Make keys of Derivation*Goal more legible

The property that substitution goals come first is still preserved

src/libstore/build/derivation-building-goal.cc

@@ -66,11 +66,7 @@ DerivationBuildingGoal::~DerivationBuildingGoal()
 
 std::string DerivationBuildingGoal::key()
 {
-    /* Ensure that derivations get built in order of their name,
-       i.e. a derivation named "aardvark" always comes before
-       "baboon". And substitution goals always happen before
-       derivation goals (due to "bd$"). */
-    return "bd$" + std::string(drvPath.name()) + "$" + worker.store.printStorePath(drvPath);
+    return "dd$" + std::string(drvPath.name()) + "$" + worker.store.printStorePath(drvPath);
 }
 
 void DerivationBuildingGoal::killChild()

src/libstore/build/derivation-goal.cc

@@ -52,11 +52,7 @@ DerivationGoal::DerivationGoal(
 
 std::string DerivationGoal::key()
 {
-    /* Ensure that derivations get built in order of their name,
-       i.e. a derivation named "aardvark" always comes before
-       "baboon". And substitution goals always happen before
-       derivation goals (due to "b$"). */
-    return "b$" + std::string(drvPath.name()) + "$" + SingleDerivedPath::Built{
+    return "db$" + std::string(drvPath.name()) + "$" + SingleDerivedPath::Built{
         .drvPath = makeConstantStorePathRef(drvPath),
         .output = wantedOutput,
     }.to_string(worker.store);

src/libstore/build/derivation-trampoline-goal.cc

@@ -58,11 +58,7 @@ static StorePath pathPartOfReq(const SingleDerivedPath & req)
 
 std::string DerivationTrampolineGoal::key()
 {
-    /* Ensure that derivations get built in order of their name,
-       i.e. a derivation named "aardvark" always comes before "baboon". And
-       substitution goals, derivation goals, and derivation building goals always happen before
-       derivation goals (due to "bt$"). */
-    return "bt$" + std::string(pathPartOfReq(*drvReq).name()) + "$" + DerivedPath::Built{
+    return "da$" + std::string(pathPartOfReq(*drvReq).name()) + "$" + DerivedPath::Built{
         .drvPath = drvReq,
         .outputs = wantedOutputs,
     }.to_string(worker.store);

src/libstore/build/drv-output-substitution-goal.cc

@@ -153,8 +153,6 @@ Goal::Co DrvOutputSubstitutionGoal::realisationFetched(
 
 std::string DrvOutputSubstitutionGoal::key()
 {
-    /* "a$" ensures substitution goals happen before derivation
-       goals. */
     return "a$" + std::string(id.to_string());
 }
 

src/libstore/include/nix/store/build/goal.hh

@@ -456,6 +456,18 @@ public:
      */
     virtual void timedOut(Error && ex) = 0;
 
+    /**
+     * Used for comparisons. The order matters a bit for scheduling. We
+     * want:
+     *
+     * 1. Substitution
+     * 2. Derivation administrativia
+     * 3. Actual building
+     *
+     * Also, ensure that derivations get processed in order of their
+     * name, i.e. a derivation named "aardvark" always comes before
+     * "baboon".
+     */
     virtual std::string key() = 0;
 
     /**

src/libstore/include/nix/store/build/substitution-goal.hh

@@ -58,10 +58,6 @@ public:
         unreachable();
     };
 
-    /**
-     * We prepend "a$" to the key name to ensure substitution goals
-     * happen before derivation goals.
-     */
     std::string key() override
     {
         return "a$" + std::string(storePath.name()) + "$" + worker.store.printStorePath(storePath);