std.crypto.tls.Client: rework the read function

Here's what I landed on for the TLS client. It's 16896 bytes
(max_ciphertext_record_len is 16640). I believe this is the theoretical
minimum size, give or take a few bytes.

These constraints are satisfied:
 * a call to the readvAdvanced() function makes at most one call to the
   underlying readv function
 * iovecs are provided by the API, and used by the implementation for
   underlying readv() calls to the socket
 * the theoretical minimum number of memcpy() calls are issued in all
   circumstances
 * decryption is only performed once for any given TLS record
 * large read buffers are fully exploited

This is accomplished by using the partial read buffer to storing both
cleartext and ciphertext.

1 files changed, 11 insertions, 0 deletions

diff --git a/lib/std/net.zig b/lib/std/net.zig
index 0112d5be8c..aa51176184 100644
--- a/lib/std/net.zig
+++ b/lib/std/net.zig
@@ -1672,6 +1672,17 @@ pub const Stream = struct {
         }
     }
 
+    pub fn readv(s: Stream, iovecs: []const os.iovec) ReadError!usize {
+        if (builtin.os.tag == .windows) {
+            // TODO improve this to use ReadFileScatter
+            if (iovecs.len == 0) return @as(usize, 0);
+            const first = iovecs[0];
+            return os.windows.ReadFile(s.handle, first.iov_base[0..first.iov_len], null, io.default_mode);
+        }
+
+        return os.readv(s.handle, iovecs);
+    }
+
     /// Returns the number of bytes read. If the number read is smaller than
     /// `buffer.len`, it means the stream reached the end. Reaching the end of
     /// a stream is not an error condition.