Support SRI hashes

SRI hashes (https://www.w3.org/TR/SRI/) combine the hash algorithm and
a base-64 hash. This allows more concise and standard hash
specifications. For example, instead of

  import <nix/fetchurl.nl> {
    url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
    sha256 = "5d22dad058d5c800d65a115f919da22938c50dd6ba98c5e3a183172d149840a4";
  };

you can write

  import <nix/fetchurl.nl> {
    url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
    hash = "sha256-XSLa0FjVyADWWhFfkZ2iKTjFDda6mMXjoYMXLRSYQKQ=";
  };

In fixed-output derivations, the outputHashAlgo is no longer mandatory
if outputHash specifies the hash (either as an SRI or in the old
"<type>:<hash>" format).

'nix hash-{file,path}' now print hashes in SRI format by default. I
also reverted them to use SHA-256 by default because that's what we're
using most of the time in Nixpkgs.

Suggested by @zimbatm.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index fd7a61df8..2dbc3b630 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -20,7 +20,7 @@ const int sha512HashSize = 64;
 
 extern const string base32Chars;
 
-enum Base : int { Base64, Base32, Base16 };
+enum Base : int { Base64, Base32, Base16, SRI };
 
 
 struct Hash
@@ -38,8 +38,9 @@ struct Hash
     Hash(HashType type) : type(type) { init(); };
 
     /* Initialize the hash from a string representation, in the format
-       "[<type>:]<base16|base32|base64>". If the 'type' argument is
-       htUnknown, then the hash type must be specified in the
+       "[<type>:]<base16|base32|base64>" or "<type>-<base64>" (a
+       Subresource Integrity hash expression). If the 'type' argument
+       is htUnknown, then the hash type must be specified in the
        string. */
     Hash(const std::string & s, HashType type = htUnknown);