Better document build failure exit codes

- Improved API docs from comment

- Exit codes are for `nix-build`, not just `nix-store --release`

- Make note in tests so the magic numbers are not surprising

Picking up where #8387 left off.

src/libstore/build/entry-points.cc

@@ -31,11 +31,11 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     }
 
     if (failed.size() == 1 && ex) {
-        ex->status = worker.exitStatus();
+        ex->status = worker.failingExitStatus();
         throw std::move(*ex);
     } else if (!failed.empty()) {
         if (ex) logError(ex->info());
-        throw Error(worker.exitStatus(), "build of %s failed", showPaths(failed));
+        throw Error(worker.failingExitStatus(), "build of %s failed", showPaths(failed));
     }
 }
 
@@ -102,10 +102,10 @@ void Store::ensurePath(const StorePath & path)
 
     if (goal->exitCode != Goal::ecSuccess) {
         if (goal->ex) {
-            goal->ex->status = worker.exitStatus();
+            goal->ex->status = worker.failingExitStatus();
             throw std::move(*goal->ex);
         } else
-            throw Error(worker.exitStatus(), "path '%s' does not exist and cannot be created", printStorePath(path));
+            throw Error(worker.failingExitStatus(), "path '%s' does not exist and cannot be created", printStorePath(path));
     }
 }
 
@@ -128,7 +128,7 @@ void Store::repairPath(const StorePath & path)
             goals.insert(worker.makeDerivationGoal(*info->deriver, OutputsSpec::All { }, bmRepair));
             worker.run(goals);
         } else
-            throw Error(worker.exitStatus(), "cannot repair path '%s'", printStorePath(path));
+            throw Error(worker.failingExitStatus(), "cannot repair path '%s'", printStorePath(path));
     }
 }
 

src/libstore/build/worker.cc

@@ -468,16 +468,9 @@ void Worker::waitForInput()
 }
 
 
-unsigned int Worker::exitStatus()
+unsigned int Worker::failingExitStatus()
 {
-    /*
-     * 1100100
-     *    ^^^^
-     *    |||`- timeout
-     *    ||`-- output hash mismatch
-     *    |`--- build failure
-     *    `---- not deterministic
-     */
+    // See API docs in header for explanation
     unsigned int mask = 0;
     bool buildFailure = permanentFailure || timedOut || hashMismatch;
     if (buildFailure)

src/libstore/build/worker.hh

@@ -280,7 +280,28 @@ public:
      */
     void waitForInput();
 
-    unsigned int exitStatus();
+    /***
+     * The exit status in case of failure.
+     *
+     * In the case of a build failure, returned value follows this
+     * bitmask:
+     *
+     * ```
+     * 0b1100100
+     *      ^^^^
+     *      |||`- timeout
+     *      ||`-- output hash mismatch
+     *      |`--- build failure
+     *      `---- not deterministic
+     * ```
+     *
+     * In other words, the failure code is at least 100 (0b1100100), but
+     * might also be greater.
+     *
+     * Otherwise (no build failure, but some other sort of failure by
+     * assumption), this returned value is 1.
+     */
+    unsigned int failingExitStatus();
 
     /**
      * Check whether the given valid path exists and has the right