Write debug output to RUST_LIBFUZZER_DEBUG_PATH, when set

And then return. This is intended for use by `cargo fuzz` for getting nice info
about an `Arbitrary` input that caused a crash.

Author: fitzgen

ci/script.sh

@@ -33,4 +33,9 @@ cargo rustc \
       -Cllvm-args=-sanitizer-coverage-prune-blocks=0 \
       -Zsanitizer=address
 (! $CARGO_TARGET_DIR/release/example_arbitrary -runs=10000000)
+RUST_LIBFUZZER_DEBUG_PATH=$(pwd)/debug_output \
+    $CARGO_TARGET_DIR/release/example_arbitrary \
+    $(ls ./crash-* | head -n 1)
+cat $(pwd)/debug_output
+grep -q Rgb $(pwd)/debug_output
 popd

example_arbitrary/.gitignore

@@ -1 +1,2 @@
 crash-*
+debug_output

src/lib.rs

@@ -118,6 +118,18 @@ macro_rules! fuzz_target {
     (|$bytes:ident| $body:block) => {
         #[no_mangle]
         pub extern "C" fn rust_fuzzer_test_input($bytes: &[u8]) {
+            // When `RUST_LIBFUZZER_DEBUG_PATH` is set, write the debug
+            // formatting of the input to that file. This is only intended for
+            // `cargo fuzz`'s use!
+            if let Ok(path) = std::env::var("RUST_LIBFUZZER_DEBUG_PATH") {
+                use std::io::Write;
+                let mut file = std::fs::File::create(path)
+                    .expect("failed to create `RUST_LIBFUZZER_DEBUG_PATH` file");
+                writeln!(&mut file, "{:?}", $bytes)
+                    .expect("failed to write to `RUST_LIBFUZZER_DEBUG_PATH` file");
+                return;
+            }
+
             $body
         }
     };
@@ -132,8 +144,24 @@ macro_rules! fuzz_target {
             use libfuzzer_sys::arbitrary::{Arbitrary, Unstructured};
 
             let mut u = Unstructured::new(bytes);
+            let data = <$dty as Arbitrary>::arbitrary_take_rest(u);
+
+            // When `RUST_LIBFUZZER_DEBUG_PATH` is set, write the debug
+            // formatting of the input to that file. This is only intended for
+            // `cargo fuzz`'s use!
+            if let Ok(path) = std::env::var("RUST_LIBFUZZER_DEBUG_PATH") {
+                use std::io::Write;
+                let mut file = std::fs::File::create(path)
+                    .expect("failed to create `RUST_LIBFUZZER_DEBUG_PATH` file");
+                (match data {
+                    Ok(data) => writeln!(&mut file, "{:#?}", data),
+                    Err(err) => writeln!(&mut file, "Arbitrary Error: {}", err),
+                })
+                .expect("failed to write to `RUST_LIBFUZZER_DEBUG_PATH` file");
+                return;
+            }
 
-            let $data: $dty = match Arbitrary::arbitrary_take_rest(u) {
+            let $data = match data {
                 Ok(d) => d,
                 Err(_) => return,
             };