Make RUST_LIBFUZZER_DEBUG_PATH "free" for non-users

Cargo.toml

@@ -10,6 +10,7 @@ version = "0.4.1"
 
 [dependencies]
 arbitrary = "1"
+once_cell = "1"
 
 [build-dependencies]
 cc = "1.0"

src/lib.rs

@@ -12,6 +12,7 @@
 #![deny(missing_docs, missing_debug_implementations)]
 
 pub use arbitrary;
+use once_cell::sync::OnceCell;
 
 extern "C" {
     // We do not actually cross the FFI bound here.
@@ -36,6 +37,9 @@ pub fn test_input_wrap(data: *const u8, size: usize) -> i32 {
     0
 }
 
+#[doc(hidden)]
+pub static RUST_LIBFUZZER_DEBUG_PATH: OnceCell<String> = OnceCell::new();
+
 #[doc(hidden)]
 #[export_name = "LLVMFuzzerInitialize"]
 pub fn initialize(_argc: *const isize, _argv: *const *const *const u8) -> isize {
@@ -52,6 +56,14 @@ pub fn initialize(_argc: *const isize, _argv: *const *const *const u8) -> isize
         default_hook(panic_info);
         ::std::process::abort();
     }));
+
+    // Initialize the `RUST_LIBFUZZER_DEBUG_PATH` cell with the path so it can be
+    // reused with little overhead.
+    if let Ok(path) = std::env::var("RUST_LIBFUZZER_DEBUG_PATH") {
+        RUST_LIBFUZZER_DEBUG_PATH
+            .set(path)
+            .expect("Since this is initialize it is only called once so can never fail");
+    }
     0
 }
 
@@ -130,7 +142,9 @@ macro_rules! fuzz_target {
             // 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") {
+
+            // `RUST_LIBFUZZER_DEBUG_PATH` is set in initialization.
+            if let Some(path) = $crate::RUST_LIBFUZZER_DEBUG_PATH.get() {
                 use std::io::Write;
                 let mut file = std::fs::File::create(path)
                     .expect("failed to create `RUST_LIBFUZZER_DEBUG_PATH` file");
@@ -151,7 +165,7 @@ macro_rules! fuzz_target {
         /// Auto-generated function
         #[no_mangle]
         pub extern "C" fn rust_fuzzer_test_input(bytes: &[u8]) {
-            use libfuzzer_sys::arbitrary::{Arbitrary, Unstructured};
+            use $crate::arbitrary::{Arbitrary, Unstructured};
 
             // Early exit if we don't have enough bytes for the `Arbitrary`
             // implementation. This helps the fuzzer avoid exploring all the
@@ -169,7 +183,9 @@ macro_rules! fuzz_target {
             // 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") {
+
+            // `RUST_LIBFUZZER_DEBUG_PATH` is set in initialization.
+            if let Some(path) = $crate::RUST_LIBFUZZER_DEBUG_PATH.get() {
                 use std::io::Write;
                 let mut file = std::fs::File::create(path)
                     .expect("failed to create `RUST_LIBFUZZER_DEBUG_PATH` file");