Auto merge of #1155 - divergentdave:shim-posix_fadivse, r=RalfJung

Add no-op shim for posix_fadvise

This function is present in the libc crate, but not exposed through the standard library anywhere, so I haven't included a test for it.

src/shims/foreign_items.rs

@@ -780,6 +780,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_null(dest)?;
             }
 
+            "posix_fadvise" => {
+                // fadvise is only informational, we can ignore it.
+                this.write_null(dest)?;
+            }
+
             "mmap" => {
                 // This is a horrible hack, but since the guard page mechanism calls mmap and expects a particular return value, we just give it that value.
                 let addr = this.read_scalar(args[0])?.not_undef()?;

tests/run-pass/libc.rs

@@ -0,0 +1,43 @@
+// ignore-windows: No libc on Windows
+// compile-flags: -Zmiri-disable-isolation
+
+#![feature(rustc_private)]
+
+#[allow(unused)] // necessary on macos due to conditional compilation
+extern crate libc;
+
+#[cfg(not(target_os = "macos"))]
+fn test_posix_fadvise() {
+    use std::convert::TryInto;
+    use std::env::temp_dir;
+    use std::fs::{File, remove_file};
+    use std::io::Write;
+    use std::os::unix::io::AsRawFd;
+
+    let path = temp_dir().join("miri_test_libc.txt");
+    // Cleanup before test
+    remove_file(&path).ok();
+
+    // Set up an open file
+    let mut file = File::create(&path).unwrap();
+    let bytes = b"Hello, World!\n";
+    file.write(bytes).unwrap();
+
+    // Test calling posix_fadvise on a file.
+    let result = unsafe {
+        libc::posix_fadvise(
+            file.as_raw_fd(),
+            0,
+            bytes.len().try_into().unwrap(),
+            libc::POSIX_FADV_DONTNEED,
+        )
+    };
+    drop(file);
+    remove_file(&path).unwrap();
+    assert_eq!(result, 0);
+}
+
+fn main() {
+    #[cfg(not(target_os = "macos"))]
+    test_posix_fadvise();
+}