Make HeaderInfo.modularHeaders lazy.

This makes later serialization work slightly simpler.

PiperOrigin-RevId: 607735291
Change-Id: I17f980c8f947d1eeb43fa0099c2434ed5590d037

src/main/java/com/google/devtools/build/lib/rules/cpp/CcCompilationContext.java

@@ -467,7 +467,7 @@ public IncludeScanningHeaderData.Builder createIncludeScanningHeaderData(
     for (HeaderInfo transitiveHeaderInfo : transitiveHeaderInfos) {
       boolean isModule = createModularHeaders && transitiveHeaderInfo.getModule(usePic) != null;
       handleHeadersForIncludeScanning(
-          transitiveHeaderInfo.modularHeaders,
+          transitiveHeaderInfo.getModularHeaders(),
           pathToLegalArtifact,
           treeArtifacts,
           isModule,
@@ -492,7 +492,7 @@ public IncludeScanningHeaderData.Builder createIncludeScanningHeaderData(
     if (transitiveHeaderCount == -1) {
       transitiveHeaderCount = pathToLegalArtifact.size();
     }
-    removeArtifactsFromSet(modularHeaders, headerInfo.modularHeaders);
+    removeArtifactsFromSet(modularHeaders, headerInfo.getModularHeaders());
     removeArtifactsFromSet(modularHeaders, headerInfo.textualHeaders);
     removeArtifactsFromSet(modularHeaders, headerInfo.separateModuleHeaders);
     return new IncludeScanningHeaderData.Builder(pathToLegalArtifact, modularHeaders);
@@ -514,8 +514,9 @@ public Set<DerivedArtifact> computeUsedModules(
       }
       // Not using range-based for loops here as often there is exactly one element in this list
       // and the amount of garbage created by SingletonImmutableList.iterator() is significant.
-      for (int i = 0; i < transitiveHeaderInfo.modularHeaders.size(); i++) {
-        Artifact header = transitiveHeaderInfo.modularHeaders.get(i);
+      ImmutableList<Artifact> modularHeaders = transitiveHeaderInfo.getModularHeaders();
+      for (int i = 0; i < modularHeaders.size(); i++) {
+        Artifact header = modularHeaders.get(i);
         if (includes.contains(header)) {
           modules.add(module);
           break;
@@ -603,7 +604,7 @@ ImmutableList<Artifact> getHeaderModuleSrcs(boolean separateModule) {
       return headerInfo.separateModuleHeaders;
     }
     return new ImmutableSet.Builder<Artifact>()
-        .addAll(headerInfo.modularHeaders)
+        .addAll(headerInfo.getModularHeaders())
         .addAll(headerInfo.textualHeaders)
         .addAll(headerInfo.separateModuleHeaders)
         .build()
@@ -918,7 +919,7 @@ public Builder addSystemIncludeDirs(Iterable<PathFragment> systemIncludeDirs) {
       return this;
     }
 
-    /** Add framewrok include directories to be added with "-F". */
+    /** Add framework include directories to be added with "-F". */
     @CanIgnoreReturnValue
     public Builder addFrameworkIncludeDirs(Iterable<PathFragment> frameworkIncludeDirs) {
       this.frameworkIncludeDirs.addAll(frameworkIncludeDirs);
@@ -1181,9 +1182,6 @@ static final class HeaderInfo {
     /** All private header files that are compiled into this module. */
     final ImmutableList<Artifact> modularPrivateHeaders;
 
-    /** All header files that are compiled into this module. */
-    private final ImmutableList<Artifact> modularHeaders;
-
     /** All textual header files that are contained in this module. */
     final ImmutableList<Artifact> textualHeaders;
 
@@ -1196,6 +1194,13 @@ static final class HeaderInfo {
     /** HeaderInfos of direct dependencies of C++ target represented by this context. */
     final ImmutableList<HeaderInfo> deps;
 
+    /**
+     * All header files that are compiled into this module.
+     *
+     * <p>Lazily initialized. Use {@link #getModularHeaders} instead.
+     */
+    private transient volatile ImmutableList<Artifact> lazyModularHeaders;
+
     /** Collection representing the memoized form of transitive information, set by flatten(). */
     private TransitiveHeaderCollection memo = null;
 
@@ -1213,8 +1218,6 @@ static final class HeaderInfo {
       this.picHeaderModule = picHeaderModule;
       this.modularPublicHeaders = modularPublicHeaders;
       this.modularPrivateHeaders = modularPrivateHeaders;
-      this.modularHeaders =
-          ImmutableList.copyOf(Iterables.concat(modularPublicHeaders, modularPrivateHeaders));
       this.textualHeaders = textualHeaders;
       this.separateModuleHeaders = separateModuleHeaders;
       this.separateModule = separateModule;
@@ -1271,6 +1274,18 @@ private void addOthers(Set<HeaderInfo> result, List<HeaderInfo> additionalDeps)
       }
     }
 
+    private ImmutableList<Artifact> getModularHeaders() {
+      if (lazyModularHeaders == null) {
+        synchronized (this) {
+          if (lazyModularHeaders == null) {
+            lazyModularHeaders =
+                ImmutableList.copyOf(Iterables.concat(modularPublicHeaders, modularPrivateHeaders));
+          }
+        }
+      }
+      return lazyModularHeaders;
+    }
+
     /** Represents the memoized transitive information for a HeaderInfo instance. */
     private class TransitiveHeaderCollection extends AbstractCollection<HeaderInfo> {
       private final int size;