[DEVOPS-999] explorer frontend: Replace @gitrev@ with actual rev

This is done in a way that avoids a webpack rebuild every time the git
revision changes.

Author: rvl

explorer/frontend/default.nix

@@ -91,8 +91,29 @@ let
       '';
     };
 
+  # Stamps the frontend with the git revision in a way that avoids
+  # a webpack rebuild when the git revision changes.
+  # This will just replace @GITREV@ in all javascript files.
+  # See also: cardano-sl/scripts/set-git-rev/default.nix
+  withGitRev = drvOut: let
+    drvOutOutputs = drvOut.outputs or ["out"];
+  in
+    pkgs.runCommand drvOut.name {
+      outputs  = drvOutOutputs;
+      passthru = drvOut.drvAttrs
+        // (drvOut.passthru or {})
+        // { inherit gitrev; };
+    }
+    (concatMapStrings (output: ''
+      cp -a "${drvOut.${output}}" "${"$"}${output}"
+      chmod -R +w "${"$"}${output}"
+      find "${"$"}${output}" -type f -name '*.js' \
+        -exec echo Setting gitrev in {} ';' \
+        -exec sed -i 's/@GITREV@/${gitrev}/g' {} ';'
+    '') drvOutOutputs);
+
 in
 
-  pkgs.callPackage frontend {
+  withGitRev (pkgs.callPackage frontend {
     inherit (yarn2nix) mkYarnPackage;
-  }
+  })