Make the --platform_mappings flag immutable.

If it cannot be changed in transitions, all errors will be caught in the first usage (in `SkyframeExecutor.createBuildConfigurationKey`), and so no further errors need to be considered.

Work towards platform-based flags: #19409.

PiperOrigin-RevId: 596918285
Change-Id: I68a1808d836716893491fd212eb2b5bc9352f541

src/main/java/com/google/devtools/build/lib/analysis/PlatformOptions.java

@@ -147,6 +147,9 @@ public static boolean platformIsDefault(Label platform) {
         OptionEffectTag.CHANGES_INPUTS,
         OptionEffectTag.LOADING_AND_ANALYSIS
       },
+      metadataTags = {
+        OptionMetadataTag.IMMUTABLE,
+      },
       help =
           "The location of a mapping file that describes which platform to use if none is set or "
               + "which flags to set when a platform already exists. Must be relative to the main "

src/main/java/com/google/devtools/build/lib/analysis/producers/BuildConfigurationKeyProducer.java

@@ -71,6 +71,8 @@ public StateMachine step(Tasks tasks) {
       Collection<String> transitionKeys = entry.getValue();
       tasks.lookUp(
           PlatformMappingValue.Key.create(entry.getKey().orElse(null)),
+          // No need to consider error handling: this flag cannot be changed and so errors were
+          // detected and handled in SkyframeExecutor.createBuildConfigurationKey.
           rawValue -> {
             var value = (PlatformMappingValue) rawValue;
             // Maps the value from all transition keys with the same platform mappings path.

src/main/java/com/google/devtools/build/lib/skyframe/SkyframeExecutor.java

@@ -1852,6 +1852,7 @@ private BuildConfigurationKey createBuildConfigurationKey(
         PlatformMappingValue.Key.create(platformMappingPath);
     EvaluationResult<SkyValue> evaluationResult =
         evaluateSkyKeys(eventHandler, ImmutableSet.of(platformMappingKey));
+    // Handle all possible errors with the platform mapping by reporting them to the user.
     if (evaluationResult.hasError()) {
       throw new InvalidConfigurationException(
           Code.PLATFORM_MAPPING_EVALUATION_FAILURE, evaluationResult.getError().getException());

src/main/java/com/google/devtools/build/lib/skyframe/config/PlatformMappingFunction.java

@@ -115,15 +115,14 @@ public PlatformMappingValue compute(SkyKey skyKey, Environment env)
                             + " '%s' but that path refers to a directory, not a file",
                         workspaceRelativeMappingPath, root),
                     Code.PLATFORM_MAPPINGS_FILE_IS_DIRECTORY),
-                Location.BUILTIN),
-            SkyFunctionException.Transience.PERSISTENT);
+                Location.BUILTIN));
       }
 
       List<String> lines;
       try {
         lines = FileSystemUtils.readLines(fileValue.realRootedPath().asPath(), UTF_8);
       } catch (IOException e) {
-        throw new PlatformMappingException(e, SkyFunctionException.Transience.TRANSIENT);
+        throw new PlatformMappingException(e);
       }
 
       Mappings parsed = parse(env, lines, mainRepoContext);
@@ -147,8 +146,7 @@ public PlatformMappingValue compute(SkyKey skyKey, Environment env)
                         + "package path roots, '%s'",
                     workspaceRelativeMappingPath, pathEntries),
                 Code.PLATFORM_MAPPINGS_FILE_NOT_FOUND),
-            Location.BUILTIN),
-        SkyFunctionException.Transience.PERSISTENT);
+            Location.BUILTIN));
   }
 
   private static FailureDetail createFailureDetail(String message, Code detailedCode) {
@@ -161,8 +159,20 @@ private static FailureDetail createFailureDetail(String message, Code detailedCo
   @VisibleForTesting
   static final class PlatformMappingException extends SkyFunctionException {
 
-    PlatformMappingException(Exception cause, Transience transience) {
-      super(cause, transience);
+    PlatformMappingException(OptionsParsingException cause) {
+      super(cause, Transience.PERSISTENT);
+    }
+
+    PlatformMappingException(MissingInputFileException cause) {
+      super(cause, Transience.PERSISTENT);
+    }
+
+    PlatformMappingException(IOException cause) {
+      super(cause, Transience.TRANSIENT);
+    }
+
+    PlatformMappingException(PlatformMappingParsingException cause) {
+      super(cause, Transience.PERSISTENT);
     }
   }
 
@@ -280,7 +290,7 @@ private static NativeAndStarlarkFlags parseStarlarkFlags(
       return NativeAndStarlarkFlags.create(
           nativeFlags.build(), fakeNativeParser.getStarlarkOptions());
     } catch (OptionsParsingException e) {
-      throw new PlatformMappingException(e, Transience.PERSISTENT);
+      throw new PlatformMappingException(e);
     }
   }
 
@@ -368,9 +378,7 @@ private static PlatformMappingException parsingException(String message) {
   }
 
   private static PlatformMappingException parsingException(String message, Exception cause) {
-    return new PlatformMappingException(
-        new PlatformMappingParsingException(message, cause),
-        SkyFunctionException.Transience.PERSISTENT);
+    return new PlatformMappingException(new PlatformMappingParsingException(message, cause));
   }
 
   /**