std.crypto.random: introduce fork safety

Everybody gets what they want!

 * AT_RANDOM is completely ignored.
 * On Linux, MADV_WIPEONFORK is used to provide fork safety.
 * On pthread systems, `pthread_atfork` is used to provide fork safety.
 * For systems that do not have the capability to provide fork safety,
   the implementation falls back to calling getrandom() every time.
 * If madvise is unavailable or returns an error, or pthread_atfork
   fails for whatever reason, it falls back to calling getrandom() every
   time.
 * Applications may choose to opt-out of fork safety.
 * Applications may choose to opt-in to unconditionally calling
   getrandom() for every call to std.crypto.random.fillFn.
 * Added `std.meta.globalOption`.
 * Added `std.os.madvise` and related bits.
 * Bumped up the size of the main thread TLS buffer. See the comment
   there for justification.
 * Simpler hot path in TLS initialization.

1 files changed, 22 insertions, 0 deletions

diff --git a/lib/std/os/bits/linux.zig b/lib/std/os/bits/linux.zig
index 583240fd4e..2bcfc89ecf 100644
--- a/lib/std/os/bits/linux.zig
+++ b/lib/std/os/bits/linux.zig
@@ -2045,3 +2045,25 @@ pub const rlimit = extern struct {
     /// Hard limit
     max: rlim_t,
 };
+
+pub const MADV_NORMAL = 0;
+pub const MADV_RANDOM = 1;
+pub const MADV_SEQUENTIAL = 2;
+pub const MADV_WILLNEED = 3;
+pub const MADV_DONTNEED = 4;
+pub const MADV_FREE = 8;
+pub const MADV_REMOVE = 9;
+pub const MADV_DONTFORK = 10;
+pub const MADV_DOFORK = 11;
+pub const MADV_MERGEABLE = 12;
+pub const MADV_UNMERGEABLE = 13;
+pub const MADV_HUGEPAGE = 14;
+pub const MADV_NOHUGEPAGE = 15;
+pub const MADV_DONTDUMP = 16;
+pub const MADV_DODUMP = 17;
+pub const MADV_WIPEONFORK = 18;
+pub const MADV_KEEPONFORK = 19;
+pub const MADV_COLD = 20;
+pub const MADV_PAGEOUT = 21;
+pub const MADV_HWPOISON = 100;
+pub const MADV_SOFT_OFFLINE = 101;