RepositoryCache: support hard linkning

Instead of copying files found in a cache hit, support adding
a hard link. This should save disk space if the same files from
cache are used in many workspaces. Fixes #5568.

Change-Id: Ie8192f9669d8420283e18e0813f3160a515ba8fe
PiperOrigin-RevId: 205034815

3 files changed, 26 insertions, 6 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java b/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
index 89f40e0a27..2b05a506e1 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
@@ -230,6 +230,7 @@ public class BazelRepositoryModule extends BlazeModule {
 
     RepositoryOptions repoOptions = env.getOptions().getOptions(RepositoryOptions.class);
     if (repoOptions != null) {
+      repositoryCache.setHardlink(repoOptions.useHardlinks);
       if (repoOptions.experimentalRepositoryCache != null) {
         Path repositoryCachePath;
         if (repoOptions.experimentalRepositoryCache.isAbsolute()) {
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/repository/RepositoryOptions.java b/src/main/java/com/google/devtools/build/lib/bazel/repository/RepositoryOptions.java
index d3ced966e2..5b06cab1a3 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/repository/RepositoryOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/repository/RepositoryOptions.java
@@ -47,6 +47,16 @@ public class RepositoryOptions extends OptionsBase {
   public PathFragment experimentalRepositoryCache;
 
   @Option(
+      name = "experimental_repository_cache_hardlinks",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.BAZEL_CLIENT_OPTIONS,
+      effectTags = {OptionEffectTag.BAZEL_INTERNAL_CONFIGURATION},
+      help =
+          "If set, the repository cache will hardlink the file in case of a"
+              + " cache hit, rather than copying. This is inteded to save disk space.")
+  public boolean useHardlinks;
+
+  @Option(
       name = "distdir",
       oldName = "experimental_distdir",
       defaultValue = "null",
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/repository/cache/RepositoryCache.java b/src/main/java/com/google/devtools/build/lib/bazel/repository/cache/RepositoryCache.java
index 956d826e9c..aa1904009c 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/repository/cache/RepositoryCache.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/repository/cache/RepositoryCache.java
@@ -76,6 +76,7 @@ public class RepositoryCache {
 
   @Nullable private Path repositoryCachePath;
   @Nullable private Path contentAddressablePath;
+  private boolean useHardlinks;
 
   public void setRepositoryCachePath(@Nullable Path repositoryCachePath) {
     this.repositoryCachePath = repositoryCachePath;
@@ -83,6 +84,10 @@ public class RepositoryCache {
         ? repositoryCachePath.getRelative(CAS_DIR) : null;
   }
 
+  public void setHardlink(boolean useHardlinks) {
+    this.useHardlinks = useHardlinks;
+  }
+
   /**
    * @return true iff the cache path is set.
    */
@@ -103,11 +108,11 @@ public class RepositoryCache {
   }
 
   /**
-   * Copies a cached value to a specified directory, if it exists.
+   * Copy or hardlink cached value to a specified directory, if it exists.
    *
-   * We're using copying instead of symlinking because symlinking require weird checks to verify
-   * that the symlink still points to an existing artifact. e.g. cleaning up the central cache but
-   * not the workspace cache.
+   * <p>We're using hardlinking instead of symlinking because symlinking require weird checks to
+   * verify that the symlink still points to an existing artifact. e.g. cleaning up the central
+   * cache but not the workspace cache.
    *
    * @param cacheKey The string key to cache the value by.
    * @param targetPath The path where the cache value should be copied to.
@@ -118,7 +123,7 @@ public class RepositoryCache {
    */
   @Nullable
   public synchronized Path get(String cacheKey, Path targetPath, KeyType keyType)
-    throws IOException {
+      throws IOException {
     Preconditions.checkState(isEnabled());
 
     assertKeyIsValid(cacheKey, keyType);
@@ -138,7 +143,11 @@ public class RepositoryCache {
     }
 
     FileSystemUtils.createDirectoryAndParents(targetPath.getParentDirectory());
-    FileSystemUtils.copyFile(cacheValue, targetPath);
+    if (useHardlinks) {
+      FileSystemUtils.createHardLink(targetPath, cacheValue);
+    } else {
+      FileSystemUtils.copyFile(cacheValue, targetPath);
+    }
     FileSystemUtils.touchFile(cacheValue);
 
     return targetPath;