Guard include scanner against null grepIncludes

The grepIncludes artifact may be null if the owning rule doesn't have a
$grep_includes attribute (see CppHelper.getGrepIncludes). This seems to
be intentional. However, the remote include scanner requires that
grepIncludes is *not* null, so we need to safe-guard against it.

Unfortunately, this is brittle, because accidentally misspelling or
removing the attributes makes all grep operations fall back to the
built-in implementation.

PiperOrigin-RevId: 277499382

src/main/java/com/google/devtools/build/lib/includescanning/IncludeParser.java

diff --git a/src/main/java/com/google/devtools/build/lib/includescanning/IncludeParser.java b/src/main/java/com/google/devtools/build/lib/includescanning/IncludeParser.java
index 0857f29..acde8ac 100644
--- a/src/main/java/com/google/devtools/build/lib/includescanning/IncludeParser.java
+++ b/src/main/java/com/google/devtools/build/lib/includescanning/IncludeParser.java
@@ -843,13 +843,16 @@
       Artifact file,
       ActionExecutionMetadata actionExecutionMetadata,
       ActionExecutionContext actionExecutionContext,
-      Artifact grepIncludes,
+      @Nullable Artifact grepIncludes,
       @Nullable SpawnIncludeScanner remoteIncludeScanner,
       boolean isOutputFile)
       throws IOException, ExecException, InterruptedException {
     Collection<Inclusion> inclusions;
 
+    // TODO(ulfjack): grepIncludes may be null if the corresponding attribute on the rule is missing
+    //  (see CppHelper.getGrepIncludes) or misspelled. It would be better to disallow this case.
     if (remoteIncludeScanner != null
+        && grepIncludes != null
         && remoteIncludeScanner.shouldParseRemotely(file, actionExecutionContext)) {
       inclusions =
           remoteIncludeScanner.extractInclusions(
@@ -866,7 +869,7 @@
             extractInclusions(
                 FileSystemUtils.readContent(actionExecutionContext.getInputPath(file)));
       } catch (IOException e) {
-        if (remoteIncludeScanner != null) {
+        if (remoteIncludeScanner != null && grepIncludes != null) {
           logger.log(
               Level.WARNING,
               "Falling back on remote parsing of " + actionExecutionContext.getInputPath(file),

src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java

diff --git a/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java b/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java
index 0c16c97..440fef0 100644
--- a/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java
+++ b/src/main/java/com/google/devtools/build/lib/includescanning/LegacyIncludeScanner.java
@@ -58,6 +58,7 @@
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
+import javax.annotation.Nullable;
 
 /**
  * C include scanner. Quickly scans C/C++ source files to determine the bounding set of transitively
@@ -650,7 +651,7 @@
   private class LegacyIncludeVisitor extends AbstractQueueVisitor implements IncludeVisitor {
     private final ActionExecutionMetadata actionExecutionMetadata;
     private final ActionExecutionContext actionExecutionContext;
-    private final Artifact grepIncludes;
+    @Nullable private final Artifact grepIncludes;
     private final Map<PathFragment, Artifact> pathToLegalOutputArtifact;
     /** The set of headers known to be part of a C++ module. Scanning can stop here. */
     private final Set<Artifact> modularHeaders;
@@ -661,7 +662,7 @@
     public LegacyIncludeVisitor(
         ActionExecutionMetadata actionExecutionMetadata,
         ActionExecutionContext actionExecutionContext,
-        Artifact grepIncludes,
+        @Nullable Artifact grepIncludes,
         final Map<PathFragment, Artifact> pathToLegalOutputArtifact,
         Set<Artifact> modularHeaders) {
       super(