store: guess the URL of failing fixed-output derivations

This is a shameless layering violation in favour of UX. It falls back
trivially to "unknown", so it's purely a UX feature.

Diagnostic sample:

```
error: hash mismatch in fixed-output derivation '/nix/store/sjfw324j4533lwnpmr5z4icpb85r63ai-x1.drv':
        likely URL: https://meow.puppy.forge/puppy.tar.gz
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-a1Qvp3FOOkWpL9kFHgugU1ok5UtRPSu+NwCZKbbaEro=
```

Change-Id: I873eedcf7984ab23f57a6754be00232b5cb5b02c

4 files changed, 27 insertions, 1 deletions

diff --git a/doc/manual/rl-next/fod-failure-includes-url.md b/doc/manual/rl-next/fod-failure-includes-url.md
new file mode 100644
index 000000000..43179aa52
--- /dev/null
+++ b/doc/manual/rl-next/fod-failure-includes-url.md
@@ -0,0 +1,16 @@
+---
+synopsis: "Hash mismatch diagnostics for fixed-output derivations include the URL"
+cls: [1536]
+credits: [jade]
+category: Improvements
+---
+
+Now, when building fixed-output derivations, Lix will guess the URL that was used in the derivation using the `url` or `urls` properties in the derivation environment.
+This is a layering violation but making these diagnostics tractable when there are multiple instances of the `AAAA` hash is too significant of an improvement to pass it up.
+
+```
+error: hash mismatch in fixed-output derivation '/nix/store/sjfw324j4533lwnpmr5z4icpb85r63ai-x1.drv':
+        likely URL: https://meow.puppy.forge/puppy.tar.gz
+         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
+            got:    sha256-a1Qvp3FOOkWpL9kFHgugU1ok5UtRPSu+NwCZKbbaEro=
+```
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 968f669ec..1dd2ad8aa 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2546,9 +2546,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                     /* Throw an error after registering the path as
                        valid. */
                     worker.hashMismatch = true;
+                    // XXX: shameless layering violation hack that makes the hash mismatch error at least not utterly worthless
+                    auto guessedUrl = getOr(drv->env, "urls", getOr(drv->env, "url", "(unknown)"));
                     delayedException = std::make_exception_ptr(
-                        BuildError("hash mismatch in fixed-output derivation '%s':\n  specified: %s\n     got:    %s",
+                        BuildError("hash mismatch in fixed-output derivation '%s':\n likely URL: %s\n  specified: %s\n     got:    %s",
                             worker.store.printStorePath(drvPath),
+                            guessedUrl,
                             wanted.to_string(SRI, true),
                             got.to_string(SRI, true)));
                 }
diff --git a/tests/functional/build.sh b/tests/functional/build.sh
index 95a20dc6a..a540cf8fd 100644
--- a/tests/functional/build.sh
+++ b/tests/functional/build.sh
@@ -142,6 +142,8 @@ test "$(<<<"$out" grep -E '^error:' | wc -l)" = 2
 <<<"$out" grepQuiet -E "hash mismatch in fixed-output derivation '.*-x1\\.drv'"
 <<<"$out" grepQuiet -vE "hash mismatch in fixed-output derivation '.*-x3\\.drv'"
 <<<"$out" grepQuiet -vE "hash mismatch in fixed-output derivation '.*-x2\\.drv'"
+<<<"$out" grepQuiet -E "likely URL: https://meow.puppy.forge/puppy.tar.gz"
+<<<"$out" grepQuiet -vE "likely URL: https://kitty.forge/cat.tar.gz"
 <<<"$out" grepQuiet -E "error: build of '.*-x[1-4]\\.drv\\^out', '.*-x[1-4]\\.drv\\^out', '.*-x[1-4]\\.drv\\^out', '.*-x[1-4]\\.drv\\^out' failed"
 
 out="$(nix build -f fod-failing.nix -L x1 x2 x3 --keep-going 2>&1)" && status=0 || status=$?
@@ -151,6 +153,9 @@ test "$(<<<"$out" grep -E '^error:' | wc -l)" = 4
 <<<"$out" grepQuiet -E "hash mismatch in fixed-output derivation '.*-x1\\.drv'"
 <<<"$out" grepQuiet -E "hash mismatch in fixed-output derivation '.*-x3\\.drv'"
 <<<"$out" grepQuiet -E "hash mismatch in fixed-output derivation '.*-x2\\.drv'"
+<<<"$out" grepQuiet -E "likely URL: https://meow.puppy.forge/puppy.tar.gz"
+<<<"$out" grepQuiet -E "likely URL: https://kitty.forge/cat.tar.gz"
+<<<"$out" grepQuiet -E "likely URL: \(unknown\)"
 <<<"$out" grepQuiet -E "error: build of '.*-x[1-3]\\.drv\\^out', '.*-x[1-3]\\.drv\\^out', '.*-x[1-3]\\.drv\\^out' failed"
 
 out="$(nix build -f fod-failing.nix -L x4 2>&1)" && status=0 || status=$?
diff --git a/tests/functional/fod-failing.nix b/tests/functional/fod-failing.nix
index 37c04fe12..b63fbd900 100644
--- a/tests/functional/fod-failing.nix
+++ b/tests/functional/fod-failing.nix
@@ -6,6 +6,7 @@ rec {
       ''
         echo $name > $out
       '';
+    url = "https://meow.puppy.forge/puppy.tar.gz";
     outputHashMode = "recursive";
     outputHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
   };
@@ -15,6 +16,7 @@ rec {
       ''
         echo $name > $out
       '';
+    urls = "https://kitty.forge/cat.tar.gz";
     outputHashMode = "recursive";
     outputHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
   };