Make the default ReadAdvice configurable by sysprop (#13264)

lucene/CHANGES.txt

@@ -163,9 +163,11 @@ Bug Fixes
 Changes in Runtime Behavior
 ---------------------
 
-* GITHUB#13244: IOContext now uses ReadAdvice#RANDOM by default for read
+* GITHUB#13244, GITHUB#13264: IOContext now uses ReadAdvice#RANDOM by default for read
   operations. An implication is that `MMapDirectory` will use POSIX_MADV_RANDOM
-  on POSIX systems. (Adrien Grand)
+  on POSIX systems. To fallback to OS default behaviour, pass system property via
+  `-Dorg.apache.lucene.store.defaultReadAdvice=normal`. This may be useful on systems
+  with lots of RAM as this increases read-ahead. (Adrien Grand, Uwe Schindler)
 
 Changes in Backwards Compatibility Policy
 -----------------------------------------

lucene/core/src/java/org/apache/lucene/store/IOContext.java

@@ -18,6 +18,7 @@ package org.apache.lucene.store;
 
 import java.util.Arrays;
 import java.util.Objects;
+import org.apache.lucene.util.Constants;
 
 /**
  * IOContext holds additional details on the merge/search context. An IOContext object can never be
@@ -48,15 +49,15 @@ public record IOContext(
   /**
    * A default context for normal reads/writes. Use {@link #withReadAdvice(ReadAdvice)} to specify
    * another {@link ReadAdvice}.
+   *
+   * <p>It will use {@link ReadAdvice#RANDOM} by default, unless set by system property {@code
+   * org.apache.lucene.store.defaultReadAdvice}.
    */
-  public static final IOContext DEFAULT = new IOContext(ReadAdvice.RANDOM);
+  public static final IOContext DEFAULT = new IOContext(Constants.DEFAULT_READADVICE);
 
   /** A default context for reads with {@link ReadAdvice#SEQUENTIAL}. */
   public static final IOContext READONCE = new IOContext(ReadAdvice.SEQUENTIAL);
 
-  private static final IOContext[] DEFAULT_READADVICE_CACHE =
-      Arrays.stream(ReadAdvice.values()).map(IOContext::new).toArray(IOContext[]::new);
-
   @SuppressWarnings("incomplete-switch")
   public IOContext {
     Objects.requireNonNull(context, "context must not be null");
@@ -90,6 +91,9 @@ public record IOContext(
     this(Context.MERGE, mergeInfo, null, ReadAdvice.SEQUENTIAL);
   }
 
+  private static final IOContext[] READADVICE_TO_IOCONTEXT =
+      Arrays.stream(ReadAdvice.values()).map(IOContext::new).toArray(IOContext[]::new);
+
   /**
    * Return an updated {@link IOContext} that has the provided {@link ReadAdvice} if the {@link
    * Context} is a {@link Context#DEFAULT} context, otherwise return this existing instance. This
@@ -99,7 +103,7 @@ public record IOContext(
    */
   public IOContext withReadAdvice(ReadAdvice advice) {
     if (context == Context.DEFAULT) {
-      return DEFAULT_READADVICE_CACHE[advice.ordinal()];
+      return READADVICE_TO_IOCONTEXT[advice.ordinal()];
     } else {
       return this;
     }

lucene/core/src/java/org/apache/lucene/util/Constants.java

@@ -18,7 +18,10 @@ package org.apache.lucene.util;
 
 import java.security.AccessController;
 import java.security.PrivilegedAction;
+import java.util.Locale;
+import java.util.Optional;
 import java.util.logging.Logger;
+import org.apache.lucene.store.ReadAdvice;
 
 /** Some useful constants. */
 public final class Constants {
@@ -152,6 +155,16 @@ public final class Constants {
     return false;
   }
 
+  /**
+   * The default {@link ReadAdvice} used for opening index files. It will be {@link
+   * ReadAdvice#RANDOM} by default, unless set by system property {@code
+   * org.apache.lucene.store.defaultReadAdvice}.
+   */
+  public static final ReadAdvice DEFAULT_READADVICE =
+      Optional.ofNullable(getSysProp("org.apache.lucene.store.defaultReadAdvice"))
+          .map(a -> ReadAdvice.valueOf(a.toUpperCase(Locale.ROOT)))
+          .orElse(ReadAdvice.RANDOM);
+
   private static String getSysProp(String property) {
     try {
       return doPrivileged(() -> System.getProperty(property));