Supporting dependsOn for extraSteps API

depends-on.patch

diff --git a/tvl-kit/buildkite/default.nix b/tvl-kit/buildkite/default.nix
index b04dc55a0..2be217937 100644
--- a/tvl-kit/buildkite/default.nix
+++ b/tvl-kit/buildkite/default.nix
@@ -340,6 +340,7 @@ rec {
     , postBuild ? null
     , skip ? false
     , agents ? null
+    , dependsOn ? [ ]
     }:
     let
       parent = overridableParent parentOverride;
@@ -368,6 +369,13 @@ rec {
         skip
         agents;
 
+      # The Buildkite API supports a string and list of strings.
+      dependsOn =
+        if builtins.isString dependsOn then
+          [ dependsOn ]
+        else
+          dependsOn;
+
       # //nix/buildkite is growing a new feature for adding different
       # "build phases" which supersedes the previous `postBuild`
       # boolean API.
@@ -419,9 +427,23 @@ rec {
           in
           if cfg.alwaysRun then false else skip';
 
-        depends_on = lib.optional
-          (buildEnabled && !cfg.alwaysRun && !cfg.needsOutput)
-          cfg.parent.key;
+        depends_on =
+          let
+            # Buildkite requires keys to be globally unique, but for extraSteps
+            # it's more convenient to define dependencies local to the
+            # extraSteps configuration.
+            #
+            # We can support this by namespacing the dependsOn keys with the key
+            # of the parent step to which they're attached to make them globally
+            # unique.
+            namespacedDependsOn = builtins.map
+              (key: "${cfg.parent.key}-${key}")
+              cfg.dependsOn;
+          in
+          if buildEnabled && !cfg.alwaysRun && !cfg.needsOutput then
+            [ cfg.parent.key ] ++ namespacedDependsOn
+          else
+            namespacedDependsOn;
 
         command = pkgs.writeShellScript "${cfg.key}-script" ''
           set -ueo pipefail
@@ -433,8 +455,10 @@ rec {
           exec ${cfg.command}
         '';
 
+        key = "${cfg.parent.key}-${cfg.key}";
         soft_fail = cfg.softFail;
-      } // (lib.optionalAttrs (cfg.agents != null) { inherit (cfg) agents; })
+      }
+      // (lib.optionalAttrs (cfg.agents != null) { inherit (cfg) agents; })
       // (lib.optionalAttrs (cfg.branches != null) {
         branches = lib.concatStringsSep " " cfg.branches;
       });