Do not crash when ctx.action.args.add map_fn returns a list of the wrong length.

PiperOrigin-RevId: 183668291

2 files changed, 12 insertions, 1 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkActionFactory.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkActionFactory.java
index 4bf9999f50..327c73697c 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkActionFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkActionFactory.java
@@ -950,7 +950,8 @@ public class SkylarkActionFactory implements SkylarkValue {
           noneable = true,
           doc =
               "The passed objects are passed through a map function. "
-                  + "For vector args the function is given a list and is expected to return a list."
+                  + "For vector args the function is given a list and is expected to "
+                  + "return a list of the same length as the input."
         )
       },
       useLocation = true
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkCustomCommandLine.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkCustomCommandLine.java
index e67ff39f09..c9e5a7ddf9 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkCustomCommandLine.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/SkylarkCustomCommandLine.java
@@ -144,6 +144,16 @@ class SkylarkCustomCommandLine extends CommandLine {
                   null));
         }
         List resultAsList = (List) result;
+        if (resultAsList.size() != count) {
+          throw new CommandLineExpansionException(
+              errorMessage(
+                  String.format(
+                      "map_fn must return a list of the same length as the input. "
+                          + "Found list of length %d, expected %d.",
+                      resultAsList.size(), count),
+                  location,
+                  null));
+        }
         mutatedValues.clear();
         mutatedValues.addAll(resultAsList);
       }