From 5bce943cdb8fc81525257413f174844d644d63c8 Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Tue, 20 Apr 2021 16:48:07 -0700
Subject: [PATCH] Use /dev/urandom instead of getrandom()

To generate the ahash crate's default hash keys, use /dev/urandom
instead of getrandom() to avoid blocking boot on systems where the
entropy pool isn't initialized in time and where the use case of this
crate doesn't actually require cryptographic randomness.

If opening or reading from /dev/urandom fails, fall back to getrandom().

Note that std::collections::HashMap doesn't block for randomness either,
for the same reason.  So this change just makes ahash work like HashMap.

Bug: 185934601
Change-Id: Ie81a1f3a893d578348db11aee114d1a8f2d9fac5
---
diff --git a/src/random_state.rs b/src/random_state.rs
index c3628bf..835467c 100644
--- a/src/random_state.rs
+++ b/src/random_state.rs
@@ -48,6 +48,15 @@ use crate::fallback_hash::*;
 #[cfg(not(all(target_arch = "arm", target_os = "none")))]
 static RAND_SOURCE: OnceBox<Box<dyn RandomSource + Send + Sync>> = OnceBox::new();
 
+#[cfg(all(feature = "runtime-rng", not(all(feature = "compile-time-rng", test))))]
+fn read_urandom(dest: &mut [u8]) -> Result<(), std::io::Error> {
+    use std::fs::File;
+    use std::io::Read;
+
+    let mut f = File::open("/dev/urandom")?;
+    f.read_exact(dest)
+}
+
 /// A supplier of Randomness used for different hashers.
 /// See [RandomState.set_random_source].
 pub trait RandomSource {
@@ -98,7 +107,9 @@ impl RandomSource for DefaultRandomSource {
 
         SEEDS.get_or_init(|| {
             let mut result: [u8; 64] = [0; 64];
-            getrandom::getrandom(&mut result).expect("getrandom::getrandom() failed.");
+            if read_urandom(&mut result).is_err() {
+                getrandom::getrandom(&mut result).expect("getrandom::getrandom() failed.");
+            }
             Box::new(result.convert())
         })
     }