In PathFragment#getRelative, return the passed-in PathFragment wh…

…en possible instead of creating a new instance.

This is possible when:

* The argument is an absolute path fragment.
* The base path is the empty path fragment.

PiperOrigin-RevId: 681455847
Change-Id: Ie7dd6687a4ebf6ad113f8cf4368ffd110316890b

src/main/java/com/google/devtools/build/lib/vfs/PathFragment.java

@@ -51,7 +51,7 @@
  * <p>Mac and Windows path fragments are case insensitive.
  */
 @Immutable
-public abstract class PathFragment
+public abstract sealed class PathFragment
  implements Comparable<PathFragment>, FileType.HasFileType, PathStrippable {
  private static final OsPathPolicy OS = OsPathPolicy.getFilePathOs();
 
@@ -62,6 +62,7 @@ public abstract class PathFragment
  private static final char ADDITIONAL_SEPARATOR_CHAR = OS.additionalSeparator();
 
  private final String normalizedPath;
+
  // DON'T add more fields here unless you know what you are doing. Adding another field will
  // increase the shallow heap of a PathFragment instance beyond the current value of 16 bytes.
  // Blaze's heap typically has many instances.
@@ -116,7 +117,7 @@ public String getPathString() {
  return normalizedPath;
  }
 
- public boolean isEmpty() {
+ public final boolean isEmpty() {
  return normalizedPath.isEmpty();
  }
 
@@ -193,7 +194,10 @@ public String getBaseName() {
  */
  public PathFragment getRelative(PathFragment other) {
  Preconditions.checkNotNull(other);
- // Fast-path: The path fragment is already normal, use cheaper normalization check
+ if (isEmpty() || other.isAbsolute()) {
+ return other;
+ }
+ // The path fragment is already normal, use cheaper normalization check.
  String otherStr = other.normalizedPath;
  return getRelative(otherStr, other.getDriveStrLength(), OS.needsToNormalizeSuffix(otherStr));
  }
@@ -210,7 +214,7 @@ public PathFragment getRelative(String other) {
  }
 
  private PathFragment getRelative(String other, int otherDriveStrLength, int normalizationLevel) {
- if (normalizedPath.isEmpty()) {
+ if (isEmpty()) {
  return create(other);
  }
  if (other.isEmpty()) {
@@ -281,7 +285,7 @@ public PathFragment getParentDirectory() {
  }
  } else {
  if (lastSeparator == -1) {
- if (!normalizedPath.isEmpty()) {
+ if (!isEmpty()) {
  return EMPTY_FRAGMENT;
  } else {
  return null;
@@ -380,7 +384,7 @@ public boolean endsWith(PathFragment other) {
  return false;
  }
  return normalizedPath.length() == other.normalizedPath.length()
- || other.normalizedPath.isEmpty()
+ || other.isEmpty()
  || normalizedPath.charAt(normalizedPath.length() - other.normalizedPath.length() - 1)
  == SEPARATOR_CHAR;
  }
@@ -608,7 +612,7 @@ public ImmutableList<String> splitToListOfSegments() {
 
  /** Returns the path string, or '.' if the path is empty. */
  public String getSafePathString() {
- return !normalizedPath.isEmpty() ? normalizedPath : ".";
+ return !isEmpty() ? normalizedPath : ".";
  }
 
  /**
@@ -622,7 +626,7 @@ public String getSafePathString() {
  public String getCallablePathString() {
  if (isAbsolute()) {
  return normalizedPath;
- } else if (normalizedPath.isEmpty()) {
+ } else if (isEmpty()) {
  return ".";
  } else if (normalizedPath.indexOf(SEPARATOR_CHAR) == -1) {
  return "." + SEPARATOR_CHAR + normalizedPath;

src/test/java/com/google/devtools/build/lib/vfs/PathFragmentTest.java

@@ -145,6 +145,20 @@ public void testGetRelative() {
  assertThat(create("a").getRelative(".").getPathString()).isEqualTo("a");
  }
 
+ @Test
+ public void getRelative_absolutePathArgument_returnsSameInstance(
+ @TestParameter({"/c/d", "c/d"}) String basePath) {
+ PathFragment absolute = PathFragment.create("/a/b");
+ assertThat(PathFragment.create(basePath).getRelative(absolute)).isSameInstanceAs(absolute);
+ }
+
+ @Test
+ public void getRelative_emptyBasePath_returnsSameInstance(
+ @TestParameter({"/a/b", "a/b"}) String argument) {
+ PathFragment instance = PathFragment.create(argument);
+ assertThat(EMPTY_FRAGMENT.getRelative(instance)).isSameInstanceAs(instance);
+ }
+
  @Test
  public void testIsNormalizedRelativePath() {
  assertThat(PathFragment.isNormalizedRelativePath("/a")).isFalse();