Only compute hostname once per server lifetime

NetUtil.getShortHostName can take seconds on mac and on windows (like, 20!),
since it performs reverse dns lookup. We already cached hostname for the
BazelWorkspaceStatusModule, let's cache it for entire bazel server. Also make
sure that users of the method understand it's cached.

Fixes #3586.

RELNOTES: None.
PiperOrigin-RevId: 168691615

3 files changed, 21 insertions, 9 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java b/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java
index bb77abc020..16f882fd11 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java
@@ -282,7 +282,6 @@ public class BazelWorkspaceStatusModule extends BlazeModule {
   }
 
   private class BazelStatusActionFactory implements WorkspaceStatusAction.Factory {
-    private String hostname;
 
     @Override
     public Map<String, String> createDummyWorkspaceStatus() {
@@ -312,11 +311,7 @@ public class BazelWorkspaceStatusModule extends BlazeModule {
      * changes during bazel server lifetime, bazel will not see the change.
      */
     private String getHostname() {
-      if (hostname == null) {
-        hostname = NetUtil.findShortHostName();
-      }
-
-      return hostname;
+      return NetUtil.getCachedShortHostName();
     }
   }
 
diff --git a/src/main/java/com/google/devtools/build/lib/exec/local/LocalSpawnRunner.java b/src/main/java/com/google/devtools/build/lib/exec/local/LocalSpawnRunner.java
index 57d757691d..9aed79b08d 100644
--- a/src/main/java/com/google/devtools/build/lib/exec/local/LocalSpawnRunner.java
+++ b/src/main/java/com/google/devtools/build/lib/exec/local/LocalSpawnRunner.java
@@ -90,7 +90,7 @@ public final class LocalSpawnRunner implements SpawnRunner {
     this.execRoot = execRoot;
     this.processWrapper = getProcessWrapper(execRoot, localOs).getPathString();
     this.localExecutionOptions = Preconditions.checkNotNull(localExecutionOptions);
-    this.hostName = NetUtil.findShortHostName();
+    this.hostName = NetUtil.getCachedShortHostName();
     this.resourceManager = resourceManager;
     this.useProcessWrapper = useProcessWrapper;
     this.productName = productName;
diff --git a/src/main/java/com/google/devtools/build/lib/util/NetUtil.java b/src/main/java/com/google/devtools/build/lib/util/NetUtil.java
index f22146a71d..dc2ce8867e 100644
--- a/src/main/java/com/google/devtools/build/lib/util/NetUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/util/NetUtil.java
@@ -21,14 +21,31 @@ import java.net.UnknownHostException;
  */
 public final class NetUtil {
 
+  private static String hostname;
+
   private NetUtil() {
   }
 
   /**
+   * Returns the *cached* short hostname (computed at most once per the lifetime of a server). Can
+   * take seconds to complete when the cache is cold.
+   */
+  public static String getCachedShortHostName() {
+    if (hostname == null) {
+      synchronized (NetUtil.class) {
+        if (hostname == null) {
+          hostname = computeShortHostName();
+        }
+      }
+    }
+    return computeShortHostName();
+  }
+
+  /**
    * Returns the short hostname or <code>unknown</code> if the host name could not be determined.
-   * Performs reverse DNS lookup and can take seconds to complete!
+   * Performs reverse DNS lookup and can take seconds to complete.
    */
-  public static String findShortHostName() {
+  private static String computeShortHostName() {
     try {
       return InetAddress.getLocalHost().getHostName();
     } catch (UnknownHostException e) {