps) {
             Ok(())
         } else {
-            Err(Error::not_capable().context(format!("desired {:?}, has {:?}", caps, self.caps,)))
+            let missing = caps & !self.caps;
+            if missing.intersects(DirCaps::READDIR) {
+                Err(Error::not_dir()
+                    .context(format!("desired rights {:?}, has {:?}", caps, self.caps)))
+            } else {
+                Err(Error::perm()
+                    .context(format!("desired rights {:?}, has {:?}", caps, self.caps)))
+            }
         }
     }
     pub fn capable_of_file(&self, caps: FileCaps) -> Result<(), Error> {
         if self.file_caps.contains(caps) {
             Ok(())
         } else {
-            Err(Error::not_capable()
-                .context(format!("desired {:?}, has {:?}", caps, self.file_caps)))
+            Err(Error::perm().context(format!(
+                "desired rights {:?}, has {:?}",
+                caps, self.file_caps
+            )))
         }
     }
     pub fn drop_caps_to(&mut self, caps: DirCaps, file_caps: FileCaps) -> Result<(), Error> {

crates/wasi-common/src/error.rs

@@ -72,6 +72,9 @@ pub enum ErrorKind {
     /// Errno::Spipe: Invalid seek
     #[error("Spipe: Invalid seek")]
     Spipe,
+    /// Errno::Perm: Permission denied
+    #[error("Permission denied")]
+    Perm,
     /// Errno::NotCapable: Not capable
     #[error("Not capable")]
     NotCapable,
@@ -92,7 +95,7 @@ pub trait ErrorExt {
     fn overflow() -> Self;
     fn range() -> Self;
     fn seek_pipe() -> Self;
-    fn not_capable() -> Self;
+    fn perm() -> Self;
 }
 
 impl ErrorExt for Error {
@@ -138,7 +141,7 @@ impl ErrorExt for Error {
     fn seek_pipe() -> Self {
         ErrorKind::Spipe.into()
     }
-    fn not_capable() -> Self {
-        ErrorKind::NotCapable.into()
+    fn perm() -> Self {
+        ErrorKind::Perm.into()
     }
 }

crates/wasi-common/src/file.rs

@@ -238,7 +238,17 @@ impl FileEntry {
         if self.caps.contains(caps) {
             Ok(())
         } else {
-            Err(Error::not_capable().context(format!("desired {:?}, has {:?}", caps, self.caps,)))
+            let missing = caps & !self.caps;
+            if missing.intersects(FileCaps::READ | FileCaps::WRITE) {
+                // `EBADF` is a little surprising here because it's also used
+                // for unknown-file-descriptor errors, but it's what POSIX uses
+                // in this situation.
+                Err(Error::badf()
+                    .context(format!("desired rights {:?}, has {:?}", caps, self.caps)))
+            } else {
+                Err(Error::perm()
+                    .context(format!("desired rights {:?}, has {:?}", caps, self.caps)))
+            }
         }
     }
 

crates/wasi-common/src/snapshots/preview_1.rs

@@ -84,6 +84,7 @@ impl From<ErrorKind> for types::Errno {
             ErrorKind::Range => Errno::Range,
             ErrorKind::Spipe => Errno::Spipe,
             ErrorKind::NotCapable => Errno::Notcapable,
+            ErrorKind::Perm => Errno::Perm,
         }
     }
 }